Far-field Imaging of Non-fluorescent Species with Sub-diffraction Resolution.

Super-resolution optical microscopy is opening a new window to unveil the unseen details on the nanoscopic scale. Current far-field super-resolution techniques rely on fluorescence as the read-out. 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 shape laser beam is then added to transiently saturate the electronic transition in the periphery of the focal volume, thus the induced modulation in the sequential probe pulse only occurs at the focal center. By raster scanning the three collinearly aligned beams, high-speed sub-diffraction-limited imaging of graphite nano-platelets was performed. This technique potentially enables super-resolution imaging of nano-materials and non-fluorescent chromophores, which may remain out of reach for fluorescence-based methods.