The Infrared Absorption Band and Vibronic Structure of the Nitrogen-Vacancy Center in Diamond

Negatively-charged nitrogen-vacancy (NV${}^{-}$) color centers in diamond have generated much interest for use in quantum technology. Despite the progress made in developing their applications, many questions about the basic properties of NV${}^{-}$ centers remain unresolved. Understanding these properties can validate theoretical models of NV${}^{-}$, improve their use in applications, and support their development into competitive quantum devices. In particular, knowledge of the phonon modes of the ${}^1{A}_1$ electronic state is key for understanding the optical pumping process. Using pump-probe spectroscopy, we measured the phonon sideband of the ${}^{1}E\to {}^1{A}_1$ electronic transition in the NV${}^{-}$ center. From this we calculated the ${}^{1}E\to {}^1{A}_1$ one-phonon absorption spectrum and found it to differ from that of the ${}^{3}E\to {}^3{A}_2$ transition, a result which is not anticipated by previous group-theoretical models of the NV${}^{-}$ electronic states. We identified a high-energy 169 meV localized phonon mode of the ${}^1{A}_1$ level.