Floquet theory and computational method for the optical absorption of laser-dressed solids

Abstract

Recent advances in laser technology now enable engineering the electronic structure of matter through strong light-matter interactions. However, the effective physicochemical properties of these laser-dressed nonequilibrium materials are not well understood. Here we develop a general theory that now enables modeling and interpreting the linear optical absorption of solids that are dressed by light of arbitrary strength and photon energy. The theory applies to any crystalline solid and quantum materials. In the theory, the dressing of Bloch electrons by the driving laser is treated exactly using Floquet theory. The effective optical properties of this laser-dressed material are probed through a weak laser whose effects are captured to first order in perturbation theory. Remarkably, in this nonequilibrium system the time- and space-periodic Floquet-Bloch modes play the role of the pristine eigenstates of matter as the optical absorption is seen to emerge from transitions among them. We implement the theoretical framework into a code FloqticS: Floquet optics in Solids) available through Github. To isolate the emergent phenomenology, we performed computations in a model solid with a cosine-shaped lattice potential driven by strong nonresonant light. The computations recover the dynamical Franz-Keldysh effect and identify novel dramatic changes in the optical absorption upon increasing the amplitude of the driving laser. The Floquet replicas open absorption sidebands separated by integer multiples of the drive photon energy. The hybridization of the Floquet-Bloch modes, create intense low-frequency absorption and stimulated emissions, and dips in the absorption spectrum. We assign these emerging effects as purely-optical tell-tale signatures of the Floquet-Bloch modes. These advances can be used to model, control and characterize the response properties of laser-dressed materials.