[Paper Review] Lasing of Moiré Trapped MoSe$_2$/WSe$_2$ Interlayer Excitons Coupled to a Nanocavity

Chenjiang Qian, Mirco Troue+9 more|arXiv (Cornell University)|Feb 14, 2023

2D Materials and Applications62 references4 citations

TL;DR

This study demonstrates lasing from moiré-trapped interlayer excitons (IXs) in a hBN-encapsulated MoSe₂/WSe₂ heterobilayer integrated into a high-Q nanocavity. By tuning the IX-cavity detuning with a magnetic field, the authors observe threshold-like behavior, superlinear power dependence, and linewidth narrowing—key signatures of lasing—confirming macroscopic coherence over the cavity mode lengthscale with a measured exciton-photon coupling strength of 78 ± 4 μeV, consistent with theoretical predictions.

ABSTRACT

We report lasing of moiré trapped interlayer excitons (IXs) by integrating a pristine hBN-encapsulated MoSe$_2$/WSe$_2$ heterobilayer into a high-$Q$ ($>10^4$) nanophotonic cavity. We control the cavity-IX detuning using a magnetic field and measure their dipolar coupling strength to be $78 \pm 4\ \mathrm{μeV}$, fully consistent with the 82 $\mathrm{μeV}$ predicted by theory. The emission from the cavity mode shows clear threshold-like behavior as the transition is tuned into resonance with the cavity. We observe a superlinear power dependence accompanied by a narrowing of the linewidth as the distinct features of lasing. The onset and prominence of these threshold-like behaviors are pronounced at resonance while weak off-resonance. Our results show that a lasing transition can be induced in interacting moiré IXs with macroscopic coherence extending over the length scale of the cavity mode. Such systems raise interesting perspectives for low-power switching and synaptic nanophotonic devices using two-dimensional materials.

Motivation & Objective

• To demonstrate lasing in moiré-trapped interlayer excitons (IXs) in a 2D transition metal dichalcogenide heterostructure.
• To achieve strong coupling between IXs and a high-Q nanocavity by integrating a pristine hBN-encapsulated MoSe₂/WSe₂ heterobilayer.
• To control the IX-cavity detuning using a magnetic field in Faraday geometry and measure the coupling strength.
• To observe lasing signatures such as superlinear power dependence and linewidth narrowing in the cavity emission.
• To establish a platform for coherent light generation and quantum photonics using 2D materials with tailored excitonic potentials.

Proposed method

• Integration of a pristine hBN-encapsulated MoSe₂/WSe₂ heterobilayer with a twist angle of 2° into a high-Q (Q > 10⁴) nanobeam photonic cavity.
• Use of a magnetic field in Faraday geometry to tune the energy of moiré-trapped interlayer excitons and control the detuning relative to the cavity mode.
• Measurement of photoluminescence (PL) spectra under varying excitation power and magnetic field to extract coupling strength and lasing thresholds.
• Analysis of power-dependent intensity and linewidth of cavity modes to identify lasing signatures such as superlinear growth and spectral narrowing.
• Comparison of experimental data with theoretical rate equation models to validate lasing behavior and coupling strength.
• Use of a 532-nm green continuous-wave laser for excitation to enhance nonlinearity and probe excitation-dependent dynamics.

Experimental results

Research questions

• RQ1 Can lasing be achieved in moiré-trapped interlayer excitons when coupled to a high-Q nanocavity?
• RQ2 What is the exciton-photon coupling strength between a single moiré-trapped IX and the cavity mode?
• RQ3 How does the excitation condition (e.g., laser wavelength) affect the lasing threshold and linewidth behavior?
• RQ4 To what extent does the magnetic field tuning enable control over the lasing transition and coherence length?
• RQ5 Can the observed lasing behavior be explained by conventional rate equation models, or does it require additional noise or dephasing effects?

Key findings

• Lasing is observed in moiré-trapped interlayer excitons via a clear threshold-like behavior in the cavity emission, marked by superlinear power dependence and linewidth narrowing.
• The measured exciton-photon coupling strength is 78 ± 4 μeV, in excellent agreement with the theoretical prediction of 82 μeV.
• The lasing threshold and associated features are most prominent when the IX is resonant with the cavity mode, while they are significantly weaker when detuned.
• Power-dependent measurements under green laser excitation show a superlinear exponent of 3.2 ± 0.1 for the dominant cavity mode (M1), indicating strong nonlinearity and enhanced lasing response.
• The re-broadening of the cavity linewidth at high excitation powers is observed and attributed to intensity-phase noise coupling, with stronger effects under green excitation due to increased dephasing.
• Mode M2, which is strongly detuned from delocalized IXs, shows suppressed re-broadening, consistent with reduced phase noise and fewer dephasing pathways.