Valley-addressable polaritons in atomically thin semiconductors

Abstract

The locking of the electron spin to the valley degree of freedom in transition metal dichalcogenide (TMD) monolayers has seen them emerge as a promising platform in valleytronics [1,2]. When embedded in optical microcavities the large oscillator strengths of excitonic transitions in TMDs allow the formation of polaritons which are part-light part-matter quasiparticles [3][4][5][6][7]. Here, we report that polaritons in MoSe2 show an efficient retention of the valley pseudospin contrasting them with excitons and trions in this material. We find that the degree of the valley pseudospin retention is dependent on the photon, exciton and trion fractions in the polariton states. This allows us to conclude that in the polaritonic regime, cavity-modified exciton relaxation inhibits loss of the valley pseudospin. The valley addressable exciton-trion-polaritons presented here offer robust valley polarised states with the potential for valleytronic devices based upon TMDs embedded in photonic structures and valley-dependent nonlinear polariton-polariton interactions.Single layers of transition metal dichalcogenides (TMDs) are two-dimensional (2D) direct band-gap semiconductors which exhibit pronounced exciton resonances with binding energies of around 0.5 eV [8][9][10]. The monolayer nature of TMDs gives rise to strongly confined excitons with Bohr radii of around 1 nm and large oscillator strengths evident from optical absorption as strong as 15% [8]. By embedding TMD monolayers in optical microcavities this huge oscillator strength has allowed the realisation of the strong light-matter coupling regime and the formation of part-light part-matter polariton eigenstates [3][4][5][6][7]. Polaritonic states inherit properties such as a strong nonlinear interaction and low effective mass from the constituent exciton and photon components. In other material systems, this has led to the experimental realisation of a wealth of rich nonlinear phenomena such as Bose-Einstein condensation [11], superfluid-like behavior [12] and optical spin switching [13]. The observation of exciton-polaritons in TMDs creates new opportunities in engineering the polariton-polariton interaction [14,15] in 2D materials. Moreover, TMD based polaritons are expected to inherit additional degrees of freedom of valley pseudospin and finite Berry curvature from their constituent excitons and trions which can be utilised in new valley-polaritonic implementations [16].In this article we report on the valley addressability of polaritons in MoSe 2 monolayers embedded in tunable microcavities. We report clear valley polarisation of both exciton-and trion-polaritons which show a strong dependence of the polarisation degree on the cavity detuning. In the bare flake fast exciton depolarisation occurs due to the Maialle-Silva-Sham (MSS) mechanism [17][18][19]. We demonstrate that in the strong coupling regime this depolarisation mechanism can be overcome and robust valleypolarised polariton states with much higher polarisation degrees can be achieved. We support this...

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

“…The well-defined optical transitions and negligible defect emission will allow us in the future to further explore these stable MoS 2 samples; for example, the exciton resonance can be tuned in resonance with optical cavity modes to explore strong coupling between light and matter in microcavities [7,8] with the added possibility of valley-specific optical excitations [88,89]. Also, all timeresolved experiments using pulsed laser, such as pumpprobe spectroscopy, time-resolved PL, and four-wave mixing [12,31,36,90,91], will benefit from these samples with much higher threshold for optical damage as compared to uncapped samples.…”

Tunable Open‐Access Microcavities for Solid‐State Quantum Photonics and Polaritonics

“…Nevertheless, upon the formation of exciton‐polaritons in open‐access microcavities, the expected valley depolarization by MSS mechanism is reduced, due to a decrease of random exciton momentum scattering by the fact that the spatial extension of polariton wavefunction is much larger than the length scale of the exciton disorder. The authors observed polarization retention of exciton‐polaritons with circular polarization degree up to 20%, compared to 2% and 5% for bare neutral and charged excitons . Furthermore, the authors reported valley coherent exciton‐polaritons in WSe 2 embedded in an open‐access microcavity in 2018 .…”

“…The use of open‐access microcavities allows to explore and modify the intrinsic electronic properties of the monolayer material, best exemplified by the demonstration of valley addressable polaritons in MoSe 2 monolayers by Dufferwiel et al. in 2017 . It had been demonstrated in earlier literature that upon nonresonant excitation with circularly polarized light, the bare excitons in MoSe 2 monolayers almost does not retain the polarization of the pumped valley state, due to the Maialle–Silva–Sham (MSS) mechanism that a combined effect of the parallel and perpendicular (LT) spin splitting of exciton states and the random exciton momentum scattering results in a valley pseudospin relaxation.…”

Recent Progress on Exciton Polaritons in Layered Transition‐Metal Dichalcogenides

“…In addition to F‐P cavities formed by DBR mirrors, several other types of F‐P cavities were reported in the studies of EPs in TMDs. For example, TMD planar waveguides could serve as F‐P microcavities due to their capability of confining photons within the waveguide .…”