Nonlocal Time-Resolved Terahertz Spectroscopy in the near Field

Scattering-type near-field optical microscopy (s-SNOM) has enabled subwavelength spectroscopy measurements on a wide variety of materials and over a large spectral range. These tip-based measurements are of particular interest in the long wavelength regimes, where the study of individual nanoscale samples is very challenging. The combination of s-SNOM techniques with short pulse durations has opened a new realm of possibilities in which nanosystems can be characterized with both high spatial and temporal resolution, for example via optical-pump, terahertz-probe measurements. Here, we demonstrate the first "nonlocal"pump-probe measurement using a scattering-type scanning near-field microscopy technique, in which the pump spot is laterally displaced from the probe location. We observe nonlocal effects corresponding to this pump-probe offset, associated with carrier drift into the s-SNOM near-field probe region.