Microwave study of photoconductivity induced by laser pulses in rare-earth-doped dielectric crystals

Abstract

Transient responses of the dielectric permittivity $\ensuremath{\varepsilon}={\ensuremath{\varepsilon}}_{1}\ensuremath{-}i{\ensuremath{\varepsilon}}_{2}$ of rare-earth-doped dielectric crystals under pulsed laser excitation were studied by the 8-mm microwave resonator technique at room temperature. The fluorite-type crystals $({\mathrm{CaF}}_{2},$ ${\mathrm{SrF}}_{2},$ and ${\mathrm{BaF}}_{2})$ which contained divalent ions of Sm, Eu, and Tm, as well as ${\mathrm{Lu}}_{2}({\mathrm{SiO}}_{4})\mathrm{O}$ and ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$ doped with trivalent Ce ions were investigated. The dielectric response to a laser pulse contains two different types of signals: electronic and heating ones. The electronic peak, which is quite fast (from 40 to 100 ns or more), is the signature of electrons released into the conduction band following an impurity photoionization. The prolonged heating signal has a sawtooth form on which oscillations are imposed. It is due to modulation of the dielectric constant by a sudden temperature rise and subsequent elastic vibrations of the sample caused by the energy absorbed from the laser pulse. In different crystals the electronic peak was caused by the transient response of either ${\ensuremath{\varepsilon}}_{1},$ ${\ensuremath{\varepsilon}}_{2},$ or a mixture of the two. The modulation of the dielectric loss factor ${\ensuremath{\varepsilon}}_{2}$ corresponds to conventional photoconductivity, i.e., the photoexcitation of mobile electrons. The modulation of the dielectric constant ${\ensuremath{\varepsilon}}_{1}$ corresponds to the photoexcitation of ``bound'' electrons, probably captured by traps. The threshold energies of photons at which the photoionization of rare-earth ions may occur, were determined for ${\mathrm{CaF}}_{2}:{\mathrm{Sm}}^{2+}$ (3.3 eV) and ${\mathrm{Lu}}_{2}({\mathrm{SiO}}_{4}){\mathrm{O}:\mathrm{C}\mathrm{e}}^{3+}$ (3.1 eV). In fluorite-type crystals doped with ${\mathrm{Sm}}^{2+}$ or ${\mathrm{Tm}}^{2+}$ ions, the significant reduction of a lifetime of electrons in a conduction band was revealed with an increase in energy of laser pulses. In ${\mathrm{SrF}}_{2}:\mathrm{Eu}$ crystal the record-high signals of ``photoconductivity'' were observed upon excitation by VIS light in the optical region of ``transparency'' of this crystal. The microwave resonant technique may be used for detail studying the photoionization dynamics of rare-earth ions and finding the location of their energy levels with respect to the host conduction band in doped insulators.