Negatively charged nitrogen-vacancy (NV${}^{\ensuremath{-}}$) 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${}^{\ensuremath{-}}$ centers remain unresolved. Understanding these properties can validate theoretical models of NV${}^{\ensuremath{-}}$, 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 ${}^{1}E\ensuremath{\rightarrow}{}^{1}{A}_{1}$ electronic transition in the NV${}^{\ensuremath{-}}$ center. From this we calculated the ${}^{1}E\ensuremath{\rightarrow}{}^{1}{A}_{1}$ one-phonon absorption spectrum and found it to differ from that of the ${}^{3}E\ensuremath{\rightarrow}{}^{3}{A}_{2}$ transition, a result which is not anticipated by previous group-theoretical models of the NV${}^{\ensuremath{-}}$ electronic states. We identified a high-energy 169-meV localized phonon mode of the
1A1 level.
We propose a hybrid laser system consisting of a semiconductor external cavity laser associated to an intra-cavity diamond etalon doped with nitrogen-vacancy color centers. We consider laser emission tuned to the infrared absorption line that is enhanced under the magnetic field dependent nitrogen-vacancy electron spin resonance and show that this architecture leads to a compact solid-state magnetometer that can be operated at room-temperature. The sensitivity to the magnetic field limited by the photonshot-noise of the output laser beam is estimated to be less than 1 pT/Hz. Unlike usual NV center infrared magnetometry, this method would not require an external frequency stabilized laser. Since the proposed system relies on the competition between the laser threshold and an intracavity absorption, such laser-based optical sensor could be easily adapted to a broad variety of sensing applications based on absorption spectroscopy.