Saturated absorption competition microscopy

Owing to the advantage of being non-invasive in observing living samples, far-field optical microscopy is widely used in the life sciences, but the existence of the diffraction barrier leads to the poor imaging of samples with spatial features smaller than approximately half the wavelength of the probes. This limit has been overcome by a number of pointwise scanning optical imaging techniques, such as stimulated emission depletion microscopy (STED) and saturated excitation microscopy (SAX). Here, we introduce the concept of saturated absorption competition (SAC) microscopy as a simple means of providing sub-diffraction spatial resolution in fluorescence imaging. Our approach can be physically implemented in a confocal microscope by dividing the input laser source into a time-modulated primary excitation beam and a doughnut-shaped saturation beam and subsequently employing a homodyne detection scheme to select the modulated fluorescence signal. Herein, we provide both a physico-chemical model of SAC and experimentally demonstrate a transverse spatial resolution of 1.5- to 2-fold that of confocal.