Tuning relaxation and nonlinear upconversion of valley-exciton-polaritons in a monolayer semiconductor

TL;DR

This paper explores how strong light-matter coupling in a tunable cavity affects energy conversion and upconversion in a monolayer MoSe2, revealing tunable nonlinear optical effects and valley polarization mechanisms.

Contribution

It demonstrates the control of exciton-polariton upconversion and relaxation pathways via cavity tuning, introducing new insights into valley polarization and nonlinear optical processes in 2D materials.

Findings

Modified photoluminescence response of cavity exciton-polaritons.

Emergence of upconversion luminescence from trion to exciton-polariton transfer.

Cavity length tuning enables control of upconversion intensity and valley polarization.

Abstract

Controlling exciton relaxation and energy conversion pathways via their coupling to photonic modes is a central task in cavity-mediated quantum materials research. In this context, the light-matter hybridization in optical cavities can lead to intriguing effects, such as modified carrier transport, enhancement of optical quantum yield, and control of chemical reaction pathways. Here, we investigate the impact of the strong light-matter coupling regime on energy conversion, both in relaxation and upconversion schemes, by utilizing a strongly charged MoSe2 monolayer embedded in a spectrally tunable open-access cavity. We find that the charge carrier gas yields a significantly modified photoluminescence response of cavity exciton-polaritons, dominated by an intra-cavity like pump scheme. In addition, upconversion luminescence emerges from a population transfer from fermionic trions to bosonic exciton-polaritons. Due to the availability of multiple optical modes in the tunable open cavity, it seamlessly meets the cavity-enhanced double resonance condition required for an efficient upconversion. The latter can be actively tuned via the cavity length in-situ, displaying nonlinear scaling in intensity and fingerprints of the valley polarization. This suggests mechanisms that include both trion-trion Auger scattering and phonon absorption as its underlying microscopic origin.