High-Energy Interlayer Exciton Ensembles in MoSe$_2$/WSe$_2$ Heterostructures by Laguerre-Gaussian Excitation

TL;DR

The paper addresses how to access and control high-energy, non-thermalized interlayer exciton ensembles in MoSe$_2$/WSe$_2$ heterobilayers. By implementing Laguerre-Gaussian excitation with an SLM, the authors generate ring-shaped IX ensembles whose diameter is tunable on the micrometer scale. Hyperspectral PL reveals that non-thermalized IXs propagate from the ring perimeter toward the center, increasing center emission and broadening the high-energy tail without significantly altering lifetime, implying incomplete thermalization and density-driven transport. This ring-based approach enables spatial separation of non-thermalized IXs from thermalized populations, offering a platform to study non-equilibrium IX dynamics, diffusion, and potential many-body effects in 2D heterostructures with controlled geometries.

Abstract

We reveal the higher energetic luminescence part of interlayer exciton ensembles in MoSe$_2$/WSe$_2$ heterostructures upon excitation by an optical Laguerre-Gaussian mode. The excitation is achieved with the help of a spatial light modulator giving rise to a ring-shaped distribution of interlayer excitons. A hyperspectral analysis of the exciton photoluminescence suggests that the excitation scheme allows the accumulation of high-energetic excitons in the rings' center. We discuss the mechanisms leading to such a distribution, including exciton-exciton interaction, phase-space filling, and an incomplete thermalization.

Figures (5)

• Figure 1: Laguerre-Gaussian (LG) excitation scheme of interlayer excitons (IXs) in a MoSe2/WSe2 heterostructure. (a) A spatial light modulator (SLM) reflects the phase-modulated laser light back into the optical circuitry. With the help of a beam splitter and an objective, the light is then focused onto the sample. (b) IX PL at an excitation power of 50nW ($E_{\mathrm{Laser}} = 1.94eV$, $T_{\mathrm{Bath}} = 1.7K$). (c) Spatial phase pattern of the SLM. Black overlay sketches the circular aperture of the optical circuitry. (d) Computed intensity pattern in the far-field corresponding to the phase pattern of (b). (e)-(g): Excitation laser profiles, as reflected from the samples' substrate and measured with a camera in the detection path for topological orders (e): $l = 0$, (f): $4$, and (g): $8$. Scale bars, 10µm.
• Figure 2: Ring-shaped pattern of IX PL from sample 1. (a) and (b): Camera images of the circular PL pattern for two different excitation powers ($E_{\mathrm{Laser}} = 1.94eV$, $T_{\mathrm{Bath}} = 1.7K$). (c) PL profiles along the dashed line in (a) with the excitation power ranging from 50nW to 63µW. Bottom panel depicts a corresponding line cut through the reflected laser illumination profile. Triangles mark the center of the excitation ($\blacktriangledown$) and the PL ($\triangledown$). Circle ($\circ$) highlights one of the PL maxima on the ring (gray shaded). Square ($\square$) marks a position outside of the ring that is equally spaced away from the excitation maximum as the center.
• Figure 3: Hyperspectral characterization of the IX PL on sample 1. (a) PL image in the sample plane recorded by a camera (similar to Fig. 2). (b) Spatio-spectral scan along the dashed line in (a) with the help of a glass fiber in the detection path (cf. Fig. 1(a) and Supporting Information). (c) Corresponding IX PL spectra for varying excitation powers collected in the center of the PL pattern as indicated by the triangle ($\triangledown$) in (b). A small arrow indicates the onset of an additional high-energy peak arising at high excitation powers. (d)-(f): Intensity $I_{\mathrm{norm.}}$ normalized to the intensity on the ring ($\circ$), energy shift $\Delta E$, and linewidth $\gamma$ of the IX PL extracted from Lorentzian fits of the acquired spectra for varying position and excitation power [cf. positions $\triangledown$, $\circ$, and $\square$ in Fig. \ref{['fig:2']} ].
• Figure 4: Comparison of an IX PL spectrum at the ring center ($\triangledown$, black) to a spectrum at the ring maximum ($\circ$, red) for sample 1. (a) Logarithmic presentation of the normalized intensities for an excitation power of 50nW. (b) and (c): Similar presentations for 1.11µW and 22.95µW. (d) Skewness shape parameter $\alpha$ for the hyperspectral data of \ref{['fig:3']} . See text for details.
• Figure 5: Comparison of intensity, emission energy, and linewidth for the center position to values at the ring maximum for sample 1 (black) and 2 (orange). (a) Ratio of IX PL intensity $I_{\mathrm{center}}/I_{\mathrm{ring}}$ vs. excitation power. (b) Ratio of PL emission energy $E_{\mathrm{center}}/E_{\mathrm{ring}}$ vs. excitation power. (c) Ratio of PL linewidth $\gamma_{\mathrm{center}}/\gamma_{\mathrm{ring}}$ vs. excitation power.