Driving corrugated donut rotors with Laguerre-Gauss beams.

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 µm and a hollow core radius of 0.5 µm, we were able to transfer approximately 0.3 h̄ per photon of the orbital angular momentum from an LG04 beam.

Comparison of T-matrix calculation methods for scattering by cylinders in optical tweezers.

The T-matrix method, or the T-matrix formulation of scattering, is a framework for mathematically describing the scattering properties of an object as a linear relationship between expansion coefficients of the incident and scattering fields in a basis of vector spherical wave functions (VSWFs). A variety of methods can be used to calculate the T-matrix. We explore the applicability of the extended boundary condition method (EBCM) and point matching (PM) method to calculate the T-matrix for scattering by cylinders in optical tweezers and hence the optical force acting on them. We compare both methods with the discrete-dipole approximation (DDA) to measure their accuracy for different sizes and aspect ratios (ARs) for Rayleigh and wavelength-size cylinders. We determine range of sizes and ARs giving errors below 1% and 10%. These results can help researchers choose the most efficient method to calculate the T-matrix for nonspherical particles with acceptable accuracy.

Surface waves and atomic force microscope probe-particle near-field coupling: discrete dipole approximation with surface interaction.

Evanescent waves on a surface form due to the collective motion of charges within the medium. They do not carry any energy away from the surface and decay exponentially as a function of the distance. However, if there is any object within the evanescent field, electromagnetic energy within the medium is tunneled away and either absorbed or scattered. In this case, the absorption is localized, and potentially it can be used for selective diagnosis or nanopatterning applications. On the other hand, scattering of evanescent waves can be employed for characterization of nanoscale structures and particles on the surface. In this paper we present a numerical methodology to study the physics of such absorption and scattering mechanisms. We developed a MATLAB implementation of discrete dipole approximation with surface interaction (DDA-SI) in combination with evanescent wave illumination to investigate the near-field coupling between particles on the surface and a probe. This method can be used to explore the effects of a number of physical, geometrical, and material properties for problems involving nanostructures on or in the proximity of a substrate under arbitrary illumination.

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

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.

Integrated optomechanical microelements.

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.