Phonon-assisted nonlinear optical processes from first principles

Phonon-assisted optical processes are some of the key physical processes that governs indirect absorption of photons in materials. Our prior work enabled insights into phonon-assisted linear optical phenomena, such as absorption across indirect band gaps and by free carriers[1,2]. Building upon our prior developments, we present a first-principles approach to investigate phonon-assisted two-photon absorption (PA-TPA), a crucial nonlinear optical process that influences applications such as waveguides under high-intensity fields. In this work, we demonstrate the ab initio theory of characterizing PA-TPA for indirect semiconductors, and its implementation into the EPW software. We accurately predict PA-TPA spectra for indirect semiconductors, including silicon and diamond, and achieve excellent agreement with experimental data. Our work offers a valuable computational tool for evaluating the limitations of indirect semiconductors in high-intensity optical applications.