Three-dimensional isotropic STED microscopy generated by 4π focusing of a radially polarized vortex Laguerre-Gaussian beam

In this study, a method is proposed to generate a three-dimensional (3D) isotropic super-resolution spot using stimulated emission depletion (STED) microscopy. A focused radially polarized Laguerre-Gaussian (LG(01)) beam is used as the pump beam. A focused depletion spot is produced by a radially polarized vortex Laguerre-Gaussian beam in a 4 pi focusing system. The influences of the topological charge, the order of LG, the maximum aperture angle, and the ratio of the pupil radius to the beam waist (beta) on the spherical symmetry of the fluorescent spot are analyzed in detail. With the optimization parameters, an isotropic fluorescent spot is realized. The full widths at half maximum (FWHM) value of the isotropic fluorescent spot are 0.0348 lambda in both the r and z directions in a saturated case. The proposed model can provide a useful and simple guideline to obtain the spherical spot in 3D microscopy, which is very useful for trapping specific pathological ultra-structures of spherical shaped molecules and related medical applications.