Sharper fluorescent super-resolution spot generated by azimuthally polarized beam in STED microscopy

A novel method is proposed for generating sharper fluorescent super-resolution spot by azimuthally polarized beam in stimulated emission depletion (STED) microscopy. The incoherent superposition of azimuthally polarized beam with five-zone binary phase plate and the same beam with quadrant 0/πphase plate can yield a tightly focused doughnut spot surrounded completely and uniformly. And azimuthally polarized beam modulated by a vortex 0-2π phase plate works as pump beam. Compared with known effective excitation spot yielded by circular polarized STED beam, the azimuthally polarized beam result is shaper, as well as energy-saving, costing only ~50% of the energy cost by circular polarized beam. A STED beam of less intensity has the potential to reduce fluorescence photobleaching and photodamage in living cell imaging. In addition, the influence of Ez absence as well as FWHM of pump beam in the focal field is discussed.