Photoluminescence excitation spectroscopy of SiV⁻ and GeV⁻ color center in diamond

Color centers in diamond are important quantum emitters for a broad range of applications ranging from quantum sensing to quantum optics. Understanding the internal energy level structure is of fundamental importance for future applications. We experimentally investigate the level structure of an ensemble of few negatively charged silicon-vacancy (SiV⁻) and germanium-vacancy (GeV⁻) centers in bulk diamond at room temperature by photoluminescence (PL) and excitation (PLE) spectroscopy over a broad wavelength range from 460 to $650nm$ and perform power-dependent saturation measurements. For SiV⁻ our experimental results confirm the presence of a higher energy transition at $\sim 2.31eV$. By comparison with detailed theoretical simulations of the imaginary dielectric function we interpret the transition as a dipole-allowed transition from ${}^2{E}_g$-state to ${}^2{A}_{2u}$-state where the corresponding a _2u-level lies deeply inside the diamond valence band. Therefore, the transition is broadened by the diamond band. At higher excitation power of $10mW$ we indicate signs of a parity-conserving transition at $\sim 2.03eV$ supported by saturation measurements. For GeV⁻ we demonstrate that the PLE spectrum is in good agreement with the mirror image of the PL spectrum of the zero-phonon line. Experimentally we do not observe a higher lying energy level up to a transition wavelength of $460nm$. The observed PL spectra are identical, independent of excitation wavelength, suggesting a rapid decay to ${}^2{E}_u$ excited state and followed by optical transition to ${}^2{E}_g$ ground state. Our investigations convey important insights for future quantum optics and quantum sensing experiments based on SiV⁻-center and GeV⁻-center in diamond. (paper)