Valley coherent exciton-polaritons in a monolayer semiconductor

TL;DR

This paper demonstrates control of valley coherence in exciton-polaritons within a monolayer WSe2 embedded in a microcavity, enhancing valley polarization and manipulating pseudospin dynamics using magnetic fields.

Contribution

It introduces a method to control valley coherence and pseudospin dynamics in TMD monolayers via strong light-matter coupling in a microcavity, surpassing limitations of short exciton lifetimes.

Findings

Enhanced linear polarization degree in polaritons compared to excitons

Magnetic field induces valley pseudospin rotation exceeding that in bare excitons

Observation of valley coherence control through cavity detuning

Abstract

Two-dimensional transition metal dichalcogenide (TMD) semiconductors provide a unique possibility to access the electronic valley degree of freedom using polarized light, opening the way to valley information transfer between distant systems. Excitons with a well-defined valley index (or valley pseudospin) as well as superpositions of the exciton valley states can be created with light having circular and linear polarization, respectively. However, the generated excitons have short lifetimes (ps) and are also subject to the electron-hole exchange interaction leading to fast relaxation of the valley pseudospin and coherence. Here we show that control of these processes can be gained by embedding a monolayer of WSe$_2$ in an optical microcavity, where part-light-part-matter exciton-polaritons are formed in the strong light-matter coupling regime. We demonstrate the optical initialization of the valley coherent polariton populations, exhibiting luminescence with a linear polarization degree up to 3 times higher than that of the excitons. We further control the evolution of the polariton valley coherence using a Faraday magnetic field to rotate the valley pseudospin by an angle defined by the exciton-cavity-mode detuning, which exceeds the rotation angle in the bare exciton. This work provides unique insight into the decoherence mechanisms in TMDs and demonstrates the potential for engineering the valley pseudospin dynamics in monolayer semiconductors embedded in photonic structures.