Abstract

Defects determine the mechanical, electrical and optical properties in solids. Often their chemical nature is very simple, comprising for example just a simple lattice vacancy or a single atom impurity. Some defects are optically active, i.e. they show allowed optical transitions. Those defect are called colour centers and often determine the optical properties of the respective solid. Famous examples are colour centers in ruby or alkali halides. By careful control of defect center concentration in combination with optical microscopy it became recently possible to investigate single defect centers optically. One of the materials that was investigated in particular detail is diamond. Pure diamond is an optically transparent medium. However more than 100 colour centers have been identified in diamond. Owing to its energy level structure the nitrogen vacancy center in diamond is a particularly interesting one to investigate. This defect center has a paramagnetic ground and first excited state. The resulting fine structure splitting can be resolved by low temperature fluorescence excitation spectroscopy. Ground as well as first excited state are spin triplets, i.e. S=1. When spin angular momentum is conserved during optical excitation one thus expects to find three optical excitation lines owing to the three spin sub levels in each electronic state. Indeed conservation of spin angular momentum during excitation seems to be fulfilled, however only a single excitation line is found. Experimentally this is due to unfavourable photophysical properties of the other transitions. Because of low electron phonon coupling of the center the linewidth of the transition is limited by the lifetime of the optically excited state and not, as in most cases, by dephasing due to electron-phonon coupling. Due to this low electron-phonon coupling all of the oscillator strength of the optical transition is concentrated in a narrow frequency interval giving rise to a large on resonance absorption cross section. This large cross section allows for ms averaging times, which are much smaller than the spin lattice relaxation times. One thus is able to determine the specific electron spin state the defect center is in. In diamonds isotopically enriched with ¹³C, the electron spin of the defect center is coupled to the nuclear spins of the surrounding ¹³C nuclei by hyperfine interaction. The hyperfine coupling depends on the distance of the center of the defect to the nuclei. For nuclei in the first coordination shell around the defect the coupling strength is around 250 MHz for nuclei in the second shell it is roughly 60 MHz. The hyperfine coupling is thus resolvable in single defect center excitation spectra. The spin relaxation times of nuclei in diamond are even longer than those of the electron. One thus is also able to determine the nuclear spin states around the defect center. Simultaneous radiation with radio frequency generates magnetic dipole transition and thus allows to control the nuclear spin states around the center. The system provides an interesting example for readout and manipulation of single spin degrees of freedom which may be interesting as hardware in future quantum information processing application.

© 2004 Optical Society of America