Phase-resolved terahertz nano-imaging of WTe₂ microcrystals

The terahertz electrodynamics of few-layer WTe₂ is dominated by the plasmon response. However, terahertz surface plasmons with long wavelengths in two-dimensional exfoliated crystals are typically confined by the lateral geometry. Direct visualization of the plasmonic standing wave patterns is challenging due to the spatial confinement and low quality factor of the surface plasmon, especially for samples that are only a few monolayers thick. Here, we resolve subtle real-space features of the plasmonic response of WTe₂ by augmenting more common scattering amplitude experiments with the phase contrast accomplished within the time-domain version of THz nano-imaging. Amplitude and phase images allow us to quantitatively evaluate the evolution of the plasmonic response at cryogenic temperatures in samples with variable thickness from 3 to 12 monolayers. The proposed imaging modality is universally applicable to the THz near-field nanoscopy of low-dimensional materials.