Label-Free Super-Resolution Microscopy by Nonlinear Photo-modulated Reflectivity

Abstract

Nonlinear photo-modulated reflectivity (NPMR) for far-field, label-free, super-resolution (SR) microscopy is based on the nonlinear changes in the reflectance of materials, induced by an ultra-short pump pulse. In NPMR, a modulated train of pump pulse is focused on the sample that photo-excites temperature and/or charge-carriers changes, spatially distributed inside the diffraction-limited spot. A spatially overlapping, delayed, and unmodulated train of probe pulse monitors the resulting nonlinear reflectance changes by the detection of the high harmonics in the probe reflectance. The resulting point spread function (PSF) of the combined pump and probe is narrower than the diffraction-limited PSF, and the improvement in resolution scales like √n, where n is the nonlinearity order. NPMR is suitable to characterize semiconductors and metals in vacuum, ambient and liquid, semi-transparent and opaque systems. In order to detect weak high-order nonlinearities at the harmonics of the modulation frequencies, pure sine modulation is required. We have succeeded in modulating our pump source with harmonic impurity down to 10−4. Examples of resolution enhancement include nanostructured silicon, plasmonic gold surfaces, and vanadium dioxide (VO2) upon photo-induction of its characteristic insulator-to-metal transition. To further reduce the PSF and improve the resolution, it is possible to couple NPMR to other resolution-enhancement approaches, such as spatial modulation and apodization of the pump beam. Additionally, under specific conditions, the method can be simplified to use a single-laser pulse. Finally, we show that the nonlinear response to photo-excitation can be coupled to other SR modalities, such as Raman scattering and nonlinear photo-acoustic signals.