# Large, valley-exclusive Bloch-Siegert shift in monolayer WS2

## TL;DR

This study reports the observation of a large, valley-specific Bloch-Siegert shift in monolayer WS2, enabling enhanced control of valleytronic properties through optical manipulation.

## Contribution

It demonstrates the first large Bloch-Siegert shift in a solid-state system and shows how to isolate it from the optical Stark shift using valley-dependent selection rules.

## Key findings

- Observed a ~10 meV Bloch-Siegert shift in monolayer WS2.
- Achieved valley-exclusive control of the Bloch-Siegert shift.
- Disentangled Bloch-Siegert shift from optical Stark shift via helicity control.

## Abstract

Coherent light-matter interaction can be used to manipulate the energy levels of atoms, molecules and solids. When light with frequency {\omega} is detuned away from a resonance {\omega}o, repulsion between the photon-dressed (Floquet) states can lead to a shift of energy resonance. The dominant effect is the optical Stark shift (1/({\omega}0-{\omega})), but there is an additional contribution from the so-called Bloch-Siegert shift (1/({\omega}o+{\omega})). Although it is common in atoms and molecules, the observation of Bloch-Siegert shift in solids has so far been limited only to artificial atoms since the shifts were small (<1 {\mu}eV) and inseparable from the optical Stark shift. Here we observe an exceptionally large Bloch-Siegert shift (~10 meV) in monolayer WS2 under infrared optical driving by virtue of the strong light-matter interaction in this system. Moreover, we can disentangle the Bloch-Siegert shift entirely from the optical Stark shift, because the two effects are found to obey opposite selection rules at different valleys. By controlling the light helicity, we can confine the Bloch-Siegert shift to occur only at one valley, and the optical Stark shift at the other valley. Such a valley-exclusive Bloch-Siegert shift allows for enhanced control over the valleytronic properties in two-dimensional materials, and offers a new avenue to explore quantum optics in solids.