Optical absorption properties of laser-dressed matter

TL;DR

This paper develops a Floquet-based theoretical framework to analyze and predict the optical absorption properties of materials driven far from equilibrium by laser fields, revealing tunable broadband absorption and spectral features.

Contribution

It introduces a novel Floquet formalism for optical absorption in laser-dressed materials, enabling prediction of emergent optical properties under strong laser driving.

Findings

Non-resonant light can turn transparent semiconductors into broadband absorbers.

Laser dressing creates strong absorption and emission bands at low frequencies.

Absorption spectra exhibit periodic features spaced by the photon energy.

Abstract

Characterizing and controlling matter driven far from equilibrium represents a major challenge for science and technology. Here we develop a theory for the optical absorption of electronic materials driven far from equilibrium by resonant and non-resonant lasers. In it, the interaction between matter and the driving light is treated exactly through a Floquet analysis, while the effects of the probing light are captured to first order in perturbation theory. The resulting equations are reminiscent to those for equilibrium absorption but with the Floquet modes playing the role of the pristine eigenstates. The formalism is employed to characterize the optical properties of a model nanoscale semiconductor dressed by non-resonant light of intermediate intensity (non-perturbative, but non-ionizing). As shown, non-resonant light can reversibly turn this transparent semiconductor into a broadband absorber and open strong absorption/stimulated emission bands at very low frequencies (~ meV). Further, the absorption spectra of the driven material exhibit periodic features energetically spaced by the photon energy of the driving light that reflect the periodic structure of the Floquet bands. These developments offer a general approach to understand and predict the emergent optical properties of materials dressed by the electric field of light, and catalyze the design of laser-dressed materials with desired optical properties.