Superresolution fluorescence microscopy using saturated modulation quenching (SMoQ)

In this work, we demonstrate a new technique which has the potential for super-resolution fluorescence imaging. In this technique, similar to STED microscopy, a tightly focused intensity-modulated excitation beam and a donut shaped cw beam are confocally raster-scanned over the sample. In contrast to STED microscopy, both beams need to be absorbed by the fluorophore. A lock-in amplifier is used to measure only the modulated fluorescence. Sufficiently high cw donut beam intensities lead to saturation of the fluorophores in the outer rim of the modulated point spread function which enables resolution enhancement. Theoretically, sub-diffraction resolution can be achieved.