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.
Serge Garnov, Sergej Pimenov, Victor Ralchenko, Sergei Klimentov, Vitali Konov, K. Korotoushenko, E. Obraztsova, S. Plotnikova, D. Sagatelyan, Sandor Holly
Photoexcitation and recombination of nonequilibrium charge carriers in both natural gemstone diamonds and CVD (chemical vapor deposition) polycrystalline diamond films in UV spectrum regions have been investigated. Transient picosecond photoconductivity technique applied permitted to conduct measurements with the time resolution better than 200 picoseconds and to register a charge carrier concentration value as low as 1020 - 1013 cm-3. The dependencies of photocurrent amplitude as a function of incident laser radiation intensity in the range from 103 to 1010 W/cm2 have been obtained. Charge carrier lifetimes had been measured and charge carrier drift mobility were estimated. It is shown that the electronic properties of high quality thick CVD diamond films are comparable to those of the most perfect natural type IIa crystals. Investigation of Raman and luminescence spectra of diamonds have been performed along with scanning electron microscopy studies to characterize bulk and surface structure of tested specimens.
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