Super-pupil beam engineering for enhanced confinement of azimuthally polarized depletion beams
Fluorescence depletion microscopy techniques require optical fields with a well-defined and spatially confined central intensity minimum to achieve sub-diffraction lateral resolution. Here, we present the design and experimental implementation of an azimuthally polarized, doughnut-shaped depletion beam based on super-pupil engineering principles. By tailoring the radial amplitude distribution at the entrance pupil to approximate a Bessel-type target function, the resulting focal field exhibits a tighter central doughnut compared to conventional circularly or azimuthally polarized beams. The designed pupil field distribution is implemented using a phase-only spatial light modulator operated in a double-pass configuration, enabling independent modulation of orthogonal polarization components via complex-field holographic encoding. Analytical modeling of the designed field predicts an estimated resolution gain of approximately 24% and a reduction in required depletion power of approximately 42% relative to a canonical circularly polarized Laguerre-Gaussian beam. Although the engineered field exhibits pronounced sidelobes, these do not preclude its use as a depletion beam, since lateral resolution is strongly influenced by the spatial confinement and effective suppression of the central intensity minimum for a given depletion intensity.