Far-field imaging of non-fluorescent species with subdiffraction resolution

Super-resolution optical microscopy is providing a new means by which to view as yet unseen details on a nanoscopic scale. Current far-field super-resolution techniques rely on fluorescence as the readout(1-5). Here, we demonstrate a scheme for breaking the diffraction limit in far-field imaging of non-fluorescent species by using spatially controlled saturation of electronic absorption. Our method is based on a pump-probe process where a modulated pump field perturbs the charge carrier density in a sample, thus modulating the transmission of a probe field. A doughnut-shaped laser beam is then added to transiently saturate the electronic transition in the periphery of the focal volume, so the induced modulation in the sequential probe pulse only occurs at the focal centre. By raster-scanning the three collinearly aligned beams, high-speed subdiffraction-limited imaging of graphite nanoplatelets is performed. This technique has the potential to enable super-resolution imaging of nanomaterials and non-fluorescent chromophores, which may remain out of reach to fluorescence-based methods