LINEAR AND NONLINEAR PHOTOCONDUCTIVITY FROM CHARGE-PRODUCING DEFECTS IN CAF2

Room-temperature measurements of linear and nonlinear photoconductivity in CaF2 containing trace amounts of yttrium, lanthanum, and several rare earths are reported and discussed. Linear spectral photoconductivity scans taken over the 1.55- to 6.20-eV range of photon energies produced significant photocurrents above 3 eV. The results are explained in terms of a combination of 4f-5d absorption in Ce+3 and Pr+3 and photoionization of interstitial F- in both nearest-neighbor charge-compensating defects and anion-Frenkel defects. Peak photocurrents induced by focused 950-ns-wide pulses from a 496.5-nm wavelength dye laser increased nonlinearly with increasing laser pulse energy. These results are interpreted as a transition form two-photon conductivity to electron avalanche and the asymptotic approach to the material's laser damage threshold at approximately 27 GW/cm2.