Fluorescence Correlation and Low Temperature Linewidth of Single Defect Centers in Diamond

Low temperature fluorescence excitation spectroscopy was carried out on single nitrogen vacancy defect centers. At temperatures below 90 K the fluorescence intensity of individual centers drastically diminishes because of the population of a metastable singlet state in near resonance with the optically excited state. Low temperature fluorescence excitation spectroscopy down to 5 K becomes possible via deshelving of this state with a second laser source. Surprisingly individual centers reveal low temperature fluorescence excitation line widths around 0.6 meV.