E. Courtade

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS[subscript 2] an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials MoSe[subscript 2] and WSe[subscript 2]. In this work, we show that encapsulation of ML MoS[subscript 2] in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T = 4 K. Narrow optical transition linewidths are also observed in encapsulated WS[subscript 2], WSe[subscript 2], and MoSe[subscript 2] MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.

Enabling valley selective exciton scattering in monolayer WSe2 through upconversion.

Excitons, Coulomb bound electron–hole pairs, are composite bosons and their interactions in traditional semiconductors lead to condensation and light amplification. The much stronger Coulomb interaction in transition metal dichalcogenides such as WSe2 monolayers combined with the presence of the valley degree of freedom is expected to provide new opportunities for controlling excitonic effects. But so far the bosonic character of exciton scattering processes remains largely unexplored in these two-dimensional materials. Here we show that scattering between B-excitons and A-excitons preferably happens within the same valley in momentum space. This leads to power dependent, negative polarization of the hot B-exciton emission. We use a selective upconversion technique for efficient generation of B-excitons in the presence of resonantly excited A-excitons at lower energy; we also observe the excited A-excitons state 2s. Detuning of the continuous wave, low-power laser excitation outside the A-exciton resonance (with a full width at half maximum of 4 meV) results in vanishing upconversion signal.

Charged excitons in monolayer WSe2: Experiment and theory

© 2017 American Physical Society. Charged excitons, or X± trions, in monolayer transition-metal dichalcogenides have binding energies of several tens of meV. Together with the neutral exciton X0 they dominate the emission spectrum at low and elevated temperatures. We use charge-tunable devices based on WSe2 monolayers encapsulated in hexagonal boron nitride to investigate the difference in binding energy between X+ and X- and the X- fine structure. We find in the charge-neutral regime, the X0 emission accompanied at lower energy by a strong peak close to the longitudinal optical (LO) phonon energy. This peak is absent in reflectivity measurements, where only the X0 and an excited state of the X0 are visible. In the n-doped regime, we find a closer correspondence between emission and reflectivity as the trion transition with a well-resolved fine-structure splitting of 6 meV for X- is observed. We present a symmetry analysis of the different X+ and X- trion states and results of the binding energy calculations. We compare the trion binding energy for the n- and p-doped regimes with our model calculations for low carrier concentrations. We demonstrate that the splitting between the X+ and X- trions as well as the fine structure of the X- state can be related to the short-range Coulomb-exchange interaction between the charge carriers.

In-Plane Propagation of Light in Transition Metal Dichalcogenide Monolayers: Optical Selection Rules

The optical selection rules for interband transitions in WSe_{2}, WS_{2}, and MoSe_{2} transition metal dichalcogenide monolayers are investigated by polarization-resolved photoluminescence experiments with a signal collection from the sample edge. These measurements reveal a strong polarization dependence of the emission lines. We see clear signatures of the emitted light with the electric field oriented perpendicular to the monolayer plane, corresponding to an interband optical transition forbidden at normal incidence used in standard optical spectroscopy measurements. The experimental results are in agreement with the optical selection rules deduced from group theory analysis, highlighting the key role played by the different symmetries of the conduction and valence bands split by the spin-orbit interaction. These studies yield a direct determination of the bright-dark exciton splitting, for which we measure 40±1  meV and 55±2  meV in WSe_{2} and WS_{2} monolayer, respectively.

Exciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure

We have combined spatially-resolved steady-state micro-photoluminescence (

Observation of exciton-phonon coupling in MoSe 2 monolayers

\mu

Spectrally narrow exciton luminescence from monolayer MoS2 and MoSe2 exfoliated onto epitaxially grown hexagonal BN

PL) with time-resolved photoluminescence (TRPL) to investigate the exciton diffusion in a WSe

Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

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Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures

monolayer encapsulated with hexagonal boron nitride (hBN). At 300 K, we extract an exciton diffusion length

Exciton States in Monolayer MoSe2 and MoTe2 Probed by Upconversion Spectroscopy

L_X= 0.36\pm 0.02 \; \mu