Study of laser induced photoconductivity, optical gain and photoacoustics

Abstract

In this work the ratio of three and two photon absorption cross-sections (g₃/g₂) has been calculated for PbI₂ using multiphoton photoconductivity by Nd: YAG laser and time dependent perturbation theory. Photoconductivity band gap has been compared with the photoacoustic energy band gap of the PbI₂. Relatively lower values of the absorption cross-section as compared to those of direct band-gap crystals have been attributed to phonon-assisted transitions. The results were found justified with earlier reports. The difference in the optical frequencies of PbI₂ obtained in photoconductivity and photoacoustics is understood in terms of Franck-Condon principle and process effect in the method of measurement. The higher value of electron mobility in the case of PbI₂ indicates that the material is suitable for x-ray and gama-ray detectors. It is possible to observe two-photon excitation by photon energy (hn) of XeCl laser higher than the band-gap (E_g) of ZnS, provided that laser field strength is more than the threshold value for two photon absorption in the material. Thermal transport parameters in Pbl₂ single crystal and photobleaching effect in case of Nile-blue doped in poly methyl metha acrylate (PMMA) were obtained using an indigeneously developed photoacoustic (PA) spectrometer. The characteristic frequency (f_c) and thermal diffusivity (a_s) are found to be 55 Hz and 34 ´ 10^-3 cm²/sec, respectively. As the photon energy of He-Ne laser is resonant with the band-gap of Nile blue, one photon absorption has been found to be responsible for the generation of good PA signal as well as its photobleaching. For nile blue dye PA results were helpful for the selection of pump frequency in the dye mixtures. Concentration of the acceptor dye should have a smaller concentration than that of the donor dye in the mixture. In a ternary dye mixture (Stilbene-420, Coumarin-540 and Nile blue) cascade transfer of energy shows that the concentration of the third dye (Nile blue) is very critical for deciding the laser output in the red region.