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 1A₁ electronic state is key for understanding the optical pumping process. Using pump-probe spectroscopy, we measured the phonon sideband of the 1E→1A₁ electronic transition in the NV^- center. From this we calculated the 1E→1A₁ one-phonon absorption spectrum and found it to differ from that of the 3E→3A₂ 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 1A₁ level.