Robust coherent dynamics of homogeneously limited anisotropic excitons in two-dimensional layered ReS2

TL;DR

This study measures the coherence time of anisotropic excitons in ReS2, revealing their robustness and potential for quantum applications at room temperature, with implications for understanding their electronic structure.

Contribution

It provides the first direct measurement of excitonic coherence times in ReS2 and demonstrates their robustness across different layer thicknesses and conditions.

Findings

Coherence time T_2 is in the sub-picosecond range.

Excitonic coherence is robust against temperature and optical density.

ReS2 exhibits a direct band gap regardless of layer thickness.

Abstract

The discovery of in-plane anisotropic excitons in two-dimensional layered semiconductors enables state-of-the-art nanophotonic applications. A fundamental yet unknown parameter of these quasiparticles is the coherence time (T_2 ), which governs the quantum dephasing timescale, over which the coherent superposition of excitons can be maintained and manipulated. Here, we report the direct measurement of T_2 within the sub-picosecond range, along with multiple population decay timescales (T_1 ) at resonance for anisotropic excitons in pristine layered rhenium disulfide (ReS2). We observe a notable weak dependence on layer thickness for T_2 , and a quasi-independence for T_1 . The excitonic coherence in few-layer ReS2 exhibits exceptional robustness against optical density and temperature compared to other two-dimensional semiconductors, enabling quantum features even at room temperature. No photon echo fingerprints were observed in pristine ReS2, highlighting the homogeneous character of the anisotropic excitonic transitions and a particularly low level of disorder in exfoliated flakes. Lastly, our results for mono- to bulk-like ReS2 support a direct gap band structure regardless their layer thickness, addressing the ongoing discussion about its nature.