Low-temperature microscopy and spectroscopy on single defect centers in diamond

Individual nitrogen-vacancy defect centers have been investigated by low-temperature confocal microscopy and fluorescence excitation spectroscopy. 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, more than two orders of magnitude larger than expected from previous high resolution laser spectroscopy on bulk samples.