Phonon-assisted linear and non-linear optical processes from first principles

The predictive understanding of phonon-assisted optical processes provides essential information on the functionality of indirect-band-gap materials in optoelectronic applications, as well as on the optical response of materials in the presence of free carriers. Recent methodological developments based on maximally localized Wannier functions have enabled the efficient calculation of the electron-phonon and the electron-photon coupling matrix elements with fine sampling of the Brillouin zone, and have catalyzed detailed investigations of the phonon-assisted indirect optical processes in materials. In this talk, I will discuss our recent advances of perturbation-theory-based theoretical characterization of phonon-assisted linear and nonlinear optical processes in both insulating and conducting materials. I will first discuss the general formalism based on second-order perturbation theory to model phonon-assisted optics. I will show that second-order perturbation theory accurately predicts the phonon-assisted absorption spectra of indirect-band-gap materials such as silicon and silicon carbide. Further, I will show that the formalism can be extended to study optical absorption by free carriers in doped semiconductors such as silicon, and metals such as silver and gold. Our results are in excellent agreement with experimental measurements, and explain the dominant absorption mechanisms at different photon wavelengths. Our work allows us to analyze the origin of optical losses in infrared optoelectronic devices and enables the design of new plasmonic materials. Last, I will present our recent development to characterize phonon-assisted two-photon absorption using higher-order perturbation theory, and I will discuss the limitation of perturbative approaches to phonon-assisted optics as well as ways to overcome them.