Photoluminescence excitation study of split-vacancy centers in diamond

Two known representatives of the split-vacancy complexes in diamond, the negatively charged silicon-vacancy ${\mathrm{SiV}}^{\ensuremath{-}}$ and recently discovered germanium-vacancy ${\mathrm{GeV}}^{\ensuremath{-}}$ defects, were comparatively studied for their photoluminescence (PL) and complementary optical absorption spectra. The observed strong difference between luminescence and absorption spectra indicates a strong frequency defect, that is the difference of binding energies of impurity atom in the ground and excited electronic states, in these color centers. The presence of frequency defect is well supported by first-principle calculations. The obtained results accompanied with isotopic effects shed light on the structure of these centers in the ground and excited electronic states that would open the doorway to their theoretical description.