Femtosecond laser pulse excitation of electrons and excitonsin CaF2 and SrF2

Abstract

Conduction band electrons, valence holes and subsequently self-trapped excitons (STEs) are createdin CaF2 and SrF2 by two-photon absorption of 5.9 eV photons. Transient absorption afterfemtosecond pulse excitation is measured at probe wavelengths of 3.0 eV and 4.4 eV,respectively. Transient absorption of 3.0 eV photons yields a fast initial response in SrF2 dueto absorption by conduction band electrons, while in CaF2 we sample the F centre part of theSTE at 3.0 eV and determine its formation time constant to be 690 fs. Excitation at 3.0 eV alsoyields a reduction in STE luminescence sampled nanoseconds after STE creation that is attributedto a forced recombination of the nearest-neighbour defect pair. Probing with 4.4 eV photons yieldsmuch weaker absorption but confirms results for 3.0 eV photons. Additionally we observe strongerabsorption due to the formation of long living defects.