Towards Efficient Modelling of Optical Micromanipulation of Complex Structures

Abstract

Computational methods for electromagnetic and light scattering can be used for the calculation of optical forces and torques. Since typical particles that are optically trapped or manipulated are on the order of the wavelength in size, approximate methods such as geometric optics or Rayleigh scattering are inapplicable, and solution or either the Maxwell equations or the vector Helmholtz equation must be resorted to. Traditionally, such solutions were only feasible for the simplest geometries; modern computational power enable the rapid solution of more general-but still simple-geometries such as axisymmetric, homogeneous, and isotropic scatterers. However, optically-driven micromachines necessarily require more complex geometries, and their computational modelling thus remains in the realm of challenging computational problems. We review our progress towards efficient computational modelling of optical tweezers and micromanipulation, including the trapping and manipulation of complex structures such as optical micromachines. In particular, we consider the exploitation of symmetry in the modelling of such devices.

Symmetry and the generation and measurement of optical torque

“…This lack of coupling between different angular momenta is widely exploited in the calculation of the T-matrix (Nieminen et al, 2003a;Mishchenko et al, 2000;Waterman, 1971;Mishchenko, 1991), in additions to the benefit of axisymmetry turning 2D surface integrals into 1D line integrals, or 3D volume problems into 2D surface problems (Loke et al, 2007). Even in the case of discrete rotational symmetry rather than axisymmetry, the restricted coupling between modes (21) can still be used to reduce computational time by orders of magnitude (Kahnert, 2005;Nieminen et al, 2006bNieminen et al, , 2007c.…”