Unified first-principles description of direct and phonon-assisted optical absorption

Current state-of-the-art ab initio approaches to study optical absorption can describe direct absorption processes for direct bandgap materials, and phonon assisted indirect absorption processes for indirect bandgap materials. One limitation of these approaches is that the formalism for phonon-assisted absorption becomes ill-defined for photon energies exceeding the direct band gap, therefore many materials where direct and indirect gaps are close in energy cannot be described adequately. In this talk, we present a new many-body formalism to capture both direct and phonon-assisted processes on the same footing. We demonstrate this approach by discussing a few test cases, such as silicon, gallium arsenide, and germanium.