Optical Absorption of Materials Driven Far from Equilibrium by Lasers

We propose a general theory for the optical absorption of matter driven far from equilibrium by lasers. Specifically, we envision a situation in which non-perturbative light drives a system out of equilibrium while a second weak probe quantifies its ability to absorb light across the electromagnetic spectrum. The interaction between matter and the driving pulse is treated exactly through a Floquet analysis, while the probe light is considered perturbatively. The procedure results in a generalization of the usual expressions for optical absorption to the non-equilibrium case. Through computations in model one-dimensional semiconductors, we unveil two striking phenomena that can reversibly be induced by non-resonant light of intermediate intensity (non-perturbative but non-ionizing). Specifically, we show that light can reversibly turn a semiconductor into a broadband absorber, and generate a strong absorption/stimulated emission band at arbitrarily low frequencies. This development offers a general approach to understand the emergent optical properties of materials dressed by the electric field of light and catalyze the design of laser-dressed materials.