Effect of concentric zones phase plate on gradient force pattern of radially polarized beam

The optical gradient force patterns in focal region of radially polarized beams is investigated. The pure phase plate consists of three concentric zones: center circle zone, inner annular zone and outer annular zone. And the phase variance of annular zone is adjustable. The tunable phase plate can be implemented through liquid crystal spatial light modulator, which can conveniently alter the phase shift and polarization rotation angle of each section of the incident laser beam in the control of computer. Numerical simulations show that the evolution of the focal shape is evident by changing polarization rotation angle of each section portion of the vector beam. Consequently, the optical gradient force patterns will change and some interesting optical trap patterns may occur, which shows th