peaks for bigger radius is very small. With decreasing decentered parameters in cosh
part of the beam, position of the three focal depth peaks shift toward to small radius,
and the peak values also fall simultaneously. With high numerical aperture, the vector
diffraction theory is used to study the effect of phase plate on intensity distribution in
axial direction. In high numerical aperture system, the focal depth increases three times
by adjusting geometric parameters. Under certain geometric parameters, the focal depth
can also increases 6 times. However, when phase plate is used to alter focal depth, the
care must be put on focal split.
The evolution of the gradient force pattern of hyperbolic-cosine-Gaussian beams
induced by the phase plate is numerically investigated. The results show that the
proposed plate may induce tunable gradient force on the particles in focal region. The
optical trap shape is tunable obviously and multiple traps may occur with changing
geometrical parameters of the phase plate. Optical trap shift may be induced by
changing the phase variance of the inner annular zone portion. The phase plate may be a
very promising method of transporting trapped particles and can be used to construct
controllable optical tweezers.
Vector diffraction theory is employed to investigate the focusing properties of the
Gaussian beam induced by phase plate with topological charge.The phase plate may
alter the wavefront phase of an incoming beam by topological charge. Both the circular
phase distribution and the annular phase distribution plates are investigated. Numerical
simulations show that the focal intensity distribution depends on topological charge.
With changing topological charge, focal intensity distribution may evolve into ring
shape, and some novel focal spots may occur. Focal intensity distribution evolving
process with integer topological charge differs considerably from that with fraction
topological charge. When the concentric annular phase plate is put in laser path, the
focal intensity distribution depends on both the inner radius and topological charge. For
small inner radius of the phase plate, doughnut shape focal pattern occurs. With
increasing inner radius, the diameter of the doughnut focal pattern decreases, and
doughnut shape disappears slowly in some cases. The evolution of gradient force
pattern induced by an annular phase distribution plate is numerically investigated in this
paper. Numerical simulations show that the proposed plate may induce 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