4-Pi Stimulated Raman Scattering for Label-free Super-resolution Chemical Imaging.

Super-resolution fluorescence microscopy has transformed the study of biological structures and functions beyond the diffraction limit. Unlike fluorescence methods, label-free chemical imaging, mostly based on Raman and infrared spectroscopy, provides intrinsic molecular contrast, enabling the study of biomolecules, nanostructures, drug molecules, and metabolites that cannot be easily tagged. However, while a wide range of fluorescence-based super-resolution techniques are well-established, extending super-resolution to label-free chemical imaging has remained challenging due to low signal levels and limited resolution improvement. Super-resolution stimulated Raman scattering microscopy (SRS) is most promising due to its high sensitivity and imaging speed. Similar to fluorescence, existing SRS super-resolution approaches are mostly based on either photoswitching/saturation of molecular labels or sample expansion, which suffers from poor sensitivity due to limitations in labeling density or signal dilution, respectively. Moreover, axial resolution is typically much worse than lateral resolution, yet most super-resolution SRS techniques focused on improving lateral resolution. In this work, we combine stimulated Raman scattering (SRS) with 4Pi-interferometry to significantly improve the axial resolution by nearly 7-fold. We report on the characterization of improvements in imaging sensitivity and axial resolution using 80 nm polystyrene beads. Harnessing the improved axial resolution, we demonstrate super-resolution 4Pi-SRS imaging in resolving small lipid droplet structures in mammalian cells and lipid membranes in cells. Because 4Pi-SRS uses interferometry to improve axial resolution, it is completely orthogonal to all previous super-resolution SRS techniques, including visible excitation, photoswitching, sample expansion, and computational approaches, thus it is straightforward to combine them to achieve much higher resolution chemical imaging than currently possible.