  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.
