Physical Origins of Extreme Cross-Polarization Extinction in Confocal Microscopy

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Confocal microscopy is an essential imaging tool for biological systems, in solid-state physics and nano-photonics. Using confocal microscopes allows performing resonant fluorescence experiments, where the emitted light has the same wavelength as the excitation laser. Theses challenging experiments are carried out under linear cross-polarization conditions, rejecting laser light from the detector. In this work we uncover the physical mechanisms that are at the origin of the yet unexplained high polarization rejection ratio which makes these measurements possible. We show in both experiment and theory that the use of a reflecting surface (i.e. the beam-splitter and mirrors) placed between the polarizer and analyzer in combination with a confocal arrangement explains the giant cross-polarization extinction ratio of 10^8 and beyond. We map the modal transformation of the polarized optical Gaussian beam. We find an intensity 'hole' in the reflected beam under cross-polarization conditions. We interpret this as a manifestation of the Imbert-Fedorov effect, which deviates the beam depending on its polarization helicity. This implies that this topological effect is amplified here from the usually observed nanometer to the micrometer scale due to our cross-polarization dark field methods. We confirm these experimental findings for a large variety of commercially available mirrors and polarization components, allowing their practical implementation in many experiments.