Temperature-Dependent Optical and Polaritonic Properties of Excitons in hBN-Encapsulated Monolayer TMDs

Abstract

Monolayer transition metal dichalcogenides (TMDs) support robust excitons in the visible to near-infrared spectral range. Their reduced dielectric screening results in large binding energies, and combined with a direct bandgap in monolayer form, these excitons dominate the optical response of TMDs. In this work, a comprehensive investigation of the temperature-dependent optical and polaritonic properties of high-quality WS2, MoS2, WSe2, and MoSe2 monolayers, encapsulated by hBN, in the range of 5-300 K, is presented. Using reflection spectroscopy measurements, the optical and polaritonic constituents of the TMD excitons in terms of oscillator strength, linewidth, and negative permittivity, are evaluated and compared. It is found that MoSe2 exhibits the most pronounced optical and polaritonic response, stemming from its rapid linewidth narrowing at low temperatures, as compared to the other TMDs. In addition, It is found that all four TMDs exhibit a temperature-dependent negative real part permittivity, thus supporting surface-exciton-polaritons. The dispersion relation, confinement factor, and losses are derived and similarly reveal that MoSe2 exhibits enhanced polaritonic properties. These findings establish a comparative framework for understanding the optical and polaritonic properties of monolayer TMDs, with implications on their utilization in optoelectronic devices based on 2D semiconductors.