Formation and Optical Properties of New Laser - Color Centers in Sodium Chloride and Potassium Chloride

Two new, stable, color-center lasers have been recently demonstrated in additively colored NaCl and KCl. Both feature stable operation, are tunable in the near-infrared spectral region and are optically pumped by the commonly available 1.06-μm output of the Nd:YAG laser. In the case of NaCl, the laser-active defect was identified as the F_2^+:O ^{2-} center, while in the case of KCl the laser was based on the N_2 center. In this thesis, optical spectroscopic techniques have been employed to study the formation of these color centers, identify their structure, and probe various properties of the particular centers that are important for the corresponding laser operation. The laser-active color center in oxygen-doped NaCl is produced through a two-step photoaggregation process in additively colored crystals. Optical spectroscopy indicates that the point defect is a perturbed F_2 ^+ center. The center has very broad absorption and emission bands peaking at 1.09 μm and 1.55 mum, respectively, with a dipole moment oriented along <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 H_2^+ model for the eight transitions observed. UV spectroscopy has identified the substitutional O^{2-} ion as being the necessary impurity for the center formation. The proposed model for the color center is an O^ {2-}-perturbed F_2^+ defect. The perturbing O^{2 -} ion serves both as a stable electron trap as well as a spatial trap for the F_2^+ center. The N_2 laser-active center in pure KCl is most efficiently formed through a photoaggregation process in heavily colored crystals exposed to light on the long wavelengths shoulder of the F band. The observed room-temperature stability of the N_2 center indicates an electrically neutral defect. Absorption and emission bands peak at 1.02 μm and 1.25 mum, respectively. The N_2-band absorbance grows proportionally to the R_1- and R_2 -band absorbances. Polarized N_2 -center fluorescence data is consistent with two degenerate orthogonal dipole moments lying on {111} planes of the crystal. Excited-state absorption spectra show higher transitions coincident in wavelength with the R _1 and R_2 absorption bands of the trigonal F_3 color center in KCl. Similar experiments on the R _1 band correlate well with the N _2-center data. Results are discussed in the context of the F_3-model predictions. Data on N and R centers in NaCl and KBr are discussed in comparison. While the nature of the N_1 band has not been conclusively determined, all experimental results are consistent with the identification of the N _2 band as a transition of the trigonal F _3 color center.