Publications

2020

Orientation of swimming cells with annular beam optical tweezers

Lenton, I. C. D., Armstrong, D. J., Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2020). Opt. Commun., 459, 124864. https://doi.org/10.1016/j.optcom.2019.124864'
Optical tweezers are a versatile tool that can be used to manipulate small particles including both motile and non-motile bacteria and cells. The orientation of a non-spherical particle within a beam depends on the shape of the particle and the shape of the light field. By using multiple beams, sculpted light fields or dynamically changing beams, it is possible to control the orientation of certain particles. In this paper we discuss the orientation of the rod-shaped bacteria Escherichia coli (E. coli) using dynamically shifting annular beam optical tweezers. We begin with examples of different beams used for the orientation of rod-shaped particles. We discuss the differences between orientation of motile and non-motile particles, and explore annular beams and the circumstances when they may be beneficial for manipulation of non-spherical particles or cells. Using simulations we map out the trajectory the E. coli takes. Estimating the trap stiffness along the trajectory gives us an insight into how stable an intermediate rotation is with respect to the desired orientation. Using this method, we predict and experimentally verify the change in the orientation of motile E. coli from vertical to near-horizontal with only one intermediate step. The method is not specific to exploring the orientation of particles and could be easily extended to quantify the stability of an arbitrary particle trajectory.
```
@article{Lenton2020Mar,
  author = {Lenton, Isaac C. D. and Armstrong, Declan J. and Stilgoe, Alexander B. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  title = {{Orientation of swimming cells with annular beam optical tweezers}},
  journal = {Opt. Commun.},
  volume = {459},
  pages = {124864},
  year = {2020},
  month = mar,
  issn = {0030-4018},
  publisher = {North-Holland},
  doi = {10.1016/j.optcom.2019.124864}
}
```
OTSLM toolbox for Structured Light Methods

Lenton, I. C. D., Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2020). Comput. Phys. Commun., 107199. https://doi.org/10.1016/j.cpc.2020.107199'
We present a new Matlab toolbox for generating phase and amplitude patterns for digital micro-mirror device (DMD) and liquid crystal (LC) based spatial light modulators (SLMs). This toolbox consists of a collection of algorithms commonly used for generating patterns for these devices with a focus on optical tweezers beam shaping applications. In addition to the algorithms provided, we have put together a range of user interfaces for simplifying the use of these patterns. The toolbox currently has functionality to generate patterns which can be saved as a image or displayed on a device/screen using the supplied interface. We have only implemented interfaces for the devices our group currently uses but we believe that extending the code we provide to other devices should be fairly straightforward. The range of algorithms included in the toolbox is not exhaustive. However, by making the toolbox open sources and available on GitHub we hope that other researchers working with these devices will contribute their patterns/algorithms to the toolbox.
```
@article{Lenton2020Feb,
  author = {Lenton, Isaac C. D. and Stilgoe, Alexander B. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  title = {{OTSLM toolbox for Structured Light Methods}},
  journal = {Comput. Phys. Commun.},
  pages = {107199},
  year = {2020},
  month = feb,
  issn = {0010-4655},
  publisher = {North-Holland},
  doi = {10.1016/j.cpc.2020.107199}
}
```

2019

Optical-trapping of particles in air using parabolic reflectors and a hollow laser beam

Pan, Y.-L., Kalume, A., Lenton, I. C. D., Nieminen, T. A., Stilgoe, A. B., Rubinsztein-Dunlop, H., Beresnev, L. A., Wang, C., & Santarpia, J. L. (2019). Opt. Express, 27(23), 33061–33069. https://doi.org/10.1364/OE.27.033061'
We present an advanced optical-trapping method that is capable of trapping arbitrary shapes of transparent and absorbing particles in air. Two parabolic reflectors were used to reflect the inner and outer parts of a single hollow laser beam, respectively, to form two counter-propagating conical beams and bring them into a focal point for trapping. This novel design demonstrated high trapping efficiency and strong trapping robustness with a simple optical configuration. Instead of using expensive microscope objectives, the parabolic reflectors can not only achieved large numerical aperture (N.A.) focusing, but were also able to focus the beam far away from optical surfaces to minimize optics contamination. This design also offered a large free space for flexible integration with other measuring techniques, such as optical-trapping Raman spectroscopy, for on-line single particle characterization.
```
@article{Pan2019Nov,
  author = {Pan, Yong-Le and Kalume, Aimable and Lenton, Isaac C. D. and Nieminen, Timo A. and Stilgoe, Alex B. and Rubinsztein-Dunlop, Halina and Beresnev, Leonid A. and Wang, Chuji and Santarpia, Joshua L.},
  title = {{Optical-trapping of particles in air using parabolic reflectors and a hollow laser beam}},
  journal = {Opt. Express},
  volume = {27},
  number = {23},
  pages = {33061--33069},
  year = {2019},
  month = nov,
  issn = {1094-4087},
  publisher = {Optical Society of America},
  doi = {10.1364/OE.27.033061}
}
```
Microscope images of strongly scattering objects via vectorial transfer matrices: Modeling and an experimental verification

Stilgoe, A. B., Loke, V. L. Y., Kashchuk, A. V., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2019). Physical Review A, 99(1). https://doi.org/10.1103/PhysRevA.99.013818'
We present an accurate and efficient method for calculating the image of a mesoscopic particle that is captured using a high-numerical-aperture objective lens. We test various scattering models of silica, plastic, and birefringent vaterite spheres in water. We show that the calculated images accurately replicate experimental observations. This method uses the idea that the optical system can be represented as a product of matrices acting on the electromagnetic field in a truncated Hilbert space representation. A general reusable matrix encapsulating the polarization, limited capture angle, or beam shaping in the microscope can be applied to find the image and not be limited to a particular particle shape or medium. We show that the image obtained from this method can be used to determine and match particle properties. We also use incoherent averaging on multiple T-matrices to produce polychromatic images. Data obtained using this method could be used as an input to track the general behavior of particles in suspension or within an optical trap.
```
@article{Stilgoe2019,
  author = {Stilgoe, Alexander B. and Loke, Vincent L.Y. and Kashchuk, Anatolii V. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevA.99.013818},
  issn = {24699934},
  journal = {Physical Review A},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jan,
  number = {1},
  publisher = {American Physical Society},
  title = {{Microscope images of strongly scattering objects via vectorial transfer matrices: Modeling and an experimental verification}},
  volume = {99},
  year = {2019}
}
```

2018

Calibration of force detection for arbitrarily shaped particles in optical tweezers

Bui, A. A. M., Kashchuk, A. V., Balanant, M. A., Nieminen, T. A., Rubinsztein-Dunlop, H., & Stilgoe, A. B. (2018). Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-28876-y'
Force measurement with an optical trap requires calibration of it. With a suitable detector, such as a position-sensitive detector (PSD), it is possible to calibrate the detector so that the force can be measured for arbitrary particles and arbitrary beams without further calibration; such a calibration can be called an "absolute calibration". Here, we present a simple method for the absolute calibration of a PSD. Very often, paired position and force measurements are required, and even if synchronous measurements are possible with the position and force detectors used, knowledge of the force-position curve for the particle in the trap can be highly beneficial. Therefore, we experimentally demonstrate methods for determining the force-position curve with and without synchronous force and position measurements, beyond the Hookean (linear) region of the trap. Unlike the absolute calibration of the force and position detectors, the force-position curve depends on the particle and the trapping beam, and needs to be determined in each individual case. We demonstrate the robustness of our absolute calibration by measuring optical forces on microspheres as commonly trapped in optical tweezers, and other particles such a birefringent vaterite microspheres, red blood cells, and a deformable "blob".
```
@article{Bui2018,
  author = {Bui, Ann A.M. and Kashchuk, Anatolii V. and Balanant, Marie Anne and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina and Stilgoe, Alexander B.},
  doi = {10.1038/s41598-018-28876-y},
  issn = {20452322},
  journal = {Scientific Reports},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = dec,
  number = {1},
  publisher = {Nature Publishing Group},
  title = {{Calibration of force detection for arbitrarily shaped particles in optical tweezers}},
  volume = {8},
  year = {2018}
}
```

Cellular-Resolution Imaging of Vestibular Processing across the Larval Zebrafish Brain

Favre-Bulle, I. A., Vanwalleghem, G., Taylor, M. A., Rubinsztein-Dunlop, H., & Scott, E. K. (2018). Current Biology, 28(23), 3711–3722.e3. https://doi.org/10.1016/j.cub.2018.09.060'

Favre-Bulle, Vanwalleghem, et al. use optical trapping to stimulate the vestibular system in stationary larval zebrafish and then use SPIM microscopy and GCaMP to describe resulting activity across the brain. The result is a brain-wide atlas of vestibular processing at cellular resolution.

@article{Favre-Bulle2018,
  author = {Favre-Bulle, Itia A. and Vanwalleghem, Gilles and Taylor, Michael A. and Rubinsztein-Dunlop, Halina and Scott, Ethan K.},
  doi = {10.1016/j.cub.2018.09.060},
  issn = {09609822},
  journal = {Current Biology},
  keywords = {GCaMP,SPIM,calcium imaging,optical physics,optical trapping,otolith,selective planar illumination microscopy,sensory processing,vestibular system,zebrafish},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = dec,
  number = {23},
  pages = {3711--3722.e3},
  pmid = {30449665},
  publisher = {Cell Press},
  title = {{Cellular-Resolution Imaging of Vestibular Processing across the Larval Zebrafish Brain}},
  volume = {28},
  year = {2018}
}

Optical tweezers toolbox: full dynamics simulations for particles of all sizes

Lenton, I. C. D., Bui, A. A. M., Nieminen, T. A., Stilgoe, A. B., & Rubinsztein-Dunlop, H. (2018). Optical Trapping and Optical Micromanipulation XV, 10723, 107232B. https://doi.org/10.1117/12.2324120'
Simulation of optical tweezers involves the calculation of optical and non-optical forces and torques, modelling the Brownian motion of the particle and combining these components to calculate the overall dynamics of the system. Here we describe two new toolboxes: an improved optical force/torque calculation toolbox and a full dynamics simulation toolbox which combines all the individual parts to simplify the process of calculating particle dynamics. The new toolbox will provide functions for simulating particle dynamics, estimating trap stiffness and calculating trap depths. The toolbox will be able to calculate optical forces/torques in different regimes including the geometric optics limit, Rayleigh limit, and intermediate regime with Vector Spherical Wave Functions/Tmatrix,VSWF/T-matrix, Discrete Dipole Approximation, DDA, and the Final Difference Time Domain method, FDTD. Using these tools, it is possible to model different types of trapping configurations that can be used to study motile particles held in optical tweezers.
```
@article{Lenton2018Sep,
  author = {Lenton, Isaac C. D. and Bui, Ann A. M. and Nieminen, Timo A. and Stilgoe, Alexander B. and Rubinsztein-Dunlop, Halina},
  title = {{Optical tweezers toolbox: full dynamics simulations for particles of all sizes}},
  journal = {Optical Trapping and Optical Micromanipulation XV},
  volume = {10723},
  pages = {107232B},
  year = {2018},
  month = sep,
  publisher = {International Society for Optics and Photonics},
  doi = {10.1117/12.2324120}
}
```
Measuring the motility and drag forces acting on biological particles using optical tweezers

Lenton, I. C. D., Armstrong, D., Calvert-Lane, J., Nieminen, T. A., Stilgoe, A. B., & Rubinsztein-Dunlop, H. (2018). Optical Trapping and Optical Micromanipulation XV, 10723, 107232C. https://doi.org/10.1117/12.2324117'
Using absolutely calibrated optical tweezers, we make quantitative measurements of the motility force of Escherichia coli (E. coli) by measuring the change in momentum of the deflected beam. By tracking the position of the particle, in addition to the optical force measurements, it should be possible to simultaneously calculate the motility force and drag. In a simple Gaussian beam optical trap away from the sample chamber E. coli tends to align and swim along the beam axis, which can make tracking the particle position and measuring the force difficult. We use a 3-D optical force detection system to measure the absolute force on the particle, allowing us to measure the motility force of E. coli in a simple Gaussian beam. By using a line-shaped trap, it is possible to align E. coli transverse to the beam axis, facilitating easy particle position measurement. The investigated methods are not specific to E. coli and could be applied to other motile organisms, the study of wall effects and bio-films.
```
@article{Lenton2018Seq,
  author = {Lenton, Isaac C. D. and Armstrong, Declan and Calvert-Lane, Jackson and Nieminen, Timo A. and Stilgoe, Alexander B. and Rubinsztein-Dunlop, Halina},
  title = {{Measuring the motility and drag forces acting on biological particles using optical tweezers}},
  journal = {Optical Trapping and Optical Micromanipulation XV},
  volume = {10723},
  pages = {107232C},
  year = {2018},
  month = sep,
  publisher = {International Society for Optics and Photonics},
  doi = {10.1117/12.2324117}
}
```

Optical Tweezers Bring Micromachines to Biology

Favre-Bulle, I. A., Zhang, S., Kashchuk, A. V., Lenton, I. C. D., Gibson, L. J., Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2018). Optics and Photonics News, 29(4), 40. https://doi.org/10.1364/opn.29.4.000040'

Optical tweezers, using focused laser beams to manipulate objects on the nano- and microscale, allow exploration of a world of curiosities at the borderline between physics and biology.

@article{Favre-Bulle2018a,
  author = {Favre-Bulle, I.A. and Zhang, S. and Kashchuk, A.V. and Lenton, I.C.D. and Gibson, L.J. and Stilgoe, A.B. and Nieminen, T.A. and Rubinsztein-Dunlop, H.},
  doi = {10.1364/opn.29.4.000040},
  issn = {1047-6938},
  journal = {Optics and Photonics News},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = apr,
  number = {4},
  pages = {40},
  publisher = {The Optical Society},
  title = {{Optical Tweezers Bring Micromachines to Biology}},
  volume = {29},
  year = {2018}
}

Impact of complex surfaces on biomicrorheological measurements using optical tweezers

Zhang, S., Gibson, L. J., Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2018). Lab on a Chip, 18(2), 315–322. https://doi.org/10.1039/c7lc01176h'
The characterisation of physical properties in biologically relevant processes and the development of novel microfluidic devices for this purpose are experiencing a great resurgence at present. In many of measurements of this type where a probe in a fluid is used, the strong influence of the boundaries of the volume used is a serious problem. In these geometries the proximity of a probe to a wall can severely influence the measurement. However, although much knowledge has been gained about flat walls, to date, the effect of non-planar surfaces at microscopic scale on rotational motion of micro-objects has not been studied. Here we present for the first time both experimental measurements and numerical computations which aim to study the drag torque on optically trapped rotating particles moving near 3D-printed conical and cylindrical walls on-chip. These results are essential for quantifying how curved walls can effect the torque on particles, and thus enable accurate hydrodynamic simulations at the micron-scale. This opens the potential for new sensing approaches under more complex conditions, allowing both dynamic and microrheological studies of biological systems and lab-on-chip devices.
```
@article{Zhang2018,
  author = {Zhang, Shu and Gibson, Lachlan J. and Stilgoe, Alexander B. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  doi = {10.1039/c7lc01176h},
  issn = {14730189},
  journal = {Lab on a Chip},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {2},
  pages = {315--322},
  publisher = {Royal Society of Chemistry},
  title = {{Impact of complex surfaces on biomicrorheological measurements using optical tweezers}},
  volume = {18},
  year = {2018}
}
```

2017

Optical trapping of otoliths drives vestibular behaviours in larval zebrafish

Favre-Bulle, I. A., Stilgoe, A. B., Rubinsztein-Dunlop, H., & Scott, E. K. (2017). Nature Communications, 8(1). https://doi.org/10.1038/s41467-017-00713-2'
The vestibular system, which detects gravity and motion, is crucial to survival, but the neural circuits processing vestibular information remain incompletely characterised. In part, this is because the movement needed to stimulate the vestibular system hampers traditional neuroscientific methods. Optical trapping uses focussed light to apply forces to targeted objects, typically ranging from nanometres to a few microns across. In principle, optical trapping of the otoliths (ear stones) could produce fictive vestibular stimuli in a stationary animal. Here we use optical trapping in vivo to manipulate 55-micron otoliths in larval zebrafish. Medial and lateral forces on the otoliths result in complementary corrective tail movements, and lateral forces on either otolith are sufficient to cause a rolling correction in both eyes. This confirms that optical trapping is sufficiently powerful and precise to move large objects in vivo, and sets the stage for the functional mapping of the resulting vestibular processing.
```
@article{Favre-Bulle2017,
  author = {Favre-Bulle, Itia A. and Stilgoe, Alexander B. and Rubinsztein-Dunlop, Halina and Scott, Ethan K.},
  doi = {10.1038/s41467-017-00713-2},
  issn = {20411723},
  journal = {Nature Communications},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = dec,
  number = {1},
  publisher = {Nature Publishing Group},
  title = {{Optical trapping of otoliths drives vestibular behaviours in larval zebrafish}},
  volume = {8},
  year = {2017}
}
```
Ultrasensitive rotating photonic probes for complex biological systems

Zhang, S., Gibson, L. J., Stilgoe, A. B., Favre-Bulle, I. A., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2017). Optica, 4(9), 1103. https://doi.org/10.1364/optica.4.001103'
\textcopyright 2017 Optical Society of America. We use rotational photonic tweezers to access local viscoelastic properties of complex fluids over a wide frequency range. This is done by monitoring both passive rotational Brownian motion and also actively driven transient rotation between two angular trapping states of a birefringent microsphere. These enable measurement of high- and low-frequency properties, respectively. Complex fluids arise frequently in microscopic biological systems, typically with length scales at the cellular level. Thus, high spatial resolution as provided by rotational photonic tweezers is important. We measure the properties of tear film on a contact lens and demonstrate variations in these properties between two subjects over time. We also show excellent agreement between our theoretical model and experimental results. We believe that this is the first time that active microrheology using rotating tweezers has been used for biologically relevant questions. Our method demonstrates potential for future applications to determine the spatial-temporal properties of biologically relevant and complex fluids that are only available in very small volumes.
```
@article{Zhang2017,
  author = {Zhang, Shu and Gibson, Lachlan J. and Stilgoe, Alexander B. and Favre-Bulle, Itia A. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/optica.4.001103},
  issn = {2334-2536},
  journal = {Optica},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = sep,
  number = {9},
  pages = {1103},
  publisher = {The Optical Society},
  title = {{Ultrasensitive rotating photonic probes for complex biological systems}},
  volume = {4},
  year = {2017}
}
```
Theory and practice of simulation of optical tweezers

Bui, A. A. M., Stilgoe, A. B., Lenton, I. C. D., Gibson, L. J., Kashchuk, A. V., Zhang, S., Rubinsztein-Dunlop, H., & Nieminen, T. A. (2017). In Journal of Quantitative Spectroscopy and Radiative Transfer (Vol. 195, pp. 66–75). Elsevier Ltd. https://doi.org/10.1016/j.jqsrt.2016.12.026'
Computational modelling has made many useful contributions to the field of optical tweezers. One aspect in which it can be applied is the simulation of the dynamics of particles in optical tweezers. This can be useful for systems with many degrees of freedom, and for the simulation of experiments. While modelling of the optical force is a prerequisite for simulation of the motion of particles in optical traps, non-optical forces must also be included; the most important are usually Brownian motion and viscous drag. We discuss some applications and examples of such simulations. We review the theory and practical principles of simulation of optical tweezers, including the choice of method of calculation of optical force, numerical solution of the equations of motion of the particle, and finish with a discussion of a range of open problems.
```
@misc{Bui2017,
  archiveprefix = {arXiv},
  arxivid = {1708.06074},
  author = {Bui, Ann A.M. and Stilgoe, Alexander B. and Lenton, Isaac C.D. and Gibson, Lachlan J. and Kashchuk, Anatolii V. and Zhang, Shu and Rubinsztein-Dunlop, Halina and Nieminen, Timo A.},
  booktitle = {Journal of Quantitative Spectroscopy and Radiative Transfer},
  doi = {10.1016/j.jqsrt.2016.12.026},
  eprint = {1708.06074},
  isbn = {9781557529541},
  issn = {00224073},
  keywords = {Laser trapping,Light scattering,Optical force,Optical torque,Optical tweezers},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jul,
  pages = {66--75},
  publisher = {Elsevier Ltd},
  title = {{Theory and practice of simulation of optical tweezers}},
  volume = {195},
  year = {2017}
}
```
Active rotational and translational microrheology beyond the linear spring regime

Gibson, L. J., Zhang, S., Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2017). Physical Review E, 95(4). https://doi.org/10.1103/PhysRevE.95.042608'
Active particle tracking microrheometers have the potential to perform accurate broadband measurements of viscoelasticity within microscopic systems. Generally, their largest possible precision is limited by Brownian motion and low frequency changes to the system. The signal to noise ratio is usually improved by increasing the size of the driven motion compared to the Brownian as well as averaging over repeated measurements. New theory is presented here whereby error in measurements of the complex shear modulus can be significantly reduced by analyzing the motion of a spherical particle driven by nonlinear forces. In some scenarios error can be further reduced by applying a variable transformation which linearizes the equation of motion. This enables normalization that eliminates error introduced by low frequency drift in the particle’s equilibrium position. Our measurements indicate that this can further resolve an additional decade of viscoelasticity at high frequencies. Using this method will easily increase the signal strength enough to significantly reduce the measurement time for the same error. Thus the method is more conducive to measuring viscoelasticity in slowly changing microscopic systems, such as a living cell.
```
@article{Gibson2017,
  archiveprefix = {arXiv},
  arxivid = {1612.07853},
  author = {Gibson, Lachlan J. and Zhang, Shu and Stilgoe, Alexander B. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevE.95.042608},
  eprint = {1612.07853},
  issn = {24700053},
  journal = {Physical Review E},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = apr,
  number = {4},
  publisher = {American Physical Society},
  title = {{Active rotational and translational microrheology beyond the linear spring regime}},
  volume = {95},
  year = {2017}
}
```
Visual guide to optical tweezers

Lenton, I. C. D., Stilgoe, A. B., Rubinsztein-Dunlop, H., & Nieminen, T. A. (2017). European Journal of Physics, 38(3). https://doi.org/10.1088/1361-6404/aa6271'
It is common to introduce optical tweezers using either geometric optics for large particles or the Rayleigh approximation for very small particles. These approaches are successful at conveying the key ideas behind optical tweezers in their respective regimes. However, they are insufficient for modelling particles of intermediate size and large particles with small features. For this, a full field approach provides greater insight into the mechanisms involved in trapping. The advances in computational capability over the last decade have led to better modelling and understanding of optical tweezers. Problems that were previously difficult to model computationally can now be solved using a variety of methods on modern systems. These advances in computational power allow for full field solutions to be visualised, leading to increased understanding of the fields and behaviour in various scenarios. In this paper we describe the operation of optical tweezers using full field simulations calculated using the finite difference time domain method. We use these simulations to visually illustrate various situations relevant to optical tweezers, from the basic operation of optical tweezers, to engineered particles and evanescent fields.
```
@article{Lenton2017,
  archiveprefix = {arXiv},
  arxivid = {1708.04394},
  author = {Lenton, Isaac C.D. and Stilgoe, Alexander B. and Rubinsztein-Dunlop, Halina and Nieminen, Timo A.},
  doi = {10.1088/1361-6404/aa6271},
  eprint = {1708.04394},
  issn = {13616404},
  journal = {European Journal of Physics},
  keywords = {FDTD,evanescent fields,optical tweezers,scattering,simulation},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = mar,
  number = {3},
  publisher = {Institute of Physics Publishing},
  title = {{Visual guide to optical tweezers}},
  volume = {38},
  year = {2017}
}
```

2015

Energy, momentum and propagation of non-paraxial high-order Gaussian beams in the presence of an aperture

Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2015). Journal of Optics (United Kingdom), 17(12). https://doi.org/10.1088/2040-8978/17/12/125601'
Non-paraxial theories of wave propagation are essential to model the interaction of highly focused light with matter. Here we investigate the energy, momentum and propagation of the Laguerre-, Hermite- and Ince-Gaussian solutions (LG, HG, and IG) of the paraxial wave equation in an apertured non-paraxial regime. We investigate the far-field relationships between the LG, HG, and IG solutions and the vector spherical wave function (VSWF) solutions of the vector Helmholtz wave equation. We investigate the convergence of the VSWF and the various Gaussian solutions in the presence of an aperture. Finally, we investigate the differences in linear and angular momentum evaluated in the paraxial and non-paraxial regimes. The non-paraxial model we develop can be applied to calculations of the focusing of high-order Gaussian modes in high-resolution microscopes. We find that the addition of an aperture in high numerical aperture optical systems does not greatly affect far-field properties except when the beam is significantly clipped by an aperture. Diffraction from apertures causes large distortions in the near-field and will influence light-matter interactions. The method is not limited to a particular solution of the paraxial wave equation. Our model is constructed in a formalism that is commonly used in scattering calculations. It is thus applicable to optical trapping and other optical investigations of matter.
```
@article{Stilgoe2015,
  author = {Stilgoe, Alexander B. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/2040-8978/17/12/125601},
  issn = {20408986},
  journal = {Journal of Optics (United Kingdom)},
  keywords = {Helmholtz wave equation,Ince gauss,optical tweezers,paraxial wave equation,wavfunctions},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = oct,
  number = {12},
  publisher = {Institute of Physics Publishing},
  title = {{Energy, momentum and propagation of non-paraxial high-order Gaussian beams in the presence of an aperture}},
  volume = {17},
  year = {2015}
}
```
Escape forces and trajectories in optical tweezers and their effect on calibration

Bui, A. A. M., Stilgoe, A. B., Khatibzadeh, N., Nieminen, T. A., Berns, M. W., & Rubinsztein-Dunlop, H. (2015). Optics Express, 23(19), 24317. https://doi.org/10.1364/oe.23.024317'
\textcopyright 2015 Optical Society of America. Whether or not an external force can make a trapped particle escape from optical tweezers can be used to measure optical forces. Combined with the linear dependence of optical forces on trapping power, a quantitative measurement of the force can be obtained. For this measurement, the particle is at the edge of the trap, away from the region near the equilbrium position where the trap can be described as a linear spring. This method provides the ability to measure higher forces for the same beam power, compared with using the linear region of the trap, with lower risk of optical damage to trapped specimens. Calibration is typically performed by using an increasing fluid flow to exert an increasing force on a trapped particle until it escapes. In this calibration technique, the particle is usually assumed to escape along a straight line in the direction of fluid-flow. Here, we show that the particle instead follows a curved trajectory, which depends on the rate of application of the force (i.e., the acceleration of the fluid flow). In the limit of very low acceleration, the particle follows the surface of zero axial optical force during the escape. The force required to produce escape depends on the trajectory, and hence the acceleration. This can result in variations in the escape force of a factor of two. This can have a major impact on calibration to determine the escape force efficiency. Even when calibration measurements are all performed in the low acceleration regime, variations in the escape force efficiency of 20% or more can still occur. We present computational simulations using generalized Lorenz-Mie theory and experimental measurements to show how the escape force efficiency depends on rate of increase of force and trapping power, and discuss the impact on calibration.
```
@article{Bui2015,
  author = {Bui, Ann A. M. and Stilgoe, Alexander B. and Khatibzadeh, Nima and Nieminen, Timo A. and Berns, Michael W. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/oe.23.024317},
  issn = {1094-4087},
  journal = {Optics Express},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = sep,
  number = {19},
  pages = {24317},
  publisher = {The Optical Society},
  title = {{Escape forces and trajectories in optical tweezers and their effect on calibration}},
  volume = {23},
  year = {2015}
}
```
Scattering of sculpted light in intact brain tissue, with implications for optogenetics

Favre-Bulle, I. A., Preece, D., Nieminen, T. A., Heap, L. A., Scott, E. K., & Rubinsztein-Dunlop, H. (2015). Scientific Reports, 5. https://doi.org/10.1038/srep11501'
Optogenetics uses light to control and observe the activity of neurons, often using a focused laser beam. As brain tissue is a scattering medium, beams are distorted and spread with propagation through neural tissue, and the beam’s degradation has important implications in optogenetic experiments. To address this, we present an analysis of scattering and loss of intensity of focused laser beams at different depths within the brains of zebrafish larvae. Our experimental set-up uses a 488 €‰nm laser and a spatial light modulator to focus a diffraction-limited spot of light within the brain. We use a combination of experimental measurements of back-scattered light in live larvae and computational modelling of the scattering to determine the spatial distribution of light. Modelling is performed using the Monte Carlo method, supported by generalised Lorenz-Mie theory in the single-scattering approximation. Scattering in areas rich in cell bodies is compared to that of regions of neuropil to identify the distinct and dramatic contributions that cell nuclei make to scattering. We demonstrate the feasibility of illuminating individual neurons, even in nucleus-rich areas, at depths beyond 100\mum using a spatial light modulator in combination with a standard laser and microscope optics.
```
@article{Favre-Bulle2015,
  author = {Favre-Bulle, Itia A. and Preece, Daryl and Nieminen, Timo A. and Heap, Lucy A. and Scott, Ethan K. and Rubinsztein-Dunlop, Halina},
  doi = {10.1038/srep11501},
  issn = {20452322},
  journal = {Scientific Reports},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jun,
  publisher = {Nature Publishing Group},
  title = {{Scattering of sculpted light in intact brain tissue, with implications for optogenetics}},
  volume = {5},
  year = {2015}
}
```
Optical Forces, Trapping and Manipulation

Rubinsztein-Dunlop, H., Stilgoe, A. B., Preece, D., Bui, A., & Nieminen, T. A. (2015). In Photonics: Scientific Foundations, Technology and Applications (Vol. 3, pp. 287–339). wiley. https://doi.org/10.1002/9781119011781.ch7'
The development of optical tweezers led to optical trapping becoming a widely used practical tool, with many applications in biology, biophysics, and other fields. The fundamental theory that one can use to describe optical tweezers is classical electromagnetic theory. This chapter presents a brief review of the relevant basic elements of electromagnetic theory, largely as a reminder about basic concepts and terminology. It presents a detailed discussion on the theory of optical forces and optical torques and the measurement of forces and torques. The force acting on a trapped particle can be calculated if the electromagnetic field is known, via the Maxwell stress tensor, or the Poynting vector in the far field. Therefore, calculation of optical forces is a task in computational light scattering.
```
@incollection{Rubinsztein-Dunlop2015,
  author = {Rubinsztein-Dunlop, Halina and Stilgoe, Alexander B. and Preece, Darryl and Bui, Ann and Nieminen, Timo A.},
  booktitle = {Photonics: Scientific Foundations, Technology and Applications},
  doi = {10.1002/9781119011781.ch7},
  isbn = {9781119011781},
  keywords = {Classical electromagnetic theory,Maxwell stress tensor,Optical forces,Optical torque,Optical trapping,Optical tweezer,Poynting vector},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = feb,
  pages = {287--339},
  publisher = {wiley},
  title = {{Optical Forces, Trapping and Manipulation}},
  volume = {3},
  year = {2015}
}
```

2014

Driving corrugated donut rotors with Laguerre-Gauss beams

Loke, V. L. Y., Asavei, T., Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2014). Optics Express, 22(16), 19692. https://doi.org/10.1364/oe.22.019692'
Tightly-focused laser beams that carry angular momentum have been used to trap and rotate microrotors. In particular, a Laguerre-Gauss mode laser beam can be used to transfer its orbital angular momentum to drive microrotors. We increase the torque efficiency by a factor of about 2 by designing the rotor such that its geometry is compatible with the driving beam, when driving the rotation with the optimum beam, rather than beams of higher or lower orbital angular momentum. Based on Floquet’s theorem, the order of discrete rotational symmetry of the rotor can be made to couple with the azimuthal mode of the Laguerre-Gauss beam. We design corrugated donut rotors, that have a flat disc-like profile, with the help of the discrete dipole approximation and the T-matrix methods in parallel with experimental demonstrations of stable trapping and torque measurement. We produce and test such a rotor using two-photon photopolymerization. With a rotor that has 8-fold discrete rotational symmetry, an outer radius of 1.85 \mum and a hollow core radius of 0.5 \mum, we were able to transfer approximately 0.3 h per photon of the orbital angular momentum from an LG04 beam. \textcopyright 2014 Optical Society of America.
```
@article{Loke2014,
  author = {Loke, Vincent L. Y. and Asavei, Theodor and Stilgoe, Alexander B. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/oe.22.019692},
  issn = {1094-4087},
  journal = {Optics Express},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = aug,
  number = {16},
  pages = {19692},
  publisher = {The Optical Society},
  title = {{Driving corrugated donut rotors with Laguerre-Gauss beams}},
  volume = {22},
  year = {2014}
}
```
Optical tweezers: Theory and modelling

Nieminen, T. A., Du Preez-Wilkinson, N., Stilgoe, A. B., Loke, V. L. Y., Bui, A. A. M., & Rubinsztein-Dunlop, H. (2014). Journal of Quantitative Spectroscopy and Radiative Transfer, 146, 59–80. https://doi.org/10.1016/j.jqsrt.2014.04.003'
Since their development in the 1980s, optical tweezers have become a widely used and versatile tool in many fields. Outstanding applications include the quantitative measurement of forces in cell biology and biophysics. Computational modelling of optical tweezers is a valuable tool in support of experimental work, especially quantitative applications. We discuss the theory, and the theoretical and computational modelling of optical tweezers. Highlights: •We discuss the theory of optical tweezers. \textcopyright 2014 Elsevier Ltd.
```
@article{Nieminen2014,
  author = {Nieminen, Timo A. and {Du Preez-Wilkinson}, Nathaniel and Stilgoe, Alexander B. and Loke, Vincent L.Y. and Bui, Ann A.M. and Rubinsztein-Dunlop, Halina},
  doi = {10.1016/j.jqsrt.2014.04.003},
  issn = {00224073},
  journal = {Journal of Quantitative Spectroscopy and Radiative Transfer},
  keywords = {Laser trapping,Light scattering,Optical force,Optical torque,Optical tweezers},
  mendeley-groups = {Optical Tweezers/UQOMG},
  pages = {59--80},
  publisher = {Elsevier Ltd},
  title = {{Optical tweezers: Theory and modelling}},
  volume = {146},
  year = {2014}
}
```

2013

Kinect the dots: 3D control of optical tweezers

Shaw, L., Preece, D., & Rubinsztein-Dunlop, H. (2013). Journal of Optics (United Kingdom), 15(7). https://doi.org/10.1088/2040-8978/15/7/075303'
Holographically generated optical traps confine micron- and sub-micron sized particles close to the center of focused light beams. They also provide a way of trapping multiple particles and moving them in three dimensions. However, in many systems the user interface is not always advantageous or intuitive especially for collaborative work and when depth information is required. We discuss and evaluate a set of multi-beam optical tweezers that utilize off the shelf gaming technology to facilitate user interaction. We use the Microsoft Kinect sensor bar as a way of getting the user input required to generate arbitrary optical force fields and control optically trapped particles. We demonstrate that the system can also be used for dynamic light control. \textcopyright 2013 IOP Publishing Ltd.
```
@article{Shaw2013,
  author = {Shaw, Lucy and Preece, Daryl and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/2040-8978/15/7/075303},
  issn = {20408978},
  journal = {Journal of Optics (United Kingdom)},
  keywords = {Microsoft Kinect,microscopy,optical tweezers,spatial light modulators,user interfaces},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jul,
  number = {7},
  title = {{Kinect the dots: 3D control of optical tweezers}},
  volume = {15},
  year = {2013}
}
```
Optically trapped and driven paddle-wheel

Asavei, T., Nieminen, T. A., Loke, V. L. Y., Stilgoe, A. B., Bowman, R., Preece, D., Padgett, M. J., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2013). New Journal of Physics, 15. https://doi.org/10.1088/1367-2630/15/6/063016'
We demonstrate the control and rotation of an optically trapped object, an optical paddle-wheel, with the rotation direction normal to the beam axis. This is in contrast to the usual situation where the rotation is about the beam axis. The paddle-wheel can be optically driven and moved to any position in the field of view of the microscope, which can be of interest for various biological applications where controlled application of a fluid flow is needed in a particular location and in a specific direction. This is of particular interest in signal transduction studies in cells, especially when a cell is flat and spread out on a surface. \textcopyright IOP Publishing and Deutsche Physikalische Gesellschaft.
```
@article{Asavei2013,
  author = {Asavei, Theodor and Nieminen, Timo A. and Loke, Vincent L.Y. and Stilgoe, Alexander B. and Bowman, Richard and Preece, Daryl and Padgett, Miles J. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/1367-2630/15/6/063016},
  issn = {13672630},
  journal = {New Journal of Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jun,
  title = {{Optically trapped and driven paddle-wheel}},
  volume = {15},
  year = {2013}
}
```
Calibration of nonspherical particles in optical tweezers using only position measurement

Bui, A. A. M., Stilgoe, A. B., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2013). Optics Letters, 38(8), 1244. https://doi.org/10.1364/ol.38.001244'
Nonspherical probe particles are an attractive choice for optically-trapped scanning probe microscopy. We show that it is possible to calibrate a trap with a nonspherical particle using only position measurements, without requiring measurement of orientation, using a pseudopotential based on the position occupation probability. It is not necessary to assume the force is linear with displacement. \textcopyright 2013 Optical Society of America.
```
@article{Bui2013,
  author = {Bui, Ann A. M. and Stilgoe, Alexander B. and Nieminen, Timo A. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/ol.38.001244},
  issn = {0146-9592},
  journal = {Optics Letters},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = apr,
  number = {8},
  pages = {1244},
  publisher = {The Optical Society},
  title = {{Calibration of nonspherical particles in optical tweezers using only position measurement}},
  volume = {38},
  year = {2013}
}
```
Spatially-resolved rotational microrheology with an optically-trapped sphere

Bennett, J. S., Gibson, L. J., Kelly, R. M., Brousse, E., Baudisch, B., Preece, D., Nieminen, T. A., Nicholson, T., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2013). Scientific Reports, 3. https://doi.org/10.1038/srep01759'
We have developed a microrheometer, based on optical tweezers, in which hydrodynamic coupling between the probe and fluid boundaries is dramatically reduced relative to existing microrheometers. Rotational Brownian motion of a birefringent microsphere within an angular optical trap is observed by measuring the polarisation shifts of transmitted light. Data gathered in this manner, in the strongly viscoelastic fluid Celluvisc, quantitatively agree with the results of conventional (bulk) rheometry. Our technique will significantly reduce the smallest sample volumes which may be reliably probed, further extending the study of rare, difficult to obtain or highly nonhomogeneous fluids.
```
@article{Bennett2013,
  author = {Bennett, James S. and Gibson, Lachlan J. and Kelly, Rory M. and Brousse, Emmanuel and Baudisch, Bastian and Preece, Daryl and Nieminen, Timo A. and Nicholson, Timothy and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1038/srep01759},
  issn = {20452322},
  journal = {Scientific Reports},
  mendeley-groups = {Optical Tweezers/UQOMG},
  title = {{Spatially-resolved rotational microrheology with an optically-trapped sphere}},
  volume = {3},
  year = {2013}
}
```

2012

Equilibrium orientations and positions of non-spherical particles in optical traps

Cao, Y., Stilgoe, A. B., Chen, L., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2012). Optics Express, 20(12), 12987. https://doi.org/10.1364/oe.20.012987'
Dynamic simulation is a powerful tool to observe the behavior of arbitrary shaped particles trapped in a focused laser beam. Here we develop a method to find equilibrium positions and orientations using dynamic simulation. This general method is applied to micro- and nano-cylinders as a demonstration of its predictive power. Orientation landscapes for particles trapped with beams of differing polarisation are presented. The torque efficiency of micro-cylinders at equilibrium in a plane is also calculated as a function of tilt angle. This systematic investigation elucidates in both the function and properties of micro- and nano-cylinders trapped in optical tweezers. \textcopyright 2012 Optical Society of America.
```
@article{Cao2012,
  author = {Cao, Yongyin and Stilgoe, Alexander B and Chen, Lixue and Nieminen, Timo A and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/oe.20.012987},
  issn = {1094-4087},
  journal = {Optics Express},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jun,
  number = {12},
  pages = {12987},
  publisher = {The Optical Society},
  title = {{Equilibrium orientations and positions of non-spherical particles in optical traps}},
  volume = {20},
  year = {2012}
}
```

2011

Phase-transition-like properties of double-beam optical tweezers

Stilgoe, A. B., Heckenberg, N. R., Nieminen, T. A., & Rubinsztein-Dunlop, H. (2011). Physical Review Letters, 107(24). https://doi.org/10.1103/PhysRevLett.107.248101'
We report on double-beam optical tweezers that undergo previously unknown phase-transition-like behavior resulting in the formation of more optical traps than the number of beams used to create them. We classify the optical force fields which produce multiple traps for a double-beam system including the critical behavior. This effect is demonstrated experimentally in orthogonally polarized (noninterfering) dual-beam optical tweezers for a silica particle of 2.32\mum diameter. Phase transitions of multiple beam trapping systems have implications for hopping rates between traps and detection of forces between biomolecules using dual-beam optical tweezers. It is an example of a novel dynamic system with multiple states where force fields undergo a series of sign inversions as a function of parameters such as size and beam separation. \textcopyright 2011 American Physical Society.
```
@article{Stilgoe2011,
  author = {Stilgoe, A. B. and Heckenberg, N. R. and Nieminen, T. A. and Rubinsztein-Dunlop, H.},
  doi = {10.1103/PhysRevLett.107.248101},
  issn = {00319007},
  journal = {Physical Review Letters},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = dec,
  number = {24},
  title = {{Phase-transition-like properties of double-beam optical tweezers}},
  volume = {107},
  year = {2011}
}
```
Optical tweezers and paradoxes in electromagnetism

Pfeifer, R. N. C., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2011). Journal of Optics, 13(4). https://doi.org/10.1088/2040-8978/13/4/044017'
The widespread application of optical forces and torques has contributed to renewed interest in the fundamentals of the electromagnetic force and torque, including long-standing paradoxes such as the Abraham-Minkowski controversy and the angular momentum density of a circularly polarized plane wave. We discuss the relationship between these electromagnetic paradoxes and optical tweezers. In particular, consideration of possible optical tweezers experiments to attempt to resolve these paradoxes strongly suggests that they are beyond experimental resolution, yielding identical observable results in all cases. \textcopyright 2011 IOP Publishing Ltd.
```
@article{Pfeifer2011,
  author = {Pfeifer, Robert N.C. and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/2040-8978/13/4/044017},
  issn = {20408978},
  journal = {Journal of Optics},
  keywords = {Abraham-Minkowski controversy,electromagnetic momentum,optical angular momentum},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = apr,
  number = {4},
  title = {{Optical tweezers and paradoxes in electromagnetism}},
  volume = {13},
  year = {2011}
}
```
T-matrix method for modelling optical tweezers

Nieminen, T. A., Loke, V. L. Y., Stilgoe, A. B., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2011). Journal of Modern Optics, 58(5-6), 528–544. https://doi.org/10.1080/09500340.2010.528565'
We review the use of the T-matrix description of scattering, or the T-matrix method, for the calculation of optical forces and torques, especially for the computational modelling of optical tweezers. We consider both simple particles such as homogeneous isotropic spheres, spherical shells, spheroids, and so on, and complex particles, including anisotropic particles, inhomogenous particles, and geometrically complex particles. \textcopyright 2011 Taylor & Francis.
```
@article{Nieminen2011,
  author = {Nieminen, Timo A. and Loke, Vincent L.Y. and Stilgoe, Alexander B. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1080/09500340.2010.528565},
  issn = {09500340},
  journal = {Journal of Modern Optics},
  keywords = {T-matrix method,light scattering,optical tweezers,optically-driven micromachines},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = mar,
  number = {5-6},
  pages = {528--544},
  title = {{T-matrix method for modelling optical tweezers}},
  volume = {58},
  year = {2011}
}
```
The difficulty of measuring orbital angular momentum

Preece, D., Nieminen, T. A., Asavei, T., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2011). AAPP Atti Della Accademia Peloritana Dei Pericolanti, Classe Di Scienze Fisiche, Matematiche e Naturali, 89(SUPPL. 1). https://doi.org/10.1478/C1V89S1P076'
Light can carry angular momentum as well as energy and momentum; the transfer of this angular momentum to an object results in an optical torque. The development of a rotational analogue to the force measurement capability of optical tweezers is hampered by the diffculty of optical measurement of orbital angular momentum. We present an experiment with encouraging results, but emphasise the diffculty of the task. \textcopyright 2011 by the Author(s); licensee Accademia Peloritana dei Pericolanti, Messina, Italy.
```
@inproceedings{Preece2011,
  author = {Preece, Daryl and Nieminen, Timo A. and Asavei, Theodor and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  booktitle = {AAPP Atti della Accademia Peloritana dei Pericolanti, Classe di Scienze Fisiche, Matematiche e Naturali},
  doi = {10.1478/C1V89S1P076},
  issn = {03650359},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {SUPPL. 1},
  title = {{The difficulty of measuring orbital angular momentum}},
  volume = {89},
  year = {2011}
}
```

2009

Highly birefringent vaterite microspheres: production, characterization and applications for optical micromanipulation

Parkin, S. J., Vogel, R., Persson, M., Funk, M., Loke, V. L., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2009). Optics Express, 17(24), 21944. https://doi.org/10.1364/oe.17.021944'
This paper reports on a simple synthesis and characterization of highly birefringent vaterite microspheres, which are composed of 20-30 nm sized nanocrystalls. Scanning electron microscopy shows a quite disordered assembly of nanocrystals within the microspheres. However, using optical tweezers, the effective birefringence of the microspheres was measured to be Deltan = 0.06, which compares to Deltan = 0.1 of vaterite single crystals. This suggests a very high orientation of the nanocrystals within the microspheres. A hyperbolic model of the direction of the optical axis throughout the vaterite spherulite best fits the experimental data. Results from polarized light microscopy further confirm the hyperbolic model.
```
@article{Parkin2009,
  author = {Parkin, Simon J. and Vogel, Robert and Persson, Martin and Funk, Maren and Loke, Vincent L. and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/oe.17.021944},
  issn = {1094-4087},
  journal = {Optics Express},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = nov,
  number = {24},
  pages = {21944},
  publisher = {The Optical Society},
  title = {{Highly birefringent vaterite microspheres: production, characterization and applications for optical micromanipulation}},
  volume = {17},
  year = {2009}
}
```
Synthesis and surface modification of birefringent vaterite microspheres

Vogel, R., Persson, M., Feng, C., Parkin, S. J., Nieminen, T. A., Wood, B., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2009). Langmuir, 25(19), 11672–11679. https://doi.org/10.1021/la901532x'
This paper reports on the synthesis of birefringent vaterite microspheres with narrow size distribution using a seeded growth method. In a post-treatment the microspheres were stabilized and functionalized through coating with a combination of organosilica and silica. The coating vastly enhanced the stability of the vaterite microspheres in biological buffers and allowed the attachment of biomolecules such as DNA or proteins. As an example, streptavidin was attached to the surface of the functionalized microspheres. These results pave the way for the use of birefringent vaterite particles for the micromanipulation of single biological molecules such as DNA or specific proteins in an optical trap capable of exerting and measuring torques. The stabilized birefringent microspheres may also find use for biosensor and biological screening applications. \textcopyright 2009 American Chemical Society.
```
@article{Vogel2009,
  author = {Vogel, Robert and Persson, Martin and Feng, Chao and Parkin, Simon J. and Nieminen, Timo A. and Wood, Barry and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1021/la901532x},
  issn = {07437463},
  journal = {Langmuir},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = oct,
  number = {19},
  pages = {11672--11679},
  title = {{Synthesis and surface modification of birefringent vaterite microspheres}},
  volume = {25},
  year = {2009}
}
```
Symmetry and the generation and measurement of optical torque

Nieminen, T. A., Asavei, T., Loke, V. L. Y., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2009). Journal of Quantitative Spectroscopy and Radiative Transfer, 110(14-16), 1472–1482. https://doi.org/10.1016/j.jqsrt.2009.03.013'
A key element in the generation of optical torque in optical traps, which occurs when electromagnetic angular momentum is transferred from the trapping beam to the trapped particle by scattering, is the symmetries of the scattering particle and the trapping beam. We discuss the effect of such symmetries on the generation and measurement of optical torque in optical tweezers, and some consequent general principles for the design of optically driven micromachines. Crown Copyright \textcopyright 2009.
```
@article{Nieminen2009,
  author = {Nieminen, Timo A. and Asavei, Theodor and Loke, Vincent L.Y. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1016/j.jqsrt.2009.03.013},
  issn = {00224073},
  journal = {Journal of Quantitative Spectroscopy and Radiative Transfer},
  keywords = {Laser trapping,Optical torque,Optical tweezers,Symmetry},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = sep,
  number = {14-16},
  pages = {1472--1482},
  title = {{Symmetry and the generation and measurement of optical torque}},
  volume = {110},
  year = {2009}
}
```
Optical angular momentum transfer to microrotors fabricated by two-photon photopolymerization

Asavei, T., Loke, V. L. Y., Barbieri, M., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2009). New Journal of Physics, 11. https://doi.org/10.1088/1367-2630/11/9/093021'
We design, fabricate and test optically driven microrotors a few microns in size. The rotors are trapped and rotated in optical tweezers using an LG 02 Laguerre-Gaussian laser beam. We verify that we can accurately measure the total optical torque by measuring the spin angular momentum transfer for three different polarizations, by comparing the optical torque with the optical torque calculated using computational electrodynamics and the viscous drag torque determined from the rotation rate and computational fluid dynamics. The torque agrees with that expected from the design principles and electromagnetic modelling of the torque within the optical trap. \textcopyright IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
```
@article{Asavei2009,
  author = {Asavei, Theodor and Loke, Vincent L.Y. and Barbieri, Marco and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/1367-2630/11/9/093021},
  issn = {13672630},
  journal = {New Journal of Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = sep,
  title = {{Optical angular momentum transfer to microrotors fabricated by two-photon photopolymerization}},
  volume = {11},
  year = {2009}
}
```

Erratum: Constraining validity of the Minkowski energy-momentum tensor (Physical Review A - Atomic, Molecular, and Optical Physics (2009) 79 (023813))

Pfeifer, R. N. C., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2009). In Physical Review A - Atomic, Molecular, and Optical Physics (Vol. 79, Number 2). https://doi.org/10.1103/PhysRevA.79.029903'

@misc{Pfeifer2009,
  author = {Pfeifer, Robert N C and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  booktitle = {Physical Review A - Atomic, Molecular, and Optical Physics},
  doi = {10.1103/PhysRevA.79.029903},
  issn = {10502947},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = feb,
  number = {2},
  title = {{Erratum: Constraining validity of the Minkowski energy-momentum tensor (Physical Review A - Atomic, Molecular, and Optical Physics (2009) 79 (023813))}},
  volume = {79},
  year = {2009}
}

Constant power optical tweezers with controllable torque

Funk, M., Parkin, S. J., Stilgoe, A. B., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2009). Optics Letters, 34(2), 139. https://doi.org/10.1364/ol.34.000139'
We describe a means for controlling the spin angular-momentum flux of a laser beam at constant power, without introducing any elliptical or linear polarization. This allows a controllable torque, acting to spin the particle uniformly, to be exerted on a birefringent particle in optical tweezers. The constant power means that transverse and axial trapping, and heating due to absorption, are unaffected by changing the torque. The torque can be computer controlled and rapidly changed. In addition, the lateral trapping is kept constant. Very low torques can be obtained such that rotational Brownian motion of birefringent particles can be observed. This has the potential to greatly extend the quantitative applications of the rotation of birefringent objects in optical tweezers.
```
@article{Funk2009,
  author = {Funk, Maren and Parkin, Simon J. and Stilgoe, Alexander B. and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/ol.34.000139},
  issn = {0146-9592},
  journal = {Optics Letters},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jan,
  number = {2},
  pages = {139},
  publisher = {The Optical Society},
  title = {{Constant power optical tweezers with controllable torque}},
  volume = {34},
  year = {2009}
}
```
Refractometry of melanocyte cell nuclei using optical scatter images recorded by digital Fourier microscopy

Seet, K. Y. T., Nieminen, T. A., & Zvyagin, A. V. (2009). Journal of Biomedical Optics, 14(4), 044031. https://doi.org/10.1117/1.3207141'
The cell nucleus is the dominant optical scatterer in the cell. Neoplastic cells are characterized by cell nucleus polymorphism and polychromism-i.e., the nuclei exhibits an increase in the distribution of both size and refractive index. The relative size parameter, and its distribution, is proportional to the product of the nucleus size and its relative refractive index and is a useful discriminant between normal and abnormal (cancerous) cells. We demonstrate a recently introduced holographic technique, digital Fourier microscopy (DFM), to provide a sensitive measure of this relative size parameter. Fourier holograms were recorded and optical scatter of individual scatterers were extracted and modeled with Mie theory to determine the relative size parameter. The relative size parameter of individual melanocyte cell nuclei were found to be 16.5+/-0.2, which gives a cell nucleus refractive index of 1.38+/-0.01 and is in good agreement with previously reported data. The relative size parameters of individual malignant melanocyte cell nuclei are expected to be greater than 16.5.
```
@article{Seet2009,
  author = {Seet, Katrina Y. T. and Nieminen, Timo A. and Zvyagin, Andrei V.},
  doi = {10.1117/1.3207141},
  issn = {10833668},
  journal = {Journal of Biomedical Optics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {4},
  pages = {044031},
  publisher = {SPIE-Intl Soc Optical Eng},
  title = {{Refractometry of melanocyte cell nuclei using optical scatter images recorded by digital Fourier microscopy}},
  volume = {14},
  year = {2009}
}
```
Fabrication of microstructures for optically driven micromachines using two-photon photopolymerization of UVcuring resins

Asavei, T., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2009). Journal of Optics A: Pure and Applied Optics, 11(3). https://doi.org/10.1088/1464-4258/11/3/034001'
Two-photon photopolymerization of UV curing resins is an attractive method for the fabrication of microscopic transparent objects with size in the tens of micrometers range. We have been using this method to produce three-dimensional (3D) structures for optical micromanipulation, in an optical system based on a femtosecond laser. By carefully adjusting the laser power and the exposure time we were able to create micro-objects with well-defined 3D features and with resolution below the diffraction limit of light. We discuss the performance and capabilities of a microfabrication system, with some examples of its products. \textcopyright 2009 IOP Publishing Ltd.
```
@article{Asavei2009a,
  author = {Asavei, Theodor and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/1464-4258/11/3/034001},
  issn = {14644258},
  journal = {Journal of Optics A: Pure and Applied Optics},
  keywords = {Optical angular momentum,Optical trapping,Two-photon photopolymerization},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {3},
  title = {{Fabrication of microstructures for optically driven micromachines using two-photon photopolymerization of UVcuring resins}},
  volume = {11},
  year = {2009}
}
```

2008

Angular momentum of a strongly focused Gaussian beam

Nieminen, T. A., Stilgoe, A. B., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2008). Journal of Optics A: Pure and Applied Optics, 10(11). https://doi.org/10.1088/1464-4258/10/11/115005'
A circularly polarized paraxial Gaussian laser beam carries angular momentum per photon as spin, with zero orbital angular momentum. Focusing the beam with a rotationally symmetric lens cannot change this angular momentum flux, yet the focused beam must have spin per photon. The remainder of the original spin is converted to orbital angular momentum, manifesting itself as a longitudinal optical vortex at the focus. We investigate the nature of this orbital angular momentum. \textcopyright 2008 IOP Publishing Ltd.
```
@article{Nieminen2008c,
  archiveprefix = {arXiv},
  arxivid = {physics/0408080},
  author = {Nieminen, Timo A. and Stilgoe, Alexander B. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/1464-4258/10/11/115005},
  eprint = {0408080},
  issn = {14644258},
  journal = {Journal of Optics A: Pure and Applied Optics},
  keywords = {Non-paraxial beams,Optical vortices,Orbital angular momentum},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = nov,
  number = {11},
  primaryclass = {physics},
  title = {{Angular momentum of a strongly focused Gaussian beam}},
  volume = {10},
  year = {2008}
}
```
The effect of Mie resonances on trapping in optical tweezers

Stilgoe, A. B., Nieminen, T. A., Knöener, G., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2008). Optics Express, 16(19), 15039. https://doi.org/10.1364/oe.16.015039'
We calculate trapping forces, trap stiffness and interference effects for spherical particles in optical tweezers using electromagnetic theory. We show the dependence of these on relative refractive index and particle size. We investigate resonance effects, especially in high refractive index particles where interference effects are expected to be strongest. We also show how these simulations can be used to assist in the optimal design of traps. ? 2008 Optical Society of America.
```
@article{Stilgoe2008,
  author = {Stilgoe, Alexander B. and Nieminen, Timo A. and Kn{\"{o}}ener, Gregor and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/oe.16.015039},
  issn = {1094-4087},
  journal = {Optics Express},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = sep,
  number = {19},
  pages = {15039},
  publisher = {The Optical Society},
  title = {{The effect of Mie resonances on trapping in optical tweezers}},
  volume = {16},
  year = {2008}
}
```
Naturalistic intervention for Asperger syndrome - A case study

Choi, S. H. J., & Nieminen, T. A. (2008). British Journal of Special Education, 35(2), 85–91. https://doi.org/10.1111/j.1467-8578.2008.00377.x'
On the basis of their cognitive abilities, children with Asperger syndrome are attractive candidates for inclusive education and, in Australia, most are in integrated settings. However, social interaction between children with Asperger syndrome and their peers remains problematic, with the children with Asperger syndrome often being left alone despite being among or near classmates. For classroom teachers, effective interventions in terms of class management are critical to facilitate positive social interaction between these two groups of children, and case studies can tell us a great deal about what works. In practice, a combination of intervention techniques may be best, in case a single one is ineffective, and in order to take advantage of potential synergies. In this article, Serene Choi, of the School of Education at the University of Newcastle (Australia) and Timo Nieminen, of the Centre for Biophotonics and Laser Science in the Department of Physics at the University of Queensland, report a naturalistic multiple-component intervention used to develop the social skills of a boy with Asperger syndrome in a mainstream school in Australia. This combined intervention, making use of social stories, simple peer modelling and individual lessons, appears to be a practical teaching method for inclusive education. \textcopyright 2008 The Author(s).
```
@article{Choi2008,
  author = {Choi, Serene Hyun Jin and Nieminen, Timo A.},
  doi = {10.1111/j.1467-8578.2008.00377.x},
  issn = {09523383},
  journal = {British Journal of Special Education},
  keywords = {Asperger syndrome,Case studies,Inclusive education,Interaction,Intervention},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jun,
  number = {2},
  pages = {85--91},
  title = {{Naturalistic intervention for Asperger syndrome - A case study}},
  volume = {35},
  year = {2008}
}
```
Binary keys for classification and taxonomy of behaviour

Nieminen, T., & Choi, S. H. J. (2008). International Journal of Research and Method in Education, 31(1), 31–44. https://doi.org/10.1080/17437270801919875'
Quantitative behaviour analysis requires the classification of behaviour to produce the basic data. This can be challenging when the theoretical taxonomy does not match observational limitations, or if a theoretical taxonomy is unavailable. Binary keys allow qualitative observation to be used to modify a theoretical taxonomy to produce a practical classification method, or to develop a pragmatic taxonomy in the absence of theory. Furthermore, inter-observer consistency can be maximized with less training, and construction of a binary key ensures that the categories into which behaviour is classified are complete and mutually exclusive. We give examples from special education research for the behaviour of children with developmental disabilities.
```
@article{Nieminen2008a,
  author = {Nieminen, Timo and Choi, Serene Hyun Jin},
  doi = {10.1080/17437270801919875},
  issn = {1743727X},
  journal = {International Journal of Research and Method in Education},
  keywords = {Autism,Binary key,Classification,Cognitive play behaviour,Dichotomous key,Social interaction},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = apr,
  number = {1},
  pages = {31--44},
  title = {{Binary keys for classification and taxonomy of behaviour}},
  volume = {31},
  year = {2008}
}
```
Forces in optical tweezers with radially and azimuthally polarized trapping beams

Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2008). Optics Letters, 33(2), 122. https://doi.org/10.1364/ol.33.000122'
It has been suggested that radially polarized beams can be used to improve the performance of optical tweezers, with reduced scattering force resulting from both the polarization and the dark center of the beam [Opt. Lett. 32, 1839 (2007)]. We calculate the forces on particles in such traps, using rigorous electromagnetic theory, comparing the results with azimuthally polarized beam, circularly polarized LG 01 beams, and Gaussian beams. Our results agree qualitatively with Opt. Lett. 32, 1839 (2007), but differ quantitatively.
```
@article{Nieminen2008b,
  author = {Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/ol.33.000122},
  issn = {0146-9592},
  journal = {Optics Letters},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jan,
  number = {2},
  pages = {122},
  pmid = {18197212},
  publisher = {The Optical Society},
  title = {{Forces in optical tweezers with radially and azimuthally polarized trapping beams}},
  volume = {33},
  year = {2008}
}
```
Tailoring particles for optical trapping and micromanipulation: An overview

Nieminen, T. A., Asavei, T., Hu, Y., Persson, M., Vogel, R., Loke, V. L. Y., Parkin, S. J., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2008). Progress in Electromagnetics Research Symposium, 2, 851–855. https://doi.org/10.2529/piers071101222308'
Optical trapping and micromanipulation has developed from an interesting novelty to a powerful and widely used tool, with the capability to move or trap microscopic live biological specimens and measure forces on the order of piconewtons, typical of forces in microbiological systems. Despite this, the range of particles typically trapped or manipulated is quite small, and it is unusual to see applications involving objects other than biological specimens or homogeneous isotropic microspheres, typically polymer or silica. However, particles can be modified or specially fabricated to expand the possible applications of optical tweezers. For example, while non-absorbing homogeneous isotropic spheres cannot be rotated, optically anisotropic spheres can, and can therefore function as microscopic torque sensors, extending the usual translational micromanipulation and force measurement to rotational manipulation and torque sensing. The development of such particles has led to applications in microscale metrology and biophysics, along with potential deployment of optically-driven micromachines in lab-on-a-chip devices. We present an overview of our work on the tailoring of microparticles for versatile optical trapping and micromanipulation. This includes approaches based on controlled chemistry - nanoassembly - and optical microfabrication. Beginning with the production of anisotropic vaterite microspheres, we review some of the applications, and difficulties encountered along the way. Some of these difficulties can be overcome by coating of the vaterite microspheres. We also discuss the use of anti-reflection coating to allow strong trapping of high refractive index particles. The alternative strategy of producing arbitrarily shaped polymer microstructures through two-photon photopolymerization is also discussed. This can be used to produce optically-driven microrotors or structurally anisotropic microspheres to replace vaterites for particular applications.
```
@inproceedings{Nieminen2008,
  author = {Nieminen, T. A. and Asavei, T. and Hu, Y. and Persson, M. and Vogel, R. and Loke, V. L.Y. and Parkin, S. J. and Heckenberg, N. R. and Rubinsztein-Dunlop, H.},
  booktitle = {Progress in Electromagnetics Research Symposium},
  doi = {10.2529/piers071101222308},
  isbn = {9781618390530},
  issn = {15599450},
  mendeley-groups = {Optical Tweezers/UQOMG},
  pages = {851--855},
  publisher = {Electromagnetics Academy},
  title = {{Tailoring particles for optical trapping and micromanipulation: An overview}},
  volume = {2},
  year = {2008}
}
```
Antireflection coating for improved optical trapping

Hu, Y., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2008). Journal of Applied Physics, 103(9). https://doi.org/10.1063/1.2919574'
An antireflection coating improves the trapping of high-index particles in optical tweezers by reducing the scattering force. This can allow the strong trapping of high-index particles that cannot normally be trapped, and the use of lower numerical aperture objectives while still obtaining strong trapping. The improvement is not overly sensitive to the refractive index or thickness of the coating. \textcopyright 2008 American Institute of Physics.
```
@article{Hu2008,
  author = {Hu, Ying and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1063/1.2919574},
  issn = {00218979},
  journal = {Journal of Applied Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {9},
  title = {{Antireflection coating for improved optical trapping}},
  volume = {103},
  year = {2008}
}
```

2007

Picoliter viscometry using optically rotated particles

Parkin, S. J., Knöner, G., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2007). Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 76(4). https://doi.org/10.1103/PhysRevE.76.041507'
Important aspects in the field of microrheology are studies of the viscosity of fluids within structures with micrometer dimensions and fluid samples where only microliter volumes are available. We have quantitatively investigated the performance and accuracy of a microviscometer based on rotating optical tweezers, which requires as little as one microliter of sample. We have characterized our microviscometer, including effects due to heating, and demonstrated its ability to perform measurements over a large dynamic range of viscosities (at least two orders of magnitude). We have also inserted a probe particle through the membrane of a cell and measured the viscosity of the intramembranous contents. Viscosity measurements of tears have also been made with our microviscometer, which demonstrate its potential use to study unstimulated eye fluid. \textcopyright 2007 The American Physical Society.
```
@article{Parkin2007,
  author = {Parkin, Simon J. and Kn{\"{o}}ner, Gregor and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevE.76.041507},
  issn = {15393755},
  journal = {Physical Review E - Statistical, Nonlinear, and Soft Matter Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = oct,
  number = {4},
  title = {{Picoliter viscometry using optically rotated particles}},
  volume = {76},
  year = {2007}
}
```
Colloquium: Momentum of an electromagnetic wave in dielectric media

Pfeifer, R. N. C., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2007). Reviews of Modern Physics, 79(4), 1197–1216. https://doi.org/10.1103/RevModPhys.79.1197'
Almost 100 years ago, two different expressions were proposed for the energy-momentum tensor of an electromagnetic wave in a dielectric. Minkowski’s tensor predicted an increase in the linear momentum of the wave on entering a dielectric medium, whereas Abraham’s tensor predicted its decrease. Theoretical arguments were advanced in favor of both sides, and experiments proved incapable of distinguishing between the two. Yet more forms were proposed, each with their advocates who considered the form that they were proposing to be the one true tensor. This paper reviews the debate and its eventual conclusion: that no electromagnetic wave energy-momentum tensor is complete on its own. When the appropriate accompanying energy-momentum tensor for the material medium is also considered, experimental predictions of all various proposed tensors will always be the same, and the preferred form is therefore effectively a matter of personal choice. \textcopyright 2007 The American Physical Society.
```
@article{Pfeifer2007,
  archiveprefix = {arXiv},
  arxivid = {0710.0461},
  author = {Pfeifer, Robert N.C. and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/RevModPhys.79.1197},
  eprint = {0710.0461},
  issn = {00346861},
  journal = {Reviews of Modern Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = oct,
  number = {4},
  pages = {1197--1216},
  title = {{Colloquium: Momentum of an electromagnetic wave in dielectric media}},
  volume = {79},
  year = {2007}
}
```
Optical tweezers computational toolbox

Nieminen, T. A., Loke, V. L. Y., Stilgoe, A. B., Knöner, G., Brańczyk, A. M., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2007). Journal of Optics A: Pure and Applied Optics, 9(8). https://doi.org/10.1088/1464-4258/9/8/S12'
We describe a toolbox, implemented in Matlab, for the computational modelling of optical tweezers. The toolbox is designed for the calculation of optical forces and torques, and can be used for both spherical and nonspherical particles, in both Gaussian and other beams. The toolbox might also be useful for light scattering using either Lorenz-Mie theory or the T-matrix method. \textcopyright IOP Publishing Ltd.
```
@article{Nieminen2007,
  author = {Nieminen, Timo A. and Loke, Vincent L.Y. and Stilgoe, Alexander B. and Kn{\"{o}}ner, Gregor and Bra{\'{n}}czyk, Agata M. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1088/1464-4258/9/8/S12},
  issn = {14644258},
  journal = {Journal of Optics A: Pure and Applied Optics},
  keywords = {Laser trapping,Light scattering,Mie theory,Optical tweezers,T-matrix method},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = aug,
  number = {8},
  title = {{Optical tweezers computational toolbox}},
  volume = {9},
  year = {2007}
}
```
FDFD/T-matrix hybrid method

Loke, V. L. Y., Nieminen, T. A., Parkin, S. J., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2007). Journal of Quantitative Spectroscopy and Radiative Transfer, 106(1-3), 274–284. https://doi.org/10.1016/j.jqsrt.2007.01.040'
The T-matrix method is particularly useful for modeling light scattering by mesoscale particles under non-plane wave illumination. Its main advantage lies where repeated calculations with different incident conditions are required, e.g. as particles move through a focused beam, or are optically trapped by a beam. The extended boundary condition method (EBCM) is the most commonly used method for calculating a T-matrix. However, the conventional EBCM is limited to modeling particles that are isotropic and homogeneous. Because scattering by anisotropic and inhomogeneous particles is of practical interest, we have implemented an FDFD T-matrix hybrid method. \textcopyright 2007 Elsevier Ltd. All rights reserved.
```
@article{Loke2007,
  author = {Loke, Vincent L.Y. and Nieminen, Timo A. and Parkin, Simon J. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1016/j.jqsrt.2007.01.040},
  issn = {00224073},
  journal = {Journal of Quantitative Spectroscopy and Radiative Transfer},
  keywords = {Birefringence,FDFD,T-matrix},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jul,
  number = {1-3},
  pages = {274--284},
  title = {{FDFD/T-matrix hybrid method}},
  volume = {106},
  year = {2007}
}
```
Refractometry of organosilica microspheres

Seet, K. Y. T., Vogel, R., Nieminen, T. A., Knöner, G., Rubinsztein-Dunlop, H., Trau, M., & Zvyagin, A. V. (2007). Applied Optics, 46(9), 1554–1561. https://doi.org/10.1364/AO.46.001554'
The refractive index of novel organosilica (nano/micro) material is determined using two methods. The first method is based on analysis of optical extinction efficiency of organosilica beads versus wavelength, which is obtained by a standard laboratory spectrometer. The second method relies on the measurable trapping potential of these beads in the focused light beam (laser tweezers). Polystyrene beads were used to test these methods, and the determined dispersion curves of refractive-index values have been found accurate. The refractive index of organosilica beads has been determined to range from 1.60 to 1.51 over the wavelength range of 300-1100 nm. \textcopyright 2007 Optical Society of America.
```
@article{Seet2007,
  archiveprefix = {arXiv},
  arxivid = {physics/0703070},
  author = {Seet, Katrina Y.T. and Vogel, Robert and Nieminen, Timo A. and Kn{\"{o}}ner, Gregor and Rubinsztein-Dunlop, Halina and Trau, Matt and Zvyagin, Andrei V.},
  doi = {10.1364/AO.46.001554},
  eprint = {0703070},
  issn = {15394522},
  journal = {Applied Optics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = mar,
  number = {9},
  pages = {1554--1561},
  primaryclass = {physics},
  publisher = {OSA - The Optical Society},
  title = {{Refractometry of organosilica microspheres}},
  volume = {46},
  year = {2007}
}
```

Toolbox for Calculation of Optical Forces and Torques

Nieminen, T. A., Loke, V. L. Y., Knoner, G., & Branczyk, A. M. (2007). PIERS Online, 3(3), 338–342. https://doi.org/10.2529/piers060908163412'

We describe a toolbox, implemented in Matlab, for the computational modelling of optical forces and torques.

@article{Nieminen2007a,
  author = {Nieminen, Timo A. and Loke, V. L. Y. and Knoner, G. and Branczyk, A. M.},
  doi = {10.2529/piers060908163412},
  issn = {1931-7360},
  journal = {PIERS Online},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {3},
  pages = {338--342},
  publisher = {PIERS Enterprise},
  title = {{Toolbox for Calculation of Optical Forces and Torques}},
  volume = {3},
  year = {2007}
}

Integrated optomechanical microelements

Knöner, G., Parkin, S., Nieminen, T. A., Loke, V. L. Y., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2007). Optics Express, 15(9), 5521. https://doi.org/10.1364/oe.15.005521'
We integrate the optical elements required to generate optical orbital angular momentum into a microdevice. This allows the rotation of either naturally occuring microparticles or specially fabricated optical rotors. We use a two photon photopolymerization process to create microscopic diffractive optical elements, customized to a wavelength of choice, which are integrated with micromachines in microfluidic devices. This enables the application of high optical torques with off-the-shelf optical tweezers systems. ? 2007 Optical Society of America.
```
@article{Knoner2007,
  author = {Kn{\"{o}}ner, Gregor and Parkin, Simon and Nieminen, Timo A. and Loke, Vincent L. Y. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/oe.15.005521},
  issn = {1094-4087},
  journal = {Optics Express},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {9},
  pages = {5521},
  publisher = {The Optical Society},
  title = {{Integrated optomechanical microelements}},
  volume = {15},
  year = {2007}
}
```
Collecting single molecules with conventional optical tweezers

Singer, W., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2007). Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 75(1). https://doi.org/10.1103/PhysRevE.75.011916'
The size of particles that can be trapped in optical tweezers ranges from tens of nanometers to tens of micrometers. This size regime also includes large single molecules. Here we present experiments demonstrating that optical tweezers can be used to collect polyethylene oxide molecules suspended in water. The molecules that accumulate in the focal volume do not aggregate and therefore represent a region of increased molecule concentration, which can be controlled by the trapping potential. We also present a model that relates the change in concentration to the trapping potential. Since many protein molecules have molecular weights for which this method is applicable the effect may be useful in assisting nucleation of protein crystals. \textcopyright 2007 The American Physical Society.
```
@article{Singer2007,
  archiveprefix = {arXiv},
  arxivid = {physics/0702044},
  author = {Singer, Wolfgang and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevE.75.011916},
  eprint = {0702044},
  issn = {15393755},
  journal = {Physical Review E - Statistical, Nonlinear, and Soft Matter Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {1},
  primaryclass = {physics},
  title = {{Collecting single molecules with conventional optical tweezers}},
  volume = {75},
  year = {2007}
}
```

2006

Towards efficient modelling of optical micromanipulation of complex structures

Nieminen, T. A., Loke, V. L. Y., Branczyk, A. M., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2006). ArXiv e-Prints, physics/0607231.

@article{2006physics...7231N,
  author = {{Nieminen}, T.~A. and {Loke}, V.~L.~Y. and {Branczyk}, A.~M. and {Heckenberg}, N.~R. and {Rubinsztein-Dunlop}, H.},
  title = {{Towards efficient modelling of optical micromanipulation of complex structures}},
  journal = {arXiv e-prints},
  keywords = {Physics - Optics, Physics - Computational Physics},
  year = {2006},
  month = jul,
  eid = {physics/0607231},
  pages = {physics/0607231},
  archiveprefix = {arXiv},
  eprint = {physics/0607231},
  primaryclass = {physics.optics},
  adsurl = {https://ui.adsabs.harvard.edu/abs/2006physics...7231N},
  adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

Resonant coupling between bulk waves and guided modes in a dielectric slab with a thick holographic grating

Mason, D. R., Goodman, S. J., Gramotnev, D. K., & Nieminen, T. A. (2006). In Applied Optics (Vol. 45, Number 8, pp. 1804–1811). OSA - The Optical Society. https://doi.org/10.1364/AO.45.001804'
What we believe to be a new type of resonant coupling of an incident bulk wave into guided modes of a slab with a thick holographic grating is shown to occur in the presence of strong frequency detunings of the Bragg condition. This happens through the reflection of the strongly noneigen +1 diffracted order with the slab-grating boundaries, the resultant reflected waves forming a guided slab mode. Rigorous coupled-wave analysis is used for the numerical analysis of the predicted resonant effects. Possible applications include enhanced options for the design of multiplexing and demultiplexing systems, optical signal-processing devices, optical sensors, and measurement techniques. \textcopyright 2006 Optical Society of America.
```
@misc{Mason2006,
  author = {Mason, D. R. and Goodman, S. J. and Gramotnev, D. K. and Nieminen, T. A.},
  booktitle = {Applied Optics},
  doi = {10.1364/AO.45.001804},
  issn = {15394522},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = mar,
  number = {8},
  pages = {1804--1811},
  publisher = {OSA - The Optical Society},
  title = {{Resonant coupling between bulk waves and guided modes in a dielectric slab with a thick holographic grating}},
  volume = {45},
  year = {2006}
}
```
Visualization of \ifmmode\checkC\elseČ\fierenkov radiation and the fields of a moving charge

Pfeifer, R. N. C., & Nieminen, T. A. (2006). Eur. J. Phys., 27(3), 521–529. https://doi.org/10.1088/0143-0807/27/3/006'
For some physics students, the concept of a particle travelling faster than the speed of light holds endless fascination, and Čerenkov radiation is a visible consequence of a charged particle travelling through a medium at locally superluminal velocities. The Heaviside–Feynman equations for calculating the magnetic and electric fields of a moving charge have been known for many decades, but it is only recently that the computing power to plot the fields of such a particle has become readily available for student use. This paper investigates and illustrates the calculation of Maxwell’s D field in homogeneous isotropic media for arbitrary, including superluminal, constant velocity, and uses the results as a basis for discussing energy transfer in the electromagnetic field.
```
@article{Pfeifer2006Mar,
  author = {Pfeifer, Robert N. C. and Nieminen, Timo A.},
  title = {{Visualization of {\ifmmode\check{C}\else\v{C}\fi}erenkov radiation and the fields of a moving charge}},
  journal = {Eur. J. Phys.},
  volume = {27},
  number = {3},
  pages = {521--529},
  year = {2006},
  month = mar,
  issn = {0143-0807},
  publisher = {IOP Publishing},
  doi = {10.1088/0143-0807/27/3/006}
}
```
Measurement of the total optical angular momentum transfer in optical tweezers

Parkin, S., Knöner, G., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2006). Optics Express, 14(15), 6963. https://doi.org/10.1364/oe.14.006963'
We describe a way to determine the total angular momentum, both spin and orbital, transferred to a particle trapped in optical tweezers. As an example an LG_02 mode of a laser beam with varying degrees of circular polarisation is used to trap and rotate an ...
```
@article{Parkin2006,
  author = {Parkin, Simon and Kn{\"{o}}ner, Gregor and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1364/oe.14.006963},
  issn = {1094-4087},
  journal = {Optics Express},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {15},
  pages = {6963},
  publisher = {The Optical Society},
  title = {{Measurement of the total optical angular momentum transfer in optical tweezers}},
  volume = {14},
  year = {2006}
}
```
Measurement of the index of refraction of single microparticles

Knöner, G., Parkin, S., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2006). Physical Review Letters, 97(15). https://doi.org/10.1103/PhysRevLett.97.157402'
The refractive index of single microparticles is derived from precise measurement and rigorous modeling of the stiffness of a laser trap. We demonstrate the method for particles of four different materials with diameters from 1.6 to 5.2\mum and achieve an accuracy of better than 1%. The method greatly contributes as a new characterization technique because it works best under conditions (small particle size, polydispersion) where other methods, such as absorption spectroscopy, start to fail. Particles need not be transferred to a particular fluid, which prevents particle degradation or alteration common in index matching techniques. Our results also show that advanced modeling of laser traps accurately reproduces experimental reality. \textcopyright 2006 The American Physical Society.
```
@article{Knoner2006,
  archiveprefix = {arXiv},
  arxivid = {physics/0610087},
  author = {Kn{\"{o}}ner, Gregor and Parkin, Simon and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevLett.97.157402},
  eprint = {0610087},
  issn = {00319007},
  journal = {Physical Review Letters},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {15},
  primaryclass = {physics},
  title = {{Measurement of the index of refraction of single microparticles}},
  volume = {97},
  year = {2006}
}
```
Optical force field mapping in microdevices

Knöner, G., Ratnapala, A., Nieminen, T. A., Vale, C. J., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2006). Lab on a Chip, 6(12), 1545–1547. https://doi.org/10.1039/b608669a'
We present a method for characterizing microscopic optical force fields. Two dimensional vector force maps are generated by measuring the optical force applied to a probe particle for a grid of particle positions. The method is used to map out the force field created by the beam from a lensed fiber inside a liquid filled microdevice. We find transverse gradient forces and axial scattering forces on the order of 2 pN per 10 mW laser power which are constant over a considerable axial range (\textgreater35 \mum). These findings suggest future useful applications of lensed fibers for particle guiding/sorting. The propulsion of a small particle at a constant velocity of 200 \mum s -1 is shown. \textcopyright The Royal Society of Chemistry.
```
@article{Knoner2006a,
  author = {Kn{\"{o}}ner, Gregor and Ratnapala, Adrian and Nieminen, Timo A. and Vale, Chris J. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1039/b608669a},
  issn = {14730189},
  journal = {Lab on a Chip},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {12},
  pages = {1545--1547},
  publisher = {Royal Society of Chemistry},
  title = {{Optical force field mapping in microdevices}},
  volume = {6},
  year = {2006}
}
```
Orientation of optically trapped nonspherical birefringent particles

Singer, W., Nieminen, T. A., Gibson, U. J., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2006). Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, 73(2). https://doi.org/10.1103/PhysRevE.73.021911'
While the alignment and rotation of microparticles in optical traps have received increased attention recently, one of the earliest examples has been almost totally neglected-the alignment of particles relative to the beam axis, as opposed to about the beam axis. However, since the alignment torques determine how particles align in a trap, they are directly relevant to practical applications. Lysozyme crystals are an ideal model system to study factors determining the orientation of nonspherical birefringent particles in a trap. Both their size and their aspect ratio can be controlled by the growth parameters, and their regular shape makes computational modeling feasible. We show that both external (shape) and internal (birefringence) anisotropy contribute to the alignment torque. Three-dimensionally trapped elongated objects either align with their long axis parallel or perpendicular to the beam axis depending on their size. The shape-dependent torque can exceed the torque due to birefringence, and can align negative uniaxial particles with their optic axis parallel to the electric field, allowing an application of optical torque about the beam axis. \textcopyright 2006 The American Physical Society.
```
@article{Singer2006,
  author = {Singer, Wolfgang and Nieminen, Timo A. and Gibson, Ursula J. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevE.73.021911},
  issn = {15393755},
  journal = {Physical Review E - Statistical, Nonlinear, and Soft Matter Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {2},
  title = {{Orientation of optically trapped nonspherical birefringent particles}},
  volume = {73},
  year = {2006}
}
```

2004

Comment on "Geometric absorption of electromagnetic angular momentum", C. Konz, G. Benford

Nieminen, T. A. (2004). In Optics Communications (Vol. 235, Numbers 1-3, pp. 227–229). Elsevier. https://doi.org/10.1016/j.optcom.2004.02.066'
The core theorem on which the above paper is centred - that a perfectly conducting body of revolution absorbs no angular momentum from an axisymmetric electromagnetic wave field - is in fact a special case of a more general result in electromagnetic scattering theory. In addition, the scaling of the efficiency of transfer of angular momentum to an object with the wavelength and object size merits further discussion. Finally, some comments are made on the choice of terminology and the erroneous statement that a circularly polarized plane wave does not carry angular momentum. \textcopyright 2004 Elsevier B.V. All rights reserved.
```
@misc{Nieminen2004,
  author = {Nieminen, Timo A.},
  booktitle = {Optics Communications},
  doi = {10.1016/j.optcom.2004.02.066},
  issn = {00304018},
  keywords = {Angular momentum,Electromagnetic scattering},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = may,
  number = {1-3},
  pages = {227--229},
  publisher = {Elsevier},
  title = {{Comment on "Geometric absorption of electromagnetic angular momentum", C. Konz, G. Benford}},
  volume = {235},
  year = {2004}
}
```

Optical microrheology using rotating laser-trapped particles

Bishop, A. I., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2004). Physical Review Letters, 92(19). https://doi.org/10.1103/PhysRevLett.92.198104'

@article{Bishop2004,
  archiveprefix = {arXiv},
  arxivid = {physics/0402021},
  author = {Bishop, Alexis I. and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevLett.92.198104},
  eprint = {0402021},
  issn = {00319007},
  journal = {Physical Review Letters},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = may,
  number = {19},
  primaryclass = {physics},
  publisher = {American Physical Society},
  title = {{Optical microrheology using rotating laser-trapped particles}},
  volume = {92},
  year = {2004}
}

Grazing-angle scattering of electromagnetic waves in gratings with varying mean parameters: grating eigenmodes

Gramotnev, D. K., Goodman, S. J., & Nieminen, T. A. (2004). Journal of Modern Optics, 51(3), 379–397. https://doi.org/10.1080/09500340310001608811'

@article{Gramotnev2004,
  author = {Gramotnev, D. K. and Goodman, S. J. and Nieminen, T. A.},
  doi = {10.1080/09500340310001608811},
  issn = {0950-0340},
  journal = {Journal of Modern Optics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = feb,
  number = {3},
  pages = {379--397},
  title = {{Grazing-angle scattering of electromagnetic waves in gratings with varying mean parameters: grating eigenmodes}},
  url = {http://journalsonline.tandf.co.uk/Index/10.1080/09500340310001608811},
  volume = {51},
  year = {2004}
}

Optical measurement of torque exerted on an elongated object by a noncircular laser beam

Parkin, S. J., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2004). Physical Review A - Atomic, Molecular, and Optical Physics, 70(2). https://doi.org/10.1103/PhysRevA.70.023816'

@article{Parkin2004,
  author = {Parkin, Simon J. and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevA.70.023816},
  issn = {10502947},
  journal = {Physical Review A - Atomic, Molecular, and Optical Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {2},
  publisher = {American Physical Society},
  title = {{Optical measurement of torque exerted on an elongated object by a noncircular laser beam}},
  volume = {70},
  year = {2004}
}

2003

Multipole expansion of strongly focussed laser beams

Nieminen, T. A., Rubinsztein-Dunlop, H., & Heckenberg, N. R. (2003). Journal of Quantitative Spectroscopy and Radiative Transfer, 79-80, 1005–1017. https://doi.org/10.1016/S0022-4073(02)00335-7'
Multipole expansion of an incident radiation field-that is, representation of the fields as sums of vector spherical wavefunctions-is essential for theoretical light scattering methods such as the T-matrix method and generalised Lorenz-Mie theory (GLMT). In general, it is theoretically straightforward to find a vector spherical wavefunction representation of an arbitrary radiation field. For example, a simple formula results in the useful case of an incident plane wave. Laser beams present some difficulties. These problems are not a result of any deficiency in the basic process of spherical wavefunction expansion, but are due to the fact that laser beams, in their standard representations, are not radiation fields, but only approximations of radiation fields. This results from the standard laser beam representations being solutions to the paraxial scalar wave equation. We present an efficient method for determining the multipole representation of an arbitrary focussed beam. \textcopyright 2003 Elsevier Science Ltd. All rights reserved.
```
@article{Nieminen2003,
  archiveprefix = {arXiv},
  arxivid = {physics/0308111},
  author = {Nieminen, T. A. and Rubinsztein-Dunlop, H. and Heckenberg, N. R.},
  doi = {10.1016/S0022-4073(02)00335-7},
  eprint = {0308111},
  issn = {00224073},
  journal = {Journal of Quantitative Spectroscopy and Radiative Transfer},
  keywords = {Expansion coefficients,Light scattering,Localised approximation,Nonparaxial beams,Optical tweezers,Shape coefficients},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jun,
  pages = {1005--1017},
  primaryclass = {physics},
  publisher = {Elsevier Ltd},
  title = {{Multipole expansion of strongly focussed laser beams}},
  volume = {79-80},
  year = {2003}
}
```
Calculation of the T-matrix: General considerations and application of the point-matching method

Nieminen, T. A., Rubinsztein-Dunlop, H., & Heckenberg, N. R. (2003). Journal of Quantitative Spectroscopy and Radiative Transfer, 79-80, 1019–1029. https://doi.org/10.1016/S0022-4073(02)00336-9'
The T-matrix method is widely used for the calculation of scattering by particles of sizes on the order of the illuminating wavelength. Although the extended boundary condition method (EBCM) is the most commonly used technique for calculating the T-matrix, a variety of methods can be used. We consider some general principles of calculating T-matrices, and apply the point-matching method to calculate the T-matrix for particles devoid of symmetry. This method avoids the time-consuming surface integrals required by the EBCM. \textcopyright 2003 Elsevier Science Ltd. All rights reserved.
```
@article{Nieminen2003a,
  archiveprefix = {arXiv},
  arxivid = {physics/0308112},
  author = {Nieminen, T. A. and Rubinsztein-Dunlop, H. and Heckenberg, N. R.},
  doi = {10.1016/S0022-4073(02)00336-9},
  eprint = {0308112},
  issn = {00224073},
  journal = {Journal of Quantitative Spectroscopy and Radiative Transfer},
  keywords = {Electromagnetic scattering,Light scattering,Nonspherical particles,T-matrix},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jun,
  pages = {1019--1029},
  primaryclass = {physics},
  publisher = {Elsevier Ltd},
  title = {{Calculation of the T-matrix: General considerations and application of the point-matching method}},
  volume = {79-80},
  year = {2003}
}
```
Anomalous absorption of bulk shear sagittal acoustic waves in a layered structure with viscous fluid

Gramotnev, D. K., Mather, M. L., & Nieminen, T. A. (2003). Ultrasonics, 41(3), 197–205. https://doi.org/10.1016/S0041-624X(03)00100-8'
It is demonstrated theoretically that the absorptivity of bulk shear sagittal waves by an ultra-thin layer of viscous fluid between two different elastic media has a strong maximum (in some cases as good as 100%) at an optimal layer thickness. This thickness is usually much smaller than the penetration depths and lengths of transverse and longitudinal waves in the fluid. The angular dependencies of the absorptivity are demonstrated to have significant and unusual structure near critical angles of incidence. The effect of non-Newtonian properties and non-uniformities of the fluid layer on the absorptivity is also investigated. In particular, it is shown that the absorption in a thin layer of viscous fluid is much more sensitive to non-zero relaxation time(s) in the fluid layer than the absorption at an isolated solid-fluid interface. \textcopyright 2003 Elsevier Science B.V. All rights reserved.
```
@article{Gramotnev2003,
  author = {Gramotnev, Dmitri K. and Mather, Melissa L. and Nieminen, Timo A.},
  doi = {10.1016/S0041-624X(03)00100-8},
  issn = {0041624X},
  journal = {Ultrasonics},
  keywords = {Absorption,Bulk waves,Frictional contacts,Viscoelastic layers},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = may,
  number = {3},
  pages = {197--205},
  title = {{Anomalous absorption of bulk shear sagittal acoustic waves in a layered structure with viscous fluid}},
  volume = {41},
  year = {2003}
}
```
Non-steady-state double-resonant extremely asymmetrical scattering of waves in periodic gratings

Gramotnev, D. K., & Nieminen, T. A. (2003). Physics Letters, Section A: General, Atomic and Solid State Physics, 310(2-3), 214–222. https://doi.org/10.1016/S0375-9601(03)00289-5'
Double-resonant extremely asymmetrical scattering (DEAS) is a strongly resonant type of Bragg scattering in two joint or separated uniform gratings with different phases. It is characterised by a very strong increase of the scattered and incident wave amplitudes inside and between the gratings at a resonant phase shift between the gratings. DEAS is realised when the first diffracted order satisfying the Bragg condition propagates parallel to the grating boundaries, and the joint or separated gratings interact by means of the diffractional divergence of the scattered waves from one grating into another. This Letter develops a theory of non-steady-state DEAS of bulk TE electromagnetic waves in holographic gratings, and investigates the process of relaxation of the incident and scattered wave amplitudes to their steady-state values inside and outside the gratings. Typical relaxation times are determined. Physical explanation of the predicted effects is presented. \textcopyright 2003 Elsevier Science B.V. All rights reserved.
```
@article{Gramotnev2003a,
  author = {Gramotnev, Dmitri K. and Nieminen, Timo A.},
  doi = {10.1016/S0375-9601(03)00289-5},
  issn = {03759601},
  journal = {Physics Letters, Section A: General, Atomic and Solid State Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = apr,
  number = {2-3},
  pages = {214--222},
  publisher = {Elsevier},
  title = {{Non-steady-state double-resonant extremely asymmetrical scattering of waves in periodic gratings}},
  volume = {310},
  year = {2003}
}
```
Rigorous analysis of grazing-angle scattering of electromagnetic waves in periodic gratings

Gramotnev, D. K., & Nieminen, T. A. (2003). Optics Communications, 219(1-6), 33–48. https://doi.org/10.1016/S0030-4018(03)01304-X'
Grazing-angle scattering (GAS) is a type of Bragg scattering of waves in slanted non-uniform periodic gratings, when the diffracted order satisfying the Bragg condition propagates at a grazing angle with respect to the boundaries of a slab-like grating. Rigorous analysis of GAS of bulk TE electromagnetic waves is undertaken in holographic gratings by means of the enhanced T-matrix algorithm. A comparison of the rigorous and the previously developed approximate theories of GAS is carried out. A complex pattern of numerous previously unknown resonances is discovered and analysed in detail for gratings with large amplitude, for which the approximate theory fails. These resonances are associated not only with the geometry of GAS, but are also typical for wide transmitting gratings. Their dependence on grating amplitude, angles of incidence and scattering, and grating width is investigated numerically. Physical interpretation of the predicted resonances is linked to the existence and the resonant generation of special new eigenmodes of slanted gratings. Main properties of these modes and their field structure are discussed. \textcopyright 2003 Elsevier Science B.V. All rights reserved.
```
@article{Gramotnev2003b,
  author = {Gramotnev, D. K. and Nieminen, T. A.},
  doi = {10.1016/S0030-4018(03)01304-X},
  issn = {00304018},
  journal = {Optics Communications},
  keywords = {Bulk optical waves,Periodic gratings,Rigorous coupled wave theory,Scattering},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = apr,
  number = {1-6},
  pages = {33--48},
  title = {{Rigorous analysis of grazing-angle scattering of electromagnetic waves in periodic gratings}},
  volume = {219},
  year = {2003}
}
```
Orientation of biological cells using plane-polarized Gaussian beam optical tweezers

Bayoudh, S., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2003). Journal of Modern Optics, 50(10), 1581–1590. https://doi.org/10.1080/0950034031000069497'
Optical tweezers are widely used for the manipulation of cells and their internal structures. However, the degree of manipulation possible is limited by poor control over the orientation of trapped cells. We show that it is possible to controllably align or rotate disc shaped cells - chloroplasts of Spinacia oleracea - in a plane polarised Gaussian beam trap, using optical torques resulting predominantly from circular polarisation induced in the transmitted beam by the non-spherical shape of the cells.
```
@article{Bayoudh2003,
  archiveprefix = {arXiv},
  arxivid = {physics/0308105},
  author = {Bayoudh, S. and Nieminen, T. A. and Heckenberg, N. R. and Rubinsztein-Dunlop, H.},
  doi = {10.1080/0950034031000069497},
  eprint = {0308105},
  issn = {0950-0340},
  journal = {Journal of Modern Optics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jan,
  number = {10},
  pages = {1581--1590},
  primaryclass = {physics},
  publisher = {Informa UK Limited},
  title = {{Orientation of biological cells using plane-polarized Gaussian beam optical tweezers}},
  volume = {50},
  year = {2003}
}
```
Optical application and measurement of torque on microparticles of isotropic nonabsorbing material

Bishop, A. I., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (2003). Physical Review A - Atomic, Molecular, and Optical Physics, 68(3), 8. https://doi.org/10.1103/PhysRevA.68.033802'
We show how it is possible to controllably rotate or align microscopic particles of isotropic nonabsorbing material in a [Formula Presented] Gaussian beam trap, with simultaneous measurement of the applied torque using purely optical means. This is a simple and general method of rotation, requiring only that the particle is elongated along one direction. Thus, this method can be used to rotate or align a wide range of naturally occurring particles. The ability to measure the applied torque enables the use of this method as a quantitative tool—the rotational equivalent of optical tweezers based force measurement. As well as being of particular value for the rotation of biological specimens, this method is also suitable for the development of optically driven micromachines. \textcopyright 2003 The American Physical Society.
```
@article{Bishop2003,
  archiveprefix = {arXiv},
  arxivid = {physics/0309122},
  author = {Bishop, Alexis I. and Nieminen, Timo A. and Heckenberg, Norman R. and Rubinsztein-Dunlop, Halina},
  doi = {10.1103/PhysRevA.68.033802},
  eprint = {0309122},
  issn = {10941622},
  journal = {Physical Review A - Atomic, Molecular, and Optical Physics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  number = {3},
  pages = {8},
  primaryclass = {physics},
  title = {{Optical application and measurement of torque on microparticles of isotropic nonabsorbing material}},
  volume = {68},
  year = {2003}
}
```

2002

Extremely asymmetrical scattering in gratings with varying mean structural parameters

Gramotnev, D. K., Nieminen, T. A., & Hopper, T. A. (2002). Journal of Modern Optics, 49(9), 1567–1585. https://doi.org/10.1080/09500340110109403'
Extremely asymmetrical scattering (EAS) is an unusual type of Bragg scattering in slanted periodic gratings with the scattered wave (the +1 diffracted order) propagating parallel to the grating boundaries. Here, a unique and strong sensitivity of EAS to small stepwise variations of mean structural parameters at the grating boundaries is predicted theoretically (by means of approximate and rigorous analyses) for bulk TE electromagnetic waves and slab optical modes of arbitrary polarization in holographic (for bulk waves) and corrugation (for slab modes) gratings. The predicted effects are explained using one of the main physical reasons for EAS - the diffractional divergence of the scattered wave (similar to divergence of a laser beam). The approximate method of analysis is based on this understanding of the role of the divergence of the scattered wave, while the rigorous analysis uses the enhanced T-matrix algorithm. The effect of small and large stepwise variations of the mean permittivity at the grating boundaries is analysed. Two distinctly different and unusual patterns of EAS are predicted in the cases of wide and narrow (compared to a critical width) gratings. Comparison between the approximate and rigorous theories is carried out.
```
@article{Gramotnev2002,
  author = {Gramotnev, D. K. and Nieminen, T. A. and Hopper, T. A.},
  doi = {10.1080/09500340110109403},
  issn = {09500340},
  journal = {Journal of Modern Optics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jul,
  number = {9},
  pages = {1567--1585},
  title = {{Extremely asymmetrical scattering in gratings with varying mean structural parameters}},
  volume = {49},
  year = {2002}
}
```
Non-steady-state extremely asymmetrical scattering of waves in periodic gratings

Nieminen, T. A., & Gramotnev, D. K. (2002). Opt. Express, 10(6), 268–273. https://doi.org/10.1364/OE.10.000268'
Extremely asymmetrical scattering (EAS) is a highly resonant type of Bragg scattering with a strong resonant increase of the scattered wave amplitude inside and outside the grating. EAS is realized when the scattered wave propagates parallel to the grating boundaries. We present a rigorous algorithm for the analysis of non-steady-state EAS, and investigate the relaxation of the incident and scattered wave amplitudes to their steady-state values. Non-steady-state EAS of bulk TE electromagnetic waves is analyzed in narrow and wide, slanted, holographic gratings. Typical relaxation times are determined and compared with previous rough estimations. Physical explanation of the predicted effects is presented.
```
@article{Nieminen2002Mar,
  author = {Nieminen, Timo A. and Gramotnev, Dmitri K.},
  title = {{Non-steady-state extremely asymmetrical scattering of waves in periodic gratings}},
  journal = {Opt. Express},
  volume = {10},
  number = {6},
  pages = {268--273},
  year = {2002},
  month = mar,
  issn = {1094-4087},
  publisher = {Optical Society of America},
  doi = {10.1364/OE.10.000268}
}
```

2001

How to SYN in seven easy steps

Nieminen, T. A., & Ross, J. E. R. (2001). Computer Physics Communications, 142(1-3), 160–163. https://doi.org/10.1016/S0010-4655(01)00358-7'

@inproceedings{Nieminen2001c,
  author = {Nieminen, T. A. and Ross, J. E.R.},
  booktitle = {Computer Physics Communications},
  doi = {10.1016/S0010-4655(01)00358-7},
  issn = {00104655},
  keywords = {Asymmetry,Line Profile,Opacity,Spectral synthesis},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = dec,
  number = {1-3},
  pages = {160--163},
  title = {{How to SYN in seven easy steps}},
  volume = {142},
  year = {2001}
}

Numerical modelling of optical trapping

Nieminen, T. A., Rubinsztein-Dunlop, H., Heckenberg, N. R., & Bishop, A. I. (2001). Computer Physics Communications, 142(1-3), 468–471. https://doi.org/10.1016/S0010-4655(01)00391-5'
Optical trapping is a widely used technique, with many important applications in biology and metrology. Complete modelling of trapping requires calculation of optical forces, primarily a scattering problem, and non-optical forces. The T-matrix method is used to calculate forces acting on spheroidal and cylindrical particles. \textcopyright 2001 Elsevier Science B.V. All rights reserved.
```
@inproceedings{Nieminen2001d,
  archiveprefix = {arXiv},
  arxivid = {physics/0308106},
  author = {Nieminen, T. A. and Rubinsztein-Dunlop, H. and Heckenberg, N. R. and Bishop, A. I.},
  booktitle = {Computer Physics Communications},
  doi = {10.1016/S0010-4655(01)00391-5},
  eprint = {0308106},
  issn = {00104655},
  keywords = {Optical tweezers,Radiation pressure,Trapping},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = dec,
  number = {1-3},
  pages = {468--471},
  primaryclass = {physics},
  title = {{Numerical modelling of optical trapping}},
  volume = {142},
  year = {2001}
}
```
Calculation and optical measurement of laser trapping forces on non-spherical particles

Nieminen, T. A., Rubinsztein-Dunlop, H., & Heckenberg, N. R. (2001). Journal of Quantitative Spectroscopy and Radiative Transfer, 70(4-6), 627–637. https://doi.org/10.1016/S0022-4073(01)00034-6'
Optical trapping, where microscopic particles are trapped and manipulated by light is a powerful and widespread technique, with the single-beam gradient trap (also known as optical tweezers) in use for a large number of biological and other applications. The forces and torques acting on a trapped particle result from the transfer of momentum and angular momentum from the trapping beam to the particle. Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult. Calculations can be performed using approximate methods, but are only applicable within their ranges of validity, such as for particles much larger than, or much smaller than, the trapping wavelength, and for spherical isotropic particles. This leaves unfortunate gaps, since wavelength-scale particles are of great practical interest because they are readily and strongly trapped and are used to probe interesting microscopic and microscopic phenomena, and non-spherical or anisotropic particles, biological, crystalline, or other, due to their frequent occurance in nature, and the possibility of rotating such objects or controlling or sensing their orientation. The systematic application of electromagnetic scattering theory can provide a general theory of laser trapping, and render results missing from existing theory. We present here calculations of force and torque on a trapped particle obtained from this theory and discuss the possible applications, including the optical measurement of the force and torque. \textcopyright 2001 Elsevier Science Ltd. All rights reserved.
```
@article{Nieminen2001,
  archiveprefix = {arXiv},
  arxivid = {physics/0308110},
  author = {Nieminen, T. A. and Rubinsztein-Dunlop, H. and Heckenberg, N. R.},
  doi = {10.1016/S0022-4073(01)00034-6},
  eprint = {0308110},
  issn = {00224073},
  journal = {Journal of Quantitative Spectroscopy and Radiative Transfer},
  keywords = {Lazer micromanipulation,Light scattering,Optical forces,Optical tweezers},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jun,
  number = {4-6},
  pages = {627--637},
  primaryclass = {physics},
  title = {{Calculation and optical measurement of laser trapping forces on non-spherical particles}},
  volume = {70},
  year = {2001}
}
```
Rigorous analysis of extremely asymmetrical scattering of electromagnetic waves in slanted periodic gratings

Nieminen, T. A., & Gramotnev, D. K. (2001). Optics Communications, 189(4-6), 175–186. https://doi.org/10.1016/S0030-4018(01)01023-9'
Extremely asymmetrical scattering (EAS) is a new type of Bragg scattering in thick, slanted, periodic gratings. It is realized when the scattered wave propagates parallel to the front boundary of the grating. Its most important feature is the strong resonant increase in the scattered wave amplitude compared to the amplitude of the incident wave: the smaller the grating amplitude, the larger the amplitude of the scattered wave. In this paper, rigorous numerical analysis of EAS is carried out by means of the enhanced T-matrix algorithm. This includes investigation of harmonic generation inside and outside the grating, unusually strong edge effects, fast oscillations of the incident wave amplitude in the grating, etc. Comparison with the previously developed approximate theory is carried out. In particular, it is demonstrated that the applicability conditions for the two-wave approximation in the case of EAS are noticeably more restrictive than those for the conventional Bragg scattering. At the same time, it is shown that the approximate theory is usually highly accurate in terms of description of EAS in the most interesting cases of scattering with strong resonant increase of the scattered wave amplitude. Physical explanation of the predicted effects is presented.
```
@article{Nieminen2001a,
  author = {Nieminen, T. A. and Gramotnev, D. K.},
  doi = {10.1016/S0030-4018(01)01023-9},
  issn = {00304018},
  journal = {Optics Communications},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = mar,
  number = {4-6},
  pages = {175--186},
  publisher = {Elsevier Science Publishers B.V.},
  title = {{Rigorous analysis of extremely asymmetrical scattering of electromagnetic waves in slanted periodic gratings}},
  volume = {189},
  year = {2001}
}
```
Optical measurement of microscopic torques

Nieminen, T. A., Heckenberg, N. R., & H., R.-D. (2001). Journal of Modern Optics, 48(3), 405–413. https://doi.org/10.1080/095003401750051370'
In recent years there has been an explosive development of interest in the measurement of forces at the microscopic level, such as within living cells, as well as the properties of fluids and suspensions on this scale, using optically trapped particles as probes. The next step would be to measure torques and associated rotational motion. This would allow measurement on very small scales since no translational motion is needed. It could also provide an absolute measurement of the forces holding a stationary non-rotating particle in place. The laser-induced torque acting on an optically trapped microscopic birefringent particle can be used for these measurements. Here we present a new method for simple, robust, accurate, simultaneous measurement of the rotation speed of a laser trapped birefringent particle, and the optical torque acting on it, by measuring the change in angular momentum of the light from passing through the particle. This method does not depend on the size or shape of the particle or the laser beam geometry, nor does it depend on the properties of the surrounding medium. This could allow accurate measurement of viscosity on a microscopic scale.
```
@article{Nieminen2001b,
  archiveprefix = {arXiv},
  arxivid = {physics/0308103},
  author = {Nieminen, T. A. and Heckenberg, N. R. and H., Rubinsztein-Dunlop},
  doi = {10.1080/095003401750051370},
  eprint = {0308103},
  issn = {0950-0340},
  journal = {Journal of Modern Optics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = mar,
  number = {3},
  pages = {405--413},
  primaryclass = {physics},
  publisher = {Informa UK Limited},
  title = {{Optical measurement of microscopic torques}},
  volume = {48},
  year = {2001}
}
```
Double-resonant extremely asymmetrical scattering of electromagnetic waves in periodic arrays separated by a gap

Gramotnev, D. K., & Nieminen, T. A. (2001). Optical and Quantum Electronics, 33(1), 1–17. https://doi.org/10.1023/A:1007019621419'
Two strong simultaneous resonances of scattering - double-resonant extremely asymmetrical scattering (DEAS) - are predicted in two parallel, oblique, periodic Bragg arrays separated by a gap, when the scattered wave propagates parallel to the arrays. One of these resonances is with respect to frequency (which is common to all types of Bragg scattering), and another is with respect to phase variation between the arrays. The diffractional divergence of the scattered wave is shown to be the main physical reason for DEAS in the considered structure. Although the arrays are separated, they are shown to interact by means of the diffractional divergence of the scattered wave across the gap from one array into the other. It is also shown that increasing separation between the two arrays results in a broader and weaker resonance with respect to phase shift. The analysis is based on a recently developed new approach allowing for the diffractional divergence of the scattered wave inside and outside the arrays. Physical interpretations of the predicted features of DEAS in separated arrays are also presented. Applicability conditions for the developed theory are derived.
```
@article{Gramotnev2001,
  author = {Gramotnev, D. K. and Nieminen, T. A.},
  doi = {10.1023/A:1007019621419},
  issn = {03068919},
  journal = {Optical and Quantum Electronics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jan,
  number = {1},
  pages = {1--17},
  publisher = {Kluwer Academic Publishers},
  title = {{Double-resonant extremely asymmetrical scattering of electromagnetic waves in periodic arrays separated by a gap}},
  volume = {33},
  year = {2001}
}
```

1999

Extremely asymmetrical scattering of electromagnetic waves in gradually varying periodic arrays

Gramotnev, D. K., & Nieminen, T. A. (1999). Journal of Optics A: Pure and Applied Optics, 1(5), 635–645. https://doi.org/10.1088/1464-4258/1/5/310'
This paper analyses theoretically and numerically the effect of varying grating amplitude on the extremely asymmetrical scattering (EAS) of bulk and guided optical modes in non-uniform strip-like periodic Bragg arrays with stepwise and gradual variations in the grating amplitude across the army. A recently developed new approach based on allowance for the diffractional divergence of the scattered wave is used for this analysis. It is demonstrated that gradual variations in magnitude of the grating amplitude may change the pattern of EAS noticeably but not radically. On the other hand, phase variations in the grating may result in a radically new type of Bragg scattering - double-resonant EAS (DEAS). In this case, a combination of two strong simultaneous resonances (one with respect to frequency, and another with respect to the phase variation) is predicted to take place in non-uniform arrays with a step-like phase and gradual magnitude variations of the grating amplitude. The tolerances of EAS and DEAS to small gradual variations in the grating amplitude are determined. The main features of these types of scattering in non-uniform arrays are explained by the diffractional divergence of the scattered wave inside and outside the array.
```
@article{Gramotnev1999,
  author = {Gramotnev, D. K. and Nieminen, T. A.},
  doi = {10.1088/1464-4258/1/5/310},
  issn = {14644258},
  journal = {Journal of Optics A: Pure and Applied Optics},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = sep,
  number = {5},
  pages = {635--645},
  publisher = {IOP},
  title = {{Extremely asymmetrical scattering of electromagnetic waves in gradually varying periodic arrays}},
  volume = {1},
  year = {1999}
}
```

1998

Optical alignment and spinning of laser-trapped microscopic particles

Friese, M. E. J., Nieminen, T. A., Heckenberg, N. R., & Rubinsztein-Dunlop, H. (1998). Nature, 394(6691), 348–350. https://doi.org/10.1038/28566'
Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams or beams with helical phase structure. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 1936, when he reported a tiny torque developed in a quartz ’wave-plate’ owing to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating, leading to rotation rates of over 350 Hz.
```
@article{Friese1998,
  archiveprefix = {arXiv},
  arxivid = {physics/0308113},
  author = {Friese, M. E.J. and Nieminen, T. A. and Heckenberg, N. R. and Rubinsztein-Dunlop, H.},
  doi = {10.1038/28566},
  eprint = {0308113},
  issn = {00280836},
  journal = {Nature},
  mendeley-groups = {Optical Tweezers/UQOMG},
  month = jul,
  number = {6691},
  pages = {348--350},
  primaryclass = {physics},
  title = {{Optical alignment and spinning of laser-trapped microscopic particles}},
  volume = {394},
  year = {1998}
}
```

Publications

2020

Orientation of swimming cells with annular beam optical tweezers

OTSLM toolbox for Structured Light Methods

2019

Optical-trapping of particles in air using parabolic reflectors and a hollow laser beam

Microscope images of strongly scattering objects via vectorial transfer matrices: Modeling and an experimental verification

2018

Calibration of force detection for arbitrarily shaped particles in optical tweezers

Cellular-Resolution Imaging of Vestibular Processing across the Larval Zebrafish Brain

Optical tweezers toolbox: full dynamics simulations for particles of all sizes

Measuring the motility and drag forces acting on biological particles using optical tweezers

Optical Tweezers Bring Micromachines to Biology

Impact of complex surfaces on biomicrorheological measurements using optical tweezers

2017

Optical trapping of otoliths drives vestibular behaviours in larval zebrafish

Ultrasensitive rotating photonic probes for complex biological systems

Theory and practice of simulation of optical tweezers

Active rotational and translational microrheology beyond the linear spring regime

Visual guide to optical tweezers

2015

Energy, momentum and propagation of non-paraxial high-order Gaussian beams in the presence of an aperture

Escape forces and trajectories in optical tweezers and their effect on calibration

Scattering of sculpted light in intact brain tissue, with implications for optogenetics

Optical Forces, Trapping and Manipulation

2014

Driving corrugated donut rotors with Laguerre-Gauss beams

Optical tweezers: Theory and modelling

2013

Kinect the dots: 3D control of optical tweezers

Optically trapped and driven paddle-wheel

Calibration of nonspherical particles in optical tweezers using only position measurement

Spatially-resolved rotational microrheology with an optically-trapped sphere

2012

Equilibrium orientations and positions of non-spherical particles in optical traps

2011

Phase-transition-like properties of double-beam optical tweezers

Optical tweezers and paradoxes in electromagnetism

T-matrix method for modelling optical tweezers

The difficulty of measuring orbital angular momentum

2009

Highly birefringent vaterite microspheres: production, characterization and applications for optical micromanipulation

Synthesis and surface modification of birefringent vaterite microspheres

Symmetry and the generation and measurement of optical torque

Optical angular momentum transfer to microrotors fabricated by two-photon photopolymerization

Erratum: Constraining validity of the Minkowski energy-momentum tensor (Physical Review A - Atomic, Molecular, and Optical Physics (2009) 79 (023813))

Constant power optical tweezers with controllable torque

Refractometry of melanocyte cell nuclei using optical scatter images recorded by digital Fourier microscopy

Fabrication of microstructures for optically driven micromachines using two-photon photopolymerization of UVcuring resins

2008

Angular momentum of a strongly focused Gaussian beam

The effect of Mie resonances on trapping in optical tweezers

Naturalistic intervention for Asperger syndrome - A case study

Binary keys for classification and taxonomy of behaviour

Forces in optical tweezers with radially and azimuthally polarized trapping beams

Tailoring particles for optical trapping and micromanipulation: An overview

Antireflection coating for improved optical trapping

2007

Picoliter viscometry using optically rotated particles

Colloquium: Momentum of an electromagnetic wave in dielectric media

Optical tweezers computational toolbox

FDFD/T-matrix hybrid method

Refractometry of organosilica microspheres

Toolbox for Calculation of Optical Forces and Torques

Integrated optomechanical microelements

Collecting single molecules with conventional optical tweezers

2006

Towards efficient modelling of optical micromanipulation of complex structures

Resonant coupling between bulk waves and guided modes in a dielectric slab with a thick holographic grating

Visualization of \ifmmode\checkC\elseČ\fierenkov radiation and the fields of a moving charge

Measurement of the total optical angular momentum transfer in optical tweezers

Measurement of the index of refraction of single microparticles

Optical force field mapping in microdevices

Orientation of optically trapped nonspherical birefringent particles

2004

Comment on "Geometric absorption of electromagnetic angular momentum", C. Konz, G. Benford

Optical microrheology using rotating laser-trapped particles

Grazing-angle scattering of electromagnetic waves in gratings with varying mean parameters: grating eigenmodes

Optical measurement of torque exerted on an elongated object by a noncircular laser beam

2003