Cavity-driven ultrafast chiral optical switching

Floquet engineering, where an oscillating electric field modifies quantum states, is a promising tool to manipulate quantum systems coherently. For example, the valley-selective A.C. Stark effect can break time-reversal symmetry in monolayer transition metal dichalcogenides by lifting valley degeneracy. However, Floquet engineering has often been limited by the requirement of an ultrashort optical pulse with a high enough pulse energy. Here, based on a cavity structure with monolayer WSe₂, we demonstrate that cavity effects can enhance effective Floquet dressing power by orders of magnitude. The strong enhancement also enables us to build an efficient chirality all-optical switch by the Floquet effect. Our study opens up the possibility to exploit Floquet engineering and other nonlinear phenomena for efficient optical devices of low-dimension materials.