Label-Free Optical Nanoscopy of Single-Layer Graphene.

The application of ultrafast pulsed laser sources and spectroscopic techniques enables label-free, deep-tissue optical microscopy. However, circumvention of the diffraction limit in this field is still an open challenge. Among such approaches, pump-probe microscopy is of increasing interest thanks to its highly specific nonfluorescent-based contrast mechanisms for the imaging of material and life science samples. In this paper, a custom femtosecond-pulsed near-infrared pump-probe microscope, which exploits transient absorption and stimulated Raman scattering interactions, is presented. The conventional pump-probe configuration is combined with a spatially shaped saturation pump beam, which allows for the reduction of the effective focal volume exploiting transient absorption saturation. By optimizing the acquisition parameters, such as power and temporal overlap of the saturation beam, we can image single-layer graphene deposited on a glass surface at the nanoscale and with increased layer sensitivity. These results suggest that saturation pump-probe nanoscopy is a promising tool for label-free high-resolution imaging.