Chirality-selective second-harmonic response with Floquet engineering

Symmetry breaking profoundly influences the macroscopic physical properties of materials. For instance, inversion symmetry breaking in monolayer transition metal dichalcogenides enables the emergence of chiral selection rules, featuring distinct optical chirality-selective electronic properties. Here, we demonstrate ultrafast, coherent manipulation of chirality-selective nonlinear optics in monolayer molybdenum disulfide (MoS₂) through time-reversal symmetry breaking via Floquet engineering. By using sub-bandgap, circularly polarized ultrafast optical pulses, we introduce nondegenerate, chirality-selective optical Stark and Bloch-Siegert shifts. This approach effectively lifts the energy degeneracy, leading to pronounced chirality-selective second-harmonic generation. Furthermore, we reveal that the Floquet-engineered chirality-selective states can be finely tuned using optical parameters such as intensity and chirality. Our results show a method for manipulating the energy degree of freedom, paving the way for ultrafast nonlinear photonics.