Theoretical Investigation Of The Improved STED Resolution Through Axially Symmetric Light Beam

Stimulated emission depletion microscope is a far field super resolution technology, which can provide direct, visible approach for researches such as biology, materials science, etc. STED microscope generally consists of two beams, one beam is the excitation beam and the other is the erase beam. Theoretically, there is no limitation on the final resolution if the intensity of the erase spot is strong enough. Nevertheless, photo bleaching confines the usage of high power laser of erase beam. Therefore, it is need to explore other methods to improve the resolution of STED.This paper mainly focuses on the focusing properties of the cylindrically polarized beam and explores ways to improve the resolution of STED, including improving the lateral resolution and accomplish the three-dimensional super-resolution.(1) Improve the lateral resolution of STED. Based on the tight focusing properties of azimuthally polarized beam, this paper firstly explores ways to improve the lateral resolution of STED by using the azimuthally polarized beam as the erase beam. We can improve the lateral resolution of STED by shrinking the erase spot. Therefore, a method is proposed for focusing azimuthally polarized beam into an ultra-small dark spot under high numerical aperture lens system. Manipulated by binary 0-π phase plate optimized by linear algorithm, an ultra small doughnut spot regardless of side-lobe intensity is generated with main-lobe peak to peak distance shrunk to 0.24λ, which is 46% compared with the case without phase plate. For potential applications, shrinking the dark spot while suppressing the side-lobe intensity is analyzed in this paper, and several cases of squeezed dark spot with size squeezed down to 84%-62% are presented. Finally, the simulation of STED is presented, and those compressed erase spots can be used to obtain ultra small effective spots, which are 60%-75% compared with the case with a compressed erase spot.(2) The three dimensional super resolution of STED. Based on the focusing properties of cylindrically polarized beam, we propose a multi-mode cylindrically polarized beam. By tight focusing of the multi zone beam, we can get a three dimensional(3D) hollow spot with uniform intensity distributions in the vicinity of the lens. The simulation of STED is presented using the 3D hollow spot as the erase spot, a quasi-spheriform super resolution spot can be generated with the full width at half maximum equals to 0.07λ and 0.09λ in the lateral direction and in the axial direction, respectively.At the end of the paper, I summarized the former work and also analyzed the potential directions of the future research.