Temperature-dependent Optical and Polaritonic Properties of hBN-encapsulated Monolayer TMDs

TL;DR

The paper investigates how temperature controls the optical and polaritonic behavior of high-quality hBN-encapsulated WS2, MoS2, WSe2, and MoSe2 monolayers in the 5–300 K range. By analyzing reflection spectra and photoluminescence with a Lorentzian oscillator model, it extracts the complex permittivity and shows that all four materials can exhibit negative real permittivity, enabling surface-exciton-polaritons; nonlocal corrections are applied to the polariton dispersion. MoSe2 stands out with the strongest optical and polaritonic response due to rapid linewidth narrowing, followed by WS2, WSe2, and MoS2, and MoSe2 achieves the largest polaritonic figure of merit and longest propagation length. The work provides a comparative framework for exciton-polariton phenomena in monolayer TMDs and informs the design of nanoscale optoelectronic devices leveraging strong light–matter interactions in 2D semiconductors.

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, we present a comprehensive investigation of the temperature-dependent optical and polaritonic properties of high-quality, flux-grown $\mathrm{WS_2}$, $\mathrm{MoS_2}$, $\mathrm{WSe_2}$, and $\mathrm{MoSe_2}$ monolayers, encapsulated by hBN, in the full temperature range between $5-300 \mathrm{K}$. Using reflection spectroscopy measurements, we evaluate and compare the optical and polaritonic constituents of the TMD excitons in terms of oscillator strength, linewidth and negative permittivity. We find that it is $\mathrm{MoSe_2}$ that exhibits the most pronounced optical and polaritonic response, stemming from its rapid linewidth narrowing at low temperatures, as compared to the temperature-dependent response of the other TMDs. In addition, we find that all four TMDs exhibits a temperature-dependent negative real part permittivity, thus supporting surface-exciton-polaritons. We derive their dispersion relation, confinement factors and losses, similarly revealing that $\mathrm{MoSe_2}$ exhibit 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.

Figures (3)

• Figure 1: Temperature-dependent optical properties of hBN-encapsulated monolayer TMDs ($\mathrm{WS_2}$, $\mathrm{MoS_2}$, $\mathrm{WSe_2}$, $\mathrm{MoSe_2}$) from $5\mathrm{K}$ to $300\mathrm{K}$. (a-d) Reflection contrast spectra showing the evolution of excitonic resonances with temperature. (e-h) Exciton linewidth evolution with temperature. (i-l) Exciton energy positions as a function of temperature.
• Figure 2: Complex permittivity of hBN-encapsulated monolayer TMDs at various temperatures. (a-d) Real part of permittivity ($\epsilon_1$), black dashed horizontal lines mark zero crossing. Inset shows a zoom-in on the energy range containing $300\mathrm{K} - 150\mathrm{K}$. (e-h) Imaginary part of the permittivity ($\epsilon_2$). (i-l) The ratio $-\frac{\epsilon_1}{\epsilon_2}$, indicating spectral regions with optimal conditions for polaritonic response.
• Figure 3: Polaritonic properties of surface-exciton-polaritons supported by the four TMDs. (a) Confinement factor, $\lambda_0/\lambda_{Polariton}$, under the local and non-local models in (dashed and full lines, respectively). Vertical dashed-dot line indicates confinement factor of unity, corresponding to the light-line. (b) The loss figure of merit, $Re(q_{Polariton})/Im(q_{Polariton})$