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Register a new userControl of Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers
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The direct gap interband transitions in transition metal dichalcogenides monolayers are governed by chiral optical selection rules. Determined by laser helicity, optical transitions in either the $K^+$ or $K^-$ valley in momentum space are induced. Linearly polarized laser excitation prepares a coherent superposition of valley states. Here we demonstrate the control of the exciton valley coherence in monolayer WSe2 by tuning the applied magnetic field perpendicular to the monolayer plane. We show rotation of this coherent superposition of valley states by angles as large as 30 degrees in applied fields up to 9 T. This exciton valley coherence control on ps time scale could be an important step towards complete control of qubits based on the valley degree of freedom.
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We study theoretically the Coulomb interaction between excitons in transition metal dichalcogenide (TMD) monolayers. We calculate direct and exchange interaction for both ground and excited states of excitons. The screening of the Coulomb interaction, specific to monolayer structures, leads to the unique behavior of the exciton-exciton scattering for excited states, characterized by the non-monotonic dependence of the interaction as function of the transferred momentum. We find that the nontrivial screening enables the description of TMD exciton interaction strength by approximate formula which includes exciton binding parameters. The influence of screening and dielectric environment on the exciton-exciton interaction was studied, showing qualitatively different behavior for ground state and excited states of excitons. Furthermore, we consider exciton-electron interaction, which for the excited states is governed by the dominant attractive contribution of the exchange component, which increases with the excitation number. The results provide a quantitative description of the exciton-exciton and exciton-electron scattering in transition metal dichalcogenides, and are of interest for the design of perspective nonlinear optical devices based on TMD monolayers.
The rise of quantum science and technologies motivates photonics research to seek new platforms with strong light-matter interactions to facilitate quantum behaviors at moderate light intensities. One promising platform to reach such strong light-matter interacting regimes is offered by polaritonic metasurfaces, which represent ultrathin artificial media structured on nano-scale and designed to support polaritons - half-light half-matter quasiparticles. Topological polaritons, or topolaritons, offer an ideal platform in this context, with unique properties stemming from topological phases of light strongly coupled with matter. Here we explore polaritonic metasurfaces based on 2D transition metal dichalcogenides (TMDs) supporting in-plane polarized exciton resonances as a promising platform for topological polaritonics. We enable a spin-Hall topolaritonic phase by strongly coupling valley polarized in-plane excitons in a TMD monolayer with a suitably engineered all-dielectric topological photonic metasurface. We first show that the strong coupling between topological photonic bands supported by the metasurface and excitonic bands in MoSe2 yields an effective phase winding and transition to a topolaritonic spin-Hall state. We then experimentally realize this phenomenon and confirm the presence of one-way spin-polarized edge topolaritons. Combined with the valley polarization in a MoSe2 monolayer, the proposed system enables a new approach to engage the photonic angular momentum and valley degree of freedom in TMDs, offering a promising platform for photonic/solid-state interfaces for valleytronics and spintronics.
We have investigated the exciton dynamics in transition metal dichalcogenide mono-layers using time-resolved photoluminescence experiments performed with optimized time-resolution. For MoSe2 monolayers, we measure $tau_{rad}=1.8pm0.2$ ps that we interpret as the intrinsic radiative recombination time. Similar values are found for WSe2 mono-layers. Our detailed analysis suggests the following scenario: at low temperature (T $leq$ 50 K), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the PL intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton- phonon interactions. Following this first non-thermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and non-radiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.
We examine the static non-linear optical response of monolayer transition metal dichalcogenides. Whereas the shift current is suppressed, we identify a strong, valley-dependent non-reciprocal response, which we term a textit{unidirectional valley-contrasting photo-current} (UVCP). It originates from Kramers symmetry breaking by trigonal warping, and its direction is set by the wave vector connecting the two valleys. The UVCP is proportional to the mobility and is enhanced by the excitonic Coulomb interaction and inter-valley scattering, enabling monitoring of inter-valley transitions. We discuss detection strategies in state-of-the-art experiments.
We show that H-phase transition metal dichalcogenides (TMDs) monolayers such as MoS$_2$ and WSe$_2$, are orbital Hall insulators. They present very large orbital Hall conductivity plateaus in their semiconducting gap, where the spin Hall conductivity vanishes. Our results open the possibility of using TMDs for orbital current injection and orbital torque transfers that surpass their spin-counterparts in spin-orbitronics devices. The orbital Hall effect (OHE) in TMD monolayers occurs even in the absence of spin-orbit coupling. It can be linked to exotic momentum-space Dresselhaus-like orbital textures, analogous to the spin-momentum locking in 2D Dirac fermions that arise from a combination of orbital attributes and lattice symmetry.
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