Twist-angle engineering of excitonic quantum interference and optical nonlinearities in stacked 2D semiconductors

TL;DR

This paper demonstrates that twisting bilayer WSe2 can significantly tune high-lying excitons and control optical nonlinearities, revealing a new degree of freedom for quantum interference in 2D semiconductors.

Contribution

It introduces twist-angle engineering as a method to manipulate excitonic states and optical nonlinearities in layered 2D materials, surpassing traditional band-edge approaches.

Findings

High-lying excitons are tunable over 235 meV by twisting.

Twist-angle susceptibility is 8.1 meV/°, much larger than band-edge excitons.

Control over excitonic quantum interference affects EIT in second-harmonic generation.

Abstract

Twist-engineering of the electronic structure of van-der-Waals layered materials relies predominantly on band hybridization between layers. Band-edge states in transition-metal-dichalcogenide semiconductors are localized around the metal atoms at the center of the three-atom layer and are therefore not particularly susceptible to twisting. Here, we report that high-lying excitons in bilayer WSe2 can be tuned over 235 meV by twisting, with a twist-angle susceptibility of 8.1 meV/{\deg}, an order of magnitude larger than that of the band-edge A-exciton. This tunability arises because the electronic states associated with upper conduction bands delocalize into the chalcogenide atoms. The effect gives control over excitonic quantum interference, revealed in selective activation and deactivation of electromagnetically induced transparency (EIT) in second-harmonic generation. Such a degree of freedom does not exist in conventional dilute atomic-gas systems, where EIT was originally established, and allows us to shape the frequency dependence, i.e. the dispersion, of the optical nonlinearity.