Optical properties and formation of oxygen-perturbedF2+color center in NaCl

The optical properties of a recently discovered laser-active color center in oxygen-doped NaCl are reported. The color center is produced through a two-step photoaggregation process in additively colored crystals. Optical spectroscopy indicates that the point defect is a perturbed $F_2$ ${\mathrm{}}^{+}$ center. The center has very broad absorption and emission bands peaking at 1.09 and 1.55 μm, respectively, with a dipole moment oriented along the 〈110〉 directions of the crystal. The center population can be fully aligned along any 〈110〉 direction by using two-photon pumping. Ground- and excited-state absorption spectra compare well with the dielectrically embedded ${\mathrm{H}}_2$ ${\mathrm{}}^{+}$ model for the eight transitions observed. uv spectroscopy has identified the substitutional ${\mathrm{O}}^{2\mathrm{-}}$ ion as being the necessary impurity for the center formation. The proposed model for the color center is an ${\mathrm{O}}^{2\mathrm{-}}$-perturbed $F_2$ ${\mathrm{}}^{+}$ defect. The perturbing ${\mathrm{O}}^{2\mathrm{-}}$ ion serves both as a stable electron trap as well as a spatial trap for the $F_2$ ${\mathrm{}}^{+}$ center.