Supercritical angle fluorescence for enhanced axial sectioning in STED microscopy

Abstract

We demonstrate subwavelength axial sectioning on biological samples with a stimulated emission depletion (STED) microscope combined with supercritical angle fluorescence (SAF) detection. SAF imaging is a powerful technique for imaging the membrane of the cell based on the direct exploitation of the fluorophore emission properties. Indeed, only when fluorophores are close to the interface can their evanescent near-field emission become propagative and be detected beyond the critical angle. Therefore, filtering out the SAF emission from the undercritical angle fluorescence (UAF) emission in the back focal plane of a high-NA objective lens permits nanometer axial sectioning of fluorescent emitters close to the coverslip. When combined with STED microscopy, a straightforward gain in axial resolution can be reached without any alteration of the STED beam path. Indeed, STED-SAF implementation only requires a modification in the detection path of the STED microscope and thus could be widely implemented.