Time-resolved SNOM via phase-domain sampling

TL;DR

This paper introduces a novel phase-domain sampling method for time-resolved SNOM, enabling dynamic optical response measurements beyond the diffraction limit with improved spectral demodulation.

Contribution

The authors present a new experimental approach using phase-domain sampling and pseudo-heterodyne modulation to measure and isolate time-resolved SNOM signals without spectral limitations.

Findings

Successfully measured and isolated time-resolved SNOM signals on WS₂ monolayer and multilayer regions.

Confirmed signal localization and spatial confinement at material boundaries.

Enabled extraction of the dynamic dielectric function of the sample.

Abstract

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 detection scheme or physical assumptions about the near-field enhancement, and is successfully measured and isolated on WS$_2$ 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 integration of more diverse and specialized light sources, growing the potential of optical ultrafast near-field measurements.