Modelling optical micro-machines

A strongly focused laser beam can be used to trap, manipulate and exert torque on a microparticle. The torque is the result of transfer of angular momentum by scattering of the laser beam. The laser could be used to drive a rotor, impeller, cog wheel or some other microdevice of a few microns in size, perhaps fabricated from a birefringent material. We review our methods of computationally simulating the torque and force imparted by a laser beam. We introduce a method of hybridizing the T-matrix with the Finite Difference Frequency Domain (FDFD) method to allow the modelling of materials that are anisotropic and inhomogeneous, and structures that have complex shapes. The high degree of symmetry of a microrotor, such as discrete or continuous rotational symmetry, can be exploited to reduce computational time and memory requirements by orders of magnitude. This is achieved by performing calculations for only a given segment or plane that is repeated across the whole structure. This can be demonstrated by modelling the optical trapping and rotation of a cube.Comment: 4 pages, 3 figure

Optical microrotors: Theory, design and fabrication

Building on the ability to exert torques in optical tweezers, optically-driven rotating micromachines have reached the verge of practical application. Prototype devices have been made, and useful applications are being sought. We outline some general principles that can be applied to the design of optically-rotated devices, and describe a method for rigorous computational modelling that is well-suited to the optimization and engineering of such micromachines. Finally, we describe a method for rapid microfabrication of prototypes for testing, and some results of such tests

Optical paddle-wheel

As an optically trapped micro-object spins in a fluid, there is a consequent flow in the fluid.. Since a free-floating optically-driven microrotor can be moved to a desired position, it can allow the controlled application of a directed flow in a particular location. Here we demonstrate the control and rotation of such a device, an optical paddle-wheel, using a multiple-beam trap. In contrast to the usual situation where rotation is around the beam axis, here we demonstrate rotation normal to this axis

Optimization of optically-driven micromachines

While a variety of different optically-driven micromachines have been demonstrated by a number of groups around the world, there is a striking similarity in the designs used. The typical optically-driven rotor consists of a number of arms attached to a central hub, or elongated stalk in the case of free-floating rotors. This is a consequence of the relationship between the symmetry of a scattering object and the transfer of optical angular momentum from a beam to the object. We use a hybrid discrete-dipole approximation/T-matrix method algorithm to computationally model the scattering by such optically-driven rotors. We systematically explore the effects of the most important parameters of rotors, such as the thickness, length, and width of the arms, in order to maximize the torque efficiency. We show that it is possible to use computational modelling to optimize the design of such devices. We also compare the computational results with experiment

Computational modelling of optical tweezers with many degrees of freedom using dynamic simulation: cylinders, nanowires, and multiple particles

Computational tasks such as the calculation and characterization of the optical force acting on a sphere are relatively straightforward in a Gaussian beam trap. Resulting properties of the trap such as the trap strength, spring constants, and equilibrium position can be easily determined. More complex systems with non-spherical particles or multiple particles add many more degrees of freedom to the problem. Extension of the simple methods used for single spherical particles could result in required computational time of months or years. Thus, alternative methods must be used. One powerful tool is to use dynamic simulation: model the dynamics and motion of a particle or particles within the trap. We demonstrate the use of dynamic simulation for non-spherical particles and multi-particle systems. Using a hybrid discrete dipole approximation (DDA) and T-matrix method, we find plausible equilibrium positions and orientations of cylinders of varying size and aspect ratio. Orientation landscapes revealing different regimes of behaviour for micro-cylinders and nanowires with different refractive indices trapped with beams of differing polarization are also presented. This investigation provides a solid background in both the function and properties of micro-cylinders and nanowires trapped in optical tweezers. This method can also be applied to particles with other shapes. We also investigate multiple-particle trapping, which is quite different from single particle systems, as they can include effects such as optical binding. We show that equilibrium positions, and the strength of interactions between particles can be found in systems of two and more particles

Optically driven micromachines: Progress and prospects

The ability to exert optical torques to rotationally manipulate microparticles has developed from an interesting curiosity to seeing deployment in practical applications. Is the next step to genuine optically-driven micromachines feasible or possible? We review the progress made towards this goal, and future prospects

Investigation of Semiconductor Quantum Dots for Waveguide Electroabsorption Modulator

In this work, we investigated the use of 10-layer InAs quantum dot (QD) as active region of an electroabsorption modulator (EAM). The QD-EAM is a p-i-n ridge waveguide structure with intrinsic layer thickness of 0.4 μm, width of 10 μm, and length of 1.0 mm. Photocurrent measurement reveals a Stark shift of ~5 meV (~7 nm) at reverse bias of 3 V (75 kV/cm) and broadening of the resonance peak due to field ionization of electrons and holes was observed for E-field larger than 25 kV/cm. Investigation at wavelength range of 1,300–1320 nm reveals that the largest absorption change occurs at 1317 nm. Optical transmission measurement at this wavelength shows insertion loss of ~8 dB, and extinction ratio of ~5 dB at reverse bias of 5 V. Consequently, methods to improve the performance of the QD-EAM are proposed. We believe that QDs are promising for EAM and the performance of QD-EAM will improve with increasing research efforts

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

Due to copyright restrictions, the access to the full text of this article is only available via subscription.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 combinationwith 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 materialproperties for problems involving nanostructures on or in the proximity of a substrate under arbitrary illumination.TÜBİTAK ; European Commissio

Discrete-dipole approximation with surface interaction: computational toolbox for MATLAB

We describe a MATLAB toolbox that utilizes the discrete-dipole approximation (DDA) method for modelling light interaction with arbitrarily-shape scatterers in free space as well with planar surface interaction (DDA-SI). The range of applicable models range from optical micromanipulation, plamonics, nano-antennae, near-field coupling and general light interaction with scatterers ranging from a few nanometers to several microns in size