Valley Phenomena in the Microcavity Polariton Regime in Transition Metal Dichalcogenides

Alexander Tartarovskii
April 9th, 2018 ALEXANDER TARTAKOVSKII University of Sheffield

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 life-times (ps) and are also subject to the electron-hole exchange interaction leading to fast relaxation of the valley pseudospin and coherence. In this talk I will show that valley pseudo-spin dynamics is strongly modified in the polariton regime. This regime is achieved in a TMD monolayer embedded in an optical microcavity: the large oscillator strength of the excitonic transition allows the formation of a hybrid quasi-particle, a ‘polariton’, a superposition of the exciton and cavity photon states.

We observe both strong trion- and exciton- polaritons in a tunable microcavity, and demonstrate the formation of valley-addressable polaritons, which are a linear superposition of excitons, trions and photons. In contrast to the very low circular polarisation degrees seen in MoSe2 exciton and trion resonances, we observe a significant enhancement of up to 20% in polarisation of polariton states, dependent on the exciton and cavity mode frequency detuning. These results indicate slower intervalley scattering in the polariton states.

Further to this, we report that control of the dynamics of the valley coherence can be gained by embedding a monolayer of WSe2 in an optical microcavity. We demonstrate the optical initialisation of the valley coherent polariton populations, exhibiting luminescence with the linear polarization degree up to 3 times higher than that of the bare excitons. We further control the evolution of the polariton valley coherence using the Faraday magnetic field rotating the valley pseudospin by an angle defined by the exciton-cavity-mode detuning, and exceeding 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. It also opens the way to valleytronic devices based upon TMDs embedded in photonic structures and valley-dependent nonlinear polariton-polariton interactions.

Seminar, April 9, 2018, 12:00. ICFO’s Seminar Room

Hosted by Prof. Frank Koppens, Prof. Maciej Lewenstein, and Prof. Valerio Pruneri