  In vectorial diffraction theory, tight focusing of a linearly polarized laser
beam produces an anisotropic field distribution around the focal plane. We
present a numerical investigation of the electromagnetic field distribution of
a focused beam in terms of the input beam polarization state and the associated
effects on the trap stiffness asymmetry of optical tweezers. We also explore
the symmetry change of a polarization-dependent optical force due to the
electromagnetic field redistribution by the presence of dielectric spheres of
selected diameters ranging from the Rayleigh scattering regime to the Mie
scattering regime.
