A common application of KTP is as a second-harmonic generator. However, scaling up second-harmonic efficiencies of KTP is limited by generation of ‘‘dark tracks.’’ It is possible to circumvent the problem of dark tracking by raising the crystal temperature. The heating extends the life time of the nonlinear medium and permits an increase of the second harmonic efficiency. At 170 °C an astigmatic beam distortion of the harmonic beam was observed. The astigmatism was strongly dependent on the beam intensities. The direction of the astigmatic distortion was parallel to the z crystallographic direction. The development of the beam distortion resulted from optically induced nonuniformity of the nz refractive index of KTP. It represented photorefractive damage to KTP. Analysis of the astigmatic distortion versus second-harmonic power indicates that the photorefractive damage resulted from multiphoton absorption involving fundamental and harmonic photons.

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Electron paramagnetic resonance (EPR), electron-nuclear double resonance, optical absorption, and thermoluminescence have been used to investigate radiation-induced point defects in a single crystal of lithium triborate (LiB3O5). Two prominent defects are observed after irradiation near liquid-nitrogen temperature with 60 kV x rays. A four-line EPR spectrum, with 12.2 G splittings, is assigned to a trapped-hole center, and another four-line EPR spectrum, with 120 G splittings, is assigned to a trapped-electron center. In each case, the nucleus responsible for the observed hyperfine is 11B. The trapped hole is localized on an oxygen ion and has a weak hyperfine interaction with one neighboring boron nucleus, whereas the trapped electron is localized primarily on a boron ion with a correspondingly larger hyperfine interaction. Both defects become thermally unstable near 125 K, and their decay (i.e., recombination) correlates with an intense thermoluminescence peak at this same temperature. An optical absorption peak at 300 nm is produced by the x rays and thermally decays at the same temperatures as the EPR spectra.

The results of an investigation of thermal effects on the fluorescence spectra of Ti3+ in sapphire are presented. As-grown samples synthesized by the Czochralski method were found to exhibit a fluorescence band characteristic of Ti4+. After thermal annealing this band was greatly reduced and the Ti3+ fluorescence was seen. The fluorescence lifetime was found to be constant between 10 and 300 K and then to decrease significantly as temperature was raised to 500 K. This decrease is shown to be associated with thermal activation to the higher Jahn–Teller component of the excited state. The temperature dependences of the intensities, widths and lifetimes of the two zero-phonon lines were monitored between 10 and 80 K. The lifetimes remained constant as the widths broadened and intensities decreased with temperature.The intensity change is associated with the increased rate of vibronic emission while the line broadening is attributed to the phonon processes coupling spin-orbit components of the ground state and to Raman scattering of phonons.

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