3D optical nanoscopy with excited state saturation at liquid helium temperatures.

We present a 3D fluorescence nanoscopy method operating at cryogenic temperatures, based on optical saturation of the excited state of individual molecules. Using a focused laser beam structured with a zero-intensity central region surrounded by intensity gradients in the three space directions, we achieve a sub-30 nm 3D optical resolution. Moreover. the analysis of the fluorescence scanning images of single molecules reveals the 3D orientation of their transition dipole with an accuracy of a few degrees. This method provides a valuable tool for locating neighboring molecules with overlapping optical transitions in order to study their interactions. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement