Excited state absorption of Cr/sup 3+/ in low field crystalline hosts

A simple single configuration coordinate crystal field model has been developed to predict the excited state absorption (ESA) spectra of Cr/sup +3/ in hosts where the /sup 4/T/sub 2g/ is the lowest excited state. Two spin allowed transitions are predicted to lie near 19,000 and 7,000 cm/sup -1/ for Cr/sup 3+/ in a typical fluoride host. On the other hand, ESA from a /sup 2/E lowest excited state is more complex having a number of near infrared and visible transitions to a rich manifold of high energy doublets. To corroborate the model, ESA measurements have been made on GSGG:Cr, GSAG:Cr and K/sub 2/NaScF/sub 6/:Cr over the range 250 nm to 1,750 nm. The higher energy ESA transition was located near 19,000 cm/sup -1/ and is accompanied by bleaching of the strong ground state transitions at 15,500 and 22,000 cm/sup -1/. The lower energy transition was found centered at 8,000 cm/sup -1/ and to be far broader than predicted by a single configuration coordinate model. In addition to these crystal field transitions, intense charge transfer bands were found at 31,000 and 32,000 cm/sup -1/ for GSGG:Cr and GSAG:Cr, respectively, but they are not present in this region for the elpasolite. The peculiar band shape of these charge-transfer transitions suggest that the terminal state of the transition is the host conduction band. Finally, saturation experiments were made at 632.8 nm in order to determine ESA cross sections. Threshold for damage and thermal lensing was found to be near 10 Mwatt/cm/sup 2/ for GSGG, consequently only partial saturation could be achieved. ESA cross sections were extracted from these data with a coupled three level kinetic model