Optical trapping forces by a focused beam through two media with different refractive indices

Dependence of optical trapping force on the trap depth is examined by calculating the intensity distribution of light focused through two media with different refractive indices. The optical trapping forces on the microsphere in the ray optics regime were measured as a function of the trap depth. As the trap depth decreased, the optical trapping force increased. These results are useful for designing a chamber to trap particles with a low-power laser and for explaining the inconsistency among the reported experimental results.