Valley addressable exciton-polaritons in atomically thin semiconductors

TL;DR

This paper demonstrates valley addressable exciton-polaritons in MoSe2 monolayers embedded in photonic cavities, showing enhanced valley polarization retention and potential for valleytronic device applications.

Contribution

It reports the first observation of controllable valley polarization in MoSe2 cavity polaritons and introduces a dynamical model to estimate fast valley pseudospin relaxation times.

Findings

Valley polarization in MoSe2 cavity polaritons can be controlled by exciton-cavity detuning.

Polaritons significantly enhance valley polarization retention compared to bare excitons.

Estimated valley pseudospin relaxation time in MoSe2 is an order of magnitude faster than in other TMDs.

Abstract

While conventional semiconductor technology relies on the manipulation of electrical charge for the implementation of computational logic, additional degrees of freedom such as spin and valley offer alternative avenues for the encoding of information. In transition metal dichalcogenide (TMD) monolayers, where spin-valley locking is present, strong retention of valley chirality has been reported for MoS$_2$, WSe$_2$ and WS$_2$ while MoSe$_2$ shows anomalously low valley polarisation retention. In this work, chiral selectivity of MoSe$_2$ cavity polaritons under helical excitation is reported with a polarisation degree that can be controlled by the exciton-cavity detuning. In contrast to the very low circular polarisation degrees seen in MoSe$_2$ exciton and trion resonances, we observe a significant enhancement of up to 7 times when in the polaritonic regime. Here, polaritons introduce a fast decay mechanism which inhibits full valley pseudospin relaxation and thus allows for increased retention of injected polarisation in the emitted light. A dynamical model applicable to cavity-polaritons in any TMD semiconductor, reproduces the detuning dependence through the incorporation of the cavity-modified exciton relaxation, allowing an estimate of the spin relaxation time in MoSe$_2$ which is an order of magnitude faster than those reported in other TMDs. The valley addressable exciton-polaritons reported here offer robust valley polarised states demonstrating the prospect of valleytronic devices based upon TMDs embedded in photonic structures, with significant potential for valley-dependent nonlinear polariton-polariton interactions.