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13 May 1997 Two-photon spectroscopy of free-carrier generation in wide-band gap crystals
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Multiphoton absorption is able to produce primary free electron concentration, launching avalanche ionization mechanism in the transparent solids. We applied transient photoconductivity technique to investigate free carrier generation, caused by two-photon absorption in the alkali halide undoped crystals. Using radiation of tunable optical parametric oscillator-amplifier, the excitation spectra and temperature dependencies for conduction band electron concentration were measured. Two variations of experiment included optical excitation by 'equal' quanta and by combination of tunable radiation with a fixed wavelength laser source. As it was shown, an efficient free electron producing was observed not only for interband optical transitions, but also for two-photon exciton absorption at room temperature. An analysis of the data obtained and comparison to conventional photoconductivity spectra indicated thermal ionization of the exciton states in the temperature range above 300 K. The corresponding exciton activation energy in KI was estimated and found to be consistent with the location of energy levels, allowed for two-photon exciton transitions. The results obtained qualify transient photoconductivity and as an effective tool for investigation of multiphoton excitation and following electron transitions in the wide-band gap crystals.
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Sergei M. Klimentov, Serge V. Garnov, Alexander S. Epifanov, Alexander A. Manenkov, and D. M. Sagatelyan "Two-photon spectroscopy of free-carrier generation in wide-band gap crystals", Proc. SPIE 2966, Laser-Induced Damage in Optical Materials: 1996, (13 May 1997);

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