Pedro Soubelet

We present evidence for the strong participation of hot phonons in the photo-physics of interlayer excitons (IXs) in 2H − and 3R − stacked MoSe₂/WSe₂ heterobilayers. Photoluminescence (PL) excitation spectroscopy reveals that excess energy associated with relaxation of intra-layer excitons towards IXs profoundly shapes the overall IX-PL lineshape, while the energy of the spectrally narrow discrete emission lines conventionally associated with trapped moiré IXs remain unaffected. A strikingly uniform line-spacing of the discrete emission lines is observed, along with characteristic temperature and excitation level dependence. Results suggest an entirely new picture of the discrete IX emission in which non-thermal phonons play a crucial role in shaping the spectrum. Excitation power and time resolved data indicate that these features are most likely polaronic in nature. Our findings extend the understanding of the photophysics of IXs beyond current interpretations based primarily on moiré-trapped IXs.

We investigate the confinement of neutral excitons in a one-dimensional (1D) potential engineered by proximizing hexagonal boron nitride (hBN)-encapsulated monolayer MoSe₂ to ferroelectric domain walls (DWs) in periodically poled LiNbO₃. Our device exploits the nanometer scale in-plane electric field gradient at the DW to induce dipolar exciton confinement via the DC Stark effect. Spatially resolved photoluminescence spectroscopy reveals the emergence of narrow emission lines redshifted from the MoSe₂ neutral exciton by up to ∼100 meV, depending on the sample structure. The spatial distribution, excitation energy response, and polarization properties of the emission are consistent with the signatures of 1D-confined excitons. The large electric-field gradients accessible via proximal ferroelectric systems open up new avenues for the creation of robust quantum-confined excitons in atomically thin materials and their heterostructures.

A spectral anomaly exhibiting pronounced asymmetry is observed in the emission spectrum for nanocavities when they are pumped by the charged boron vacancy VB− and have a high Q-factor above a threshold of 10 000. In contrast, cavities without VB− centers always exhibit trivial Lorentzian emission. These observations are well explained by the feature of VB− that its light–matter interaction is induced by local phonon modes. Moreover, we find that in nanocavities the local phonon modes arise from the coupling between VB− lattice phonons and cavity mechanical modes, supported by the spatially resolved spectroscopy of VB− and resonant spectroscopy of the cavity photonic mode. Our results reveal a novel system involving the coupling in both the photonic and phononic degrees of freedom between the active material and the nanocavity. Such an emitter–optomechanical interaction system shows great potential in non-trivial quantum photonic devices and provides a platform to interface spin defects, photons, and phonons.

We investigate the confinement of neutral excitons in a one-dimensional (1D) potential, engineered by proximizing hBN-encapsulated monolayer MoSe$_2$ to ferroelectric domain walls (DW) in periodically poled LiNbO$_3$. Our device exploits the nanometer scale in-plane electric field gradient at the DW to induce the dipolar exciton confinement via the Stark effect. Spatially resolved photoluminescence (PL) spectroscopy reveals the emergence of narrow emission lines redshifted from the MoSe$_2$ neutral exciton by up to $\sim100\,$meV, depending on the sample structure. The spatial distribution, excitation energy response and polarization properties of the emission is consistent with signatures of 1D-confined excitons. The large electric field gradients accessible via proximal ferroelectric systems open up new avenues for the creation of robust quantum-confined excitons in atomically thin materials and their heterostructures.

We investigate the coupling between an ensemble of individual emitters and multiple photons in a high-$Q$ cavity at the mesoscopic excitation level. The master equation theory is used to calculate the emission spectrum of the cavity QED system. The increasing excitation level not only pumps the system to the high-energy multi-emitter-photon states, but also introduces the pump-induced dephasing that suppresses the coherent energy exchange (coupling) between photons and emitters. When the emitter lifetime exceeds a threshold, we observe the mesoscopic excitation level i.e., the system is pumped to high energy states whilst the coherent couplings between these states are not yet suppressed. The mesoscopic excitation enables the couplings between multi-emitter-photon states, and thereby, paves the way to building quantum photonic devices based on multiple photons and nonlinear effects.

Combining a precise sputter etching method with subsequent AlO_x growth within an atomic layer deposition chamber enables the fabrication of atomically flat lateral patterns of SiO₂ and AlO_x. The transfer of MoSe₂ monolayers onto these dielectrically modulated substrates results in the formation of lateral heterojunctions due to the interaction with alternating regions of SiO₂ and AlO_x, with the flat substrate topography leading to minimal strain across the junction. Kelvin probe force microscopy measurements show significant variations in the contact potential difference (CPD) across the interface, with AlO_x regions inducing a 230 mV increase in CPD. Photoluminescence spectroscopy reveals shifts in spectral weight of neutral and charged exciton species across the different dielectric regions. On the AlO_x side, the Fermi energy moves closer to the conduction band, leading to a higher trion‐to‐exciton ratio, indicating a bandgap shift consistent with CPD changes. In addition, transient reflection spectroscopy highlights the influence of the dielectric environment on carrier dynamics, with the SiO₂ side exhibiting rapid carrier decay typical of neutral exciton recombination. In contrast, the AlO_x side shows slower, mixed decay behavior consistent with conversion of trions back into excitons. These results demonstrate how dielectric substrate engineering can tune 2D materials, allowing scalable fabrication of advanced junctions for novel (opto)electronics applications.

We present time-resolved and CW optical spectroscopy studies of interlayer excitons (IXs) in 2$H-$ and 3$R-$stacked MoSe$_2$/WSe$_2$ heterobilayers and obtain evidence for the strong participation of hot phonons in the underlying photo-physics. Photoluminescence excitation spectroscopy reveals that excess energy associated with optical excitation of \textit{intra}-layer excitons and relaxation to IXs affects the overall IX-PL lineshape, while the spectrally narrow emission lines conventionally associated with moir\'e IXs are unaffected. A striking uniform line-spacing of the sharp emission lines is observed together with temperature and excitation level dependent spectra suggesting an entirely new picture that photo-generated phonons lead to phonon-replicas shaping the IX-emission. Excitation power and time resolved data indicate that these features are polaronic in nature. Our experimental findings modify our current understanding of the photophysics of IXs beyond current interpretations based on moir\'e-trapped IXs.

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 ± 4 micro–electron volts, fully consistent with the 82 micro–electron volts 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.

The increasing role of two-dimensional (2D) devices requires the development of new techniques for ultrafast control of physical properties in 2D van der Waals (vdW) nanolayers. A special feature of heterobilayers assembled from vdW monolayers is femtosecond separation of photoexcited electrons and holes between the neighboring layers, resulting in the formation of Coulomb force. Using laser pulses, we generate a 0.8 THz coherent breathing mode in MoSe₂/WSe₂ heterobilayers, which modulates the thickness of the heterobilayer and should modulate the photogenerated electric field in the vdW gap. While the phonon frequency and decay time are independent of the stacking angle between the MoSe₂ and WSe₂ monolayers, the amplitude decreases at intermediate angles, which is explained by a decrease in the photogenerated electric field between the layers. The modulation of the vdW gap by coherent phonons enables a new technology for the generation of THz radiation in 2D nanodevices with vdW heterobilayers.