Analytical description of sub-diffraction dark spot

Abstract

Optical vortices have developed rapidly in recent times owing to their theoretical significance and attractive potential applications in extensive realms. Composite optical vortices using two or more coherently superimposed vortex beams have been widely researched. Here, we propose a novel beam pattern incoherently generated by laser beams with two distinct wavelengths. We apply this beam pattern in fluorescence super-resolution imaging based on the stimulated emission effect, and find that the size of the resulting dark spot in the focal plane lies below the diffraction limit. Crucial factors affecting the sub-diffraction dark spot, such as beam intensities, spatial displacement, pulse width, polarization, and aberration, are studied in detail. We believe that the results are beneficial for experimental studies and will serve as useful references in other circumstances where they may be utilized.