Theoretical investigation of indirect optical processes in metals and plasmonic ceramics*

Understanding optical response for metallic materials is essential for understanding the physics and functionality of plasmonic devices. Traditional plasmonic materials such as noble metals are limited by their low melting points. Recently, several nitrides have been considered as potential high-temperature plasmonic materials due to their thermal stability. In this work, we demonstrate a first-principles investigation of the optical processes in these metallic systems, treating the direct and phonon-assisted single particle absorption and the collective oscillation of the electrons on the same footing, implemented in the open-source code EPW. Our simulated optical responses for the noble metals Ag, Au, and Cu, and for the plasmonic ceramic material TiN shows excellent agreement with previous experimental measurements. We further show that single-particle phonon-assisted indirect transitions and the Drude contribution are equally important in understanding optical responses in the IR region.