Robust Purely Optical Signatures of Floquet States in Laser-Dressed Crystals

Strong light-matter interactions can create nonequilibrium materials with on-demand novel functionalities. For periodically driven solids, the Floquet-Bloch theory provides the natural states to characterize the physical properties of these laser-dressed systems. However, signatures of such Floquet states are needed, as common experimental conditions, such as pulsed laser excitation and dissipative many-body dynamics, can disrupt their formation and survival. Here, we identify a tell-tale signature of Floquet states in the linear optical response of laser-dressed solids that remains prominent even in the presence of the strong spectral congestion of bulk matter. To do so, we introduce a computationally efficient strategy based on the Floquet formalism to finally capture the full frequency dependence in the optical response properties of realistic laser-dressed crystals, and use it investigate the Floquet engineering in a first-principle model for ZnO of full dimensionality. The computations reveal intense, spectrally isolated, laser-controllable, absorption and stimulated emission features at midinfrared energies present for a wide range of laser-driving conditions that arise due to the hybridization of the Floquet states. As such, these spectral features open a purely optical pathway to investigate the birth and survival of Floquet states in crystals while avoiding the experimental challenges of fully reconstructing the band structure of laser-dressed materials.