Time-Resolved SNOM via Phase-Domain Sampling.

Time-resolved scanning near-field optical microscopy (tr-SNOM) enables the measurement of the dynamic optical response of functional surfaces beyond the diffraction limit. Experimental challenges are imposed both by the use of a pulsed light source and by the need for interferometric signal modulation to isolate the near-field contribution. We present a novel experimental approach to retrieve the tr-SNOM signal using a 200-kHz laser system and pseudo-heterodyne modulation. We circumvent the Nyquist limit for spectral demodulation by sampling modulation phases, pump intensity, and SNOM signal for every laser shot. A general time-resolved SNOM signal is derived, independent of the detection scheme or physical assumptions about the near-field enhancement, and is successfully measured and isolated on WS2 monolayer and multilayer regions. We confirm localization by signal-distance curves, spatial confinement at material boundaries, and by identifying signal contributions at individual modulation harmonics. Disentangling the dynamic contributions enables us to extract the dynamic dielectric function of the sample. Showing the capability of phase-domain sampling paves the way to the integration of more diverse and specialized light sources, growing the potential of optical ultrafast near-field measurements.