Individual N-vacancy defect centers were investigated by using low-temperature confocal microscopy and fluorescence excitation spectroscopy. At temperatures below 90K, the fluorescence intensity of individual centers decreased markedly because of the population of a metastable singlet state which was in near-resonance with the optically excited state. Low-temperature fluorescence excitation spectroscopy down to 5K became possible via de-shelving of this state using a second laser source. Individual centers exhibited low-temperature fluorescence excitation line-widths of about 0.0006eV. This was more than 2 orders of magnitude greater than that expected on the basis of previous high-resolution laser spectroscopic studies of bulk samples.

Low-Temperature Microscopy and Spectroscopy of Single Defect Centers in Diamond. A.Dräbenstedt, L.Fleury, C.Tietz, F.Jelezko, S.Kilin, A.Nizovtzev, J.Wrachtrup: Physical Review B, 1999, 60[16], 11503-8