Electrifying Control Over Light-Matter Interactions in 2D Materials

In the intricate dance between light and matter, the optical Stark effect (OSE) plays a starring role. This phenomenon occurs when a strong light field creates hybrid states of photons and matter, temporarily shifting the material's energy bands. Historically, scientists have controlled this effect by modulating the light source itself—adjusting its intensity, energy, or polarisation. However, a new study introduces a game-changing variable: electricity.

Focusing on monolayer tungsten disulphide (WS₂), a promising two-dimensional material, researchers utilised a back-gated device to achieve electrical tuning of the valley-selective OSE. By applying a voltage, the team successfully modified the transition dipole moment and the strength of excitons—bound pairs of electrons and holes that are fundamental to the material's optical properties.

The findings revealed a complex landscape. While electrical control proved effective, the material's behaviour at high negative voltages defied standard predictions based on steady-state exciton density. The observed 'blue shift'—a shift towards higher energy levels—deviated from expected trends, hinting at a more intricate underlying origin. This breakthrough demonstrates that electrical tuning serves as a powerful additional tool for 'Floquet engineering', allowing physicists to fine-tune coherent light-matter interactions with unprecedented precision.