Generation of a dark spot beyond the diffraction limit with a radially polarized vortex beam

Generation of a dark spot beyond the diffraction limit with a radially polarized vortex beam

Jianghua Xu, Tao Geng, Xiumin Gao, and Songlin Zhuang

We propose and demonstrate that a dark spot can be generated by a radially polarized beam with a first-order vortex after phase modulation by an optimal binary phase element. The distance between two mainlobe intensity peaks along the radial direction is 0.62?? on the focal plane. The proposed system is easy to implement in practice and has low energy loss, since it uses phase-only modulation. Then, the fluorescent spot is calculated for stimulated emission depletion microscopy when a radially polarized beam works as the pump beam. The FWHM of the fluorescent spot can achieve 0.008?? for a saturated case. This is much smaller than that achieved using a circular polarized vortex erase beam (0.0422??) or an azimuthally polarized erase beam with optimizing amplitude-phase modulation (0.0279??).