In-plane anisotropy in biaxial ReS2 crystals probed by nano-optical imaging of waveguide modes

TL;DR

This study uses near-field imaging to analyze in-plane anisotropic waveguide modes in thin ReS2 crystals, revealing the full dielectric tensor and excitonic effects across a broad near-infrared spectrum.

Contribution

It demonstrates a method to extract the complete dielectric tensor of biaxial ReS2 using nano-optical imaging of waveguide modes over a wide frequency range.

Findings

Resolved in-plane anisotropic waveguide modes in ReS2

Determined the full dielectric tensor including out-of-plane response

Identified excitonic absorption effects on mode dispersion

Abstract

Near-field imaging has emerged as a reliable probe of the dielectric function of van der Waals crystals. In principle, analyzing the propagation patterns of subwavelength waveguide modes (WMs) allows for extraction of the full dielectric tensor. Yet previous studies have mostly been restricted to high-symmetry materials or narrowband probing. Here, we resolve in-plane anisotropic WMs in thin rhenium disulfide (ReS2) crystals across a wide range of near-infrared frequencies. By tracing the evolution of these modes as a function of crystallographic direction, polarization of the electric field and sample thickness, we have determined the anisotropic dielectric tensor including the elusive out-of-plane response. The excitonic absorption at ~1.5 eV manifests itself as a clear backbending feature in the WM dispersion and a reduction of the quality factors as fully supported by numerical calculations. Our results extend the sensitivity of near-field microscopy towards biaxial anisotropy and provide key insights into the optoelectronic properties of ReS2.