We report ultraslow group velocities of light in an optically dense crystal of Pr doped Y2SiO5. Light speeds as slow as 45 m/s were observed, corresponding to a group delay of 66 micros. Deceleration and "stopping" or trapping of the light pulse was also observed. These reductions of the group velocity are accomplished by using a sharp spectral feature in absorption and dispersion that is produced by resonance Raman excitation of a ground-state spin coherence.
Raman-excited spin coherences were experimentally observed in nitrogen-vacancy (N-V) diamond color centers by means of nondegenerate four-wave mixing and electromagnetically induced transparency. The maximal absorption suppression was found to be 17%, which corresponds to 70% of what is possible given the random geometric orientation of the N-V center in diamond. In the context of quantum computing in solids, this level of transparency represents efficient preparation of quantum bits, as well as the ability to perform arbitrary single-quantum-bit rotations.
Photoluminescence spectra from the Si impurity center in diamond films grown by chemical-vapor deposition techniques were studied at temperatures between 9 and 300 K. Laser excitation at 514.5 nm, and resonant with the zero-phonon absorption line at 737 nm (1.6823 eV), were used. Luminescence lines become narrowed at resonance excitation, and the vibrational structure of the Si center was observed with a major phonon replica at 767 nm (1.6165 eV). The observed vibrational energy of 515 cm−1 supports a diatomic quasi-molecular Si2 center structure. Temperature dependencies of the linewidth of the zero-phonon line and its phonon replica were measured and the origins of the broadening are discussed.
Photoluminescence decay times and quantum yields were measured at 77 K in the spectra of two Si-doped diamond films grown by the chemical-vapor deposition technique. A decay time of 950 ps for both films and a quantum yield of 0.05 for one film were determined for the Si center zero-phonon optical transition at 737 nm ͑1.6823 eV͒. Two broad subbands with decay times of 55 and 650 ps and yields of 8ϫ10 Ϫ4 were observed in the luminescence at 600, 650, and 720 nm and ascribed to an sp 2 nondiamond carbon phase.
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