Gradient force pattern of Gaussian beam adjusted by an annular phase plate

The gradient force pattern adjusted by an annular phase distribution plate is numerically investigated by vector diffraction theory. The phase plate may alter the wavefront phase of incident Gaussian beam with tunable topological charge. Numerical simulations show that the proposed plate may adjust the tunable gradient force on the particles in the focal region. By adjusting the geometrical parameters or changing the topological charge of the phase-shifting plate, some novel polygon trap patterns may occur. For example, a triangle, quadrangle, pentagon and hexagon traps can be obained when the topological charge equals to 3, 4, 5 and 6, respectivily. Therefore, the phase plate may be very advantageous for constructing tunable optical traps. The method is versatile in that it allows precise