Toward Label-Free Super-Resolution Microscopy

We propose and implement a far-field spectroscopic system for imaging below the diffraction limit without the need for fluorescence labeling. Our technique combines concepts from Stimulated Emission Depletion (STED) microscopy and Femtosecond Stimulated Raman Spectroscopy (FSRS). The FSRS process generates signal through the creation of vibrational coherences, and here we use a toroidal-shaped decoherence pulse to eliminate vibrational signal from the edges of the focal spot. The nonlinear dependence on decoherence pulse power enables subdiffraction imaging. As in STED, the resolution is in theory infinitely small given infinite decoherence pulse power. Here, we first experimentally demonstrate that the photophysical principles behind our super-resolution Raman imaging method are sound. We then prove that addition of the decoherence pulse significantly improves the spatial resolution of our microscope, achieving values beyond the diffraction limit. We discuss future directions for this technique, including methods to reach resolution on the order of ten nanometers.