Radiation forces on a microsphere in an arbitrary refractive index profile

( The force equations describing the radiation forces on a microsphere in an arbitrary refractive index profile are derived here by using the photon-stream method in a ray-optics regime. A loosely focused Gaussian beam was employed as the radiating illumination beam. The radiation forces on a spherical microsphere were calculated in a time-varying refractive index profile. The refractive index profile of the surrounding medium was evaluated according to the concentration distribution obtained from the diffusion equation. The scattering and gradient forces on a microsphere were calculated for different refractive indices (1.22, 1.33, 1.43, and 1.59), and the radiation forces on a perfectly reflecting microsphere were calculated. The results were compared with previous results to validate the derivation. (C) 2012 Optical Society of America