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Transition metal dichalcogenide monolayers are promising candidates for exploring new electronic and optical phenomena and for realizing atomically thin optoelectronic devices. They host tightly bound electron-hole pairs (excitons) that can be efficiently excited by resonant light fields. Here, we demonstrate that a single monolayer of molybdenum diselenide (MoSe2) can dramatically modify light transmission near the excitonic resonance, acting as an electrically switchable mirror that reflects up to 85% of incident light at cryogenic temperatures. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor, encapsulated by hexagonal boron nitride. Furthermore, we show that the MoSe2 monolayer exhibits power- and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used. These observations open up new possibilities for studying quantum nonlinear optical phenomena and topological photonics, and for miniaturizing optical devices.
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We show that a transition metal dichalcogenide monolayer with a radiatively broadened exciton resonance would exhibit perfect extinction of a transmitted field. This result holds for s- or p-polarized weak resonant light fields at any incidence angle, due to the conservation of in-plane momentum of excitons and photons in a flat defect-free two dimensional crystal. In contrast to extinction experiments with single quantum emitters, exciton-exciton interactions lead to an enhancement of reflection with increasing power for incident fields that are blue detuned with respect to the exciton resonance. We show that the interactions limit the maximum reflection that can be achieved by depleting the incoming coherent state into an outgoing two-mode squeezed state.
The realization of mixtures of excitons and charge carriers in van-der-Waals materials presents a new frontier for the study of the many-body physics of strongly interacting Bose-Fermi mixtures. In order to derive an effective low-energy model for such systems, we develop an exact diagonalization approach based on a discrete variable representation that predicts the scattering and bound state properties of three charges in two-dimensional transition metal dichalcogenides. From the solution of the quantum mechanical three-body problem we thus obtain the bound state energies of excitons and trions within an effective mass model which are in excellent agreement with Quantum Monte Carlo predictions. The diagonalization approach also gives access to excited states of the three-body system. This allows us to predict the scattering phase shifts of electrons and excitons that serve as input for a low-energy theory of interacting mixtures of excitons and charge carriers at finite density. To this end we derive an effective exciton-electron scattering potential that is directly applicable for Quantum Monte-Carlo or diagrammatic many-body techniques. As an example, we demonstrate the approach by studying the many-body physics of exciton Fermi polarons in transition-metal dichalcogenides, and we show that finite-range corrections have a substantial impact on the optical absorption spectrum. Our approach can be applied to a plethora of many-body phenomena realizable in atomically thin semiconductors ranging from exciton localization to induced superconductivity.
Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.
We perform absorption and photoluminescence spectroscopy of trions in hBN-encapsulated WSe$_2$, WS$_2$, MoSe$_2$, and MoS$_2$ monolayers, depending on temperature. The different trends for W- and Mo-based materials are excellently reproduced considering a Fermi-Dirac distribution of bright and dark trions. We find a dark trion, $rm{X_D^-}$ 19 meV $textit{below}$ the lowest bright trion, $rm{X}_1^-$ in WSe$_2$ and WS$_2$. In MoSe$_2$, $rm{X_D^-}$ lies 6 meV $textit{above}$ $rm{X}_1^-$, while $rm{X_D^-}$ and $rm{X}_1^-$ almost coincide in MoS$_2$. Our results agree with GW-BSE $textit{ab-initio}$ calculations and quantitatively explain the optical response of doped monolayers with temperature.
Electronic states in two-dimensional layered materials can exhibit a remarkable variety of correlated phases including Wigner-crystals, Mott insulators, charge density waves, and superconductivity. Recent experimental and theoretical research has indicated that ferromagnetic phases can exist in electronically-doped transition metal dichalcogenide (TMD) semiconductors, but a stable magnetic state at zero magnetic field has eluded detection. Here, we experimentally demonstrate that mesoscopic ferromagnetic order can be generated and controlled by local optical pumping in monolayer WSe2 at zero applied magnetic field. In a spatially resolved pump-probe experiment, we use polarization-resolved reflectivity from excitonic states as a probe of charge-carrier spin polarization. When the sample is electron-doped at density $n_e = 10^{12} cm^{-2}$, we observe that a local, circularly-polarized, microwatt-power optical pump breaks the symmetry between equivalent ferromagnetic spin configurations and creates magnetic order which extends over mesoscopic regions as large as 8 um x 5 um, bounded by sample edges and folds in the 2D semiconductor. The experimental signature of magnetic order is circular dichroism (CD) in reflectivity from the excitonic states, with magnitude exceeding 20% at resonant wavelengths. The helicity of the pump determines the orientation of the magnetic state, which can be aligned along the two principle out-of-plane axes. In contrast to previous studies in 2D materials that have required non-local, slowly varying magnetic fields to manipulate magnetic phases, the demonstrated capability to control long-range magnetism and corresponding strong CD with local and tunable optical pumps is highly versatile. This discovery will unlock new TMD-based spin and optical technologies and enable sophisticated control of correlated electron phases in two-dimensional electron gases (2DEGs).
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