Intrinsic exciton states mixing and non-linear optical properties in transition metal dichalcogenide monolayers

TL;DR

This paper develops a theory for excitonic states in transition metal dichalcogenide monolayers, revealing mixing of exciton shells due to symmetry, which enhances nonlinear optical processes like second harmonic generation.

Contribution

It introduces a detailed model of exciton fine structure and predicts shell mixing caused by symmetry, impacting optical activity in these materials.

Findings

p- and s-shell excitons are mixed due to symmetry

Both exciton shells are active in nonlinear optical processes

Calculated contribution to second harmonic generation susceptibility

Abstract

Optical properties of transition metal dichalcogenides monolayers are controlled by the Wannier-Mott excitons forming a series of $1s$, $2s$, $2p$,... hydrogen-like states. We develop the theory of the excited excitonic states energy spectrum fine structure. We predict that $p$- and $s$-shell excitons are mixed due to the specific $D_{3h}$ point symmetry of the transition metal dichalcogenide monolayers. Hence, both $s$- and $p$-shell excitons are active in both single- and two-photon processes providing an efficient mechanism of second harmonic generation. The corresponding contribution to the nonlinear susceptibility is calculated.