# Observation of intervalley biexcitonic optical Stark effect in monolayer WS2

## TL;DR

This study reports a novel optical Stark effect in monolayer WS2, mediated by intervalley biexcitons, enabling enhanced control of valley degrees of freedom for valleytronics applications.

## Contribution

It demonstrates a new type of optical Stark effect involving intervalley biexcitons, revealing their role in valley coupling and control in monolayer WS2.

## Key findings

- Observation of energy downshift at the excitation valley
- Anomalous energy upshift at the opposite valley
- Intervalley biexcitons enable coupling between valleys

## Abstract

Coherent optical dressing of quantum materials offers technological advantages to control their electronic properties, such as the electronic valley degree of freedom in monolayer transition metal dichalcogenides (TMDs). Here, we observe a new type of optical Stark effect in monolayer WS2, one that is mediated by intervalley biexcitons under the blue-detuned driving with circularly polarized light. We found that such helical optical driving not only induces an exciton energy downshift at the excitation valley, but also causes an anomalous energy upshift at the opposite valley, which is normally forbidden by the exciton selection rules but now made accessible through the intervalley biexcitons. These findings reveal the critical, but hitherto neglected, role of biexcitons to couple the two seemingly independent valleys, and to enhance the optical control in valleytronics.