Analysis of the photoconduction in CaF2:Eu2+ crystals using the microwave resonant cavity technique

The microwave resonant cavity technique (MRCT) was used to measure the room-temperature photoconductivity spectrum of a CaF(2):Eu(2+) single crystal between 275 and 450 nm, with the aim of positioning the Eu(2+) levels relatively to the bottom of the host conduction band. A photoconductivity signal was detected at laser wavelengths lambda(l)<= 430 nm (h nu(l)>= 2.9 eV). Its intensity was observed to exhibit a superlinear dependence on the laser mean power for lambda(l)>280 nm and an almost linear one at shorter wavelengths, showing that Eu(2+) photoionization may involve either a one-photon or a two-step two-photon absorption process. The probabilities of both linear and quadratic processes were determined from measurements of the dependences of the photoconductivity signal intensity versus the mean laser power for several laser wavelengths within the spectral range that is under investigation. The Eu(2+) photoionization threshold was estimated at 4.9 eV from the comparison between the MRCT photoconductivity spectrum, the Eu(2+) 4f(6)5d(e(g)) excited-state absorption spectrum, and the calculated density of states of the CaF(2) conduction band. In addition, the photoconduction dynamics in two CaF(2):Eu(2+) samples grown under different experimental conditions was studied. The MRCT signals from the two samples were observed to exhibit different thermal behaviors. This observation is interpreted in terms of differences in trap densities and depths, in connection with thermoluminescence measurements.