Brightening of a dark monolayer semiconductor via strong light-matter coupling in a cavity

TL;DR

This paper demonstrates that strong light-matter coupling in a cavity can invert the excitonic band order in a monolayer WSe2, effectively brightening an intrinsically dark material and preventing luminescence quenching.

Contribution

It introduces a method to invert excitonic band order in monolayer WSe2 using cavity-induced strong coupling, enabling brightening of dark excitons.

Findings

Bright polaritonic modes are created in the optical bandgap.

Luminescence quenching is prevented by band inversion.

Experimental results match theoretical models of band inversion.

Abstract

Engineering the properties of quantum materials via strong light-matter coupling is a compelling research direction with a multiplicity of modern applications. Those range from modifying charge transport in organic molecules, steering particle correlation and interactions, and even controlling chemical reactions. Here, we study the modification of the material properties via strong coupling and demonstrate an effective inversion of the excitonic band-ordering in a monolayer of WSe2 with spin-forbidden, optically dark ground state. In our experiments, we harness the strong light-matter coupling between cavity photon and the high energy, spin-allowed bright exciton, and thus creating two bright polaritonic modes in the optical bandgap with the lower polariton mode pushed below the WSe2 dark state. We demonstrate that in this regime the commonly observed luminescence quenching stemming from the fast relaxation to the dark ground state is prevented, which results in the brightening of this intrinsically dark material. We probe this effective brightening by temperature-dependent photoluminescence, and we find an excellent agreement with a theoretical model accounting for the inversion of the band ordering and phonon-assisted polariton relaxation.