Resolving self-cavity effects in two-dimensional quantum materials

TL;DR

This paper develops an analytical framework to account for self-cavity effects in 2D quantum materials, enabling precise extraction of THz conductivity and collective mode dynamics using near-field THz spectroscopy.

Contribution

The paper introduces a new analytical approach to resolve self-cavity effects in 2D materials, improving interpretation of near-field THz measurements.

Findings

Self-cavity effects significantly influence THz responses in 2D materials.

The analytical framework allows extraction of conductivity and mode dynamics outside the light cone.

Enhanced spatial resolution in probing quantum phases of 2D materials.

Abstract

Two-dimensional materials and van der Waals (vdW) heterostructures host many strongly correlated and topological quantum phases on the $\sim$ meV energy scale. Direct electrodynamical signatures of such states are thus expected to appear in the terahertz (THz) frequency range (1 THz $\sim$ 4 meV). Because the typical size of vdW heterostructures ($\sim$10 $\mu m$) is much smaller than the diffraction limit of THz light, probing THz optical conductivities necessitates the use of near-field optical probes. However, interpreting the response of such near-field probes is complicated by finite-size effects, the presence of electrostatic gates, and the influence of the probe itself on material dynamics -- all of which conspire to form polaritonic self-cavities, in which interactions between THz electromagnetic fields and material excitations form discretized standing waves. In this paper, we demonstrate the relevance of self-cavity effects in 2D materials and derive an analytical framework to resolve these effects using the emerging experimental technique of time-domain on-chip THz spectroscopy. We show that by pairing experiments with the analytical theory, it is possible to extract the THz conductivity and resolve collective mode dynamics far outside the light cone, with $\sim \mu m$ in-plane and $\sim nm$ out-of-plane resolution. This study lays the groundwork for studying quantum phases and cavity effects in vdW heterostructures and 2D quantum materials.