| Methods for enhancing the optically-generated torque on laser driven micromachines, by altering the physical properties of the system, were explored. The three primary techniques examined were (a) modification of the index of refraction of the scattering object (i.e. a gear, or "rotor") to inhomogeneous profiles, (b) application of higher order transverse modes of the incident laser beam, and (c) modification of the geometry of the gear. A 3D Finite Difference Time Domain (FDTD) code was written, using MATLABRTM software, to numerically solve Maxwell's equations for the electric and magnetic fields in the vicinity of the gears. Since the geometries involved were generally rectangular in nature, and the physical dimensions of the micro-gears were of the order of the wavelength of the laser source, the numerical FDTD approach was suitable due to its mathematical simplicity and ability to account for the wave nature of light. Maxwell's stress tensor was employed to calculate the electromagnetic forces on the gears resulting from scattering of the incident laser light at the material boundaries.; Simulations were performed for a reference case, consisting of an asymmetric gear with four arms and a homogeneous index of refraction of n = 1.55, which was illuminated by a TEM0,0 fundamental mode laser beam. Each of the three enhancement techniques separately provided a means to significantly increase the optically-generated torque over that obtained from the reference case. One inhomogeneous index of refraction profile resulted in an increase in torque by a factor of 3. Modification of gear geometry also resulted in an increase by a factor of 3 and the angular polarization of the Laguerra-Gaussian doughnut mode resulted in an increase by a factor of 21. Finally, the three techniques were combined in a successful attempt to generate the largest possible applied torque on a microgear. The optically-generated torque was increased by factors as large as 72 for such cases. |
