Nonlinear Coloration Effects in Transition-Metal-Doped SrTiO3Crystals

Nonlinear coloration effects were discovered in doped crystals of SrTi${\mathrm{O}}_3$ and were attributed to twophoton excited electron-hole pairs that subsequently become trapped by color centers. An induced absorption coefficient of 2.6 ${\mathrm{cm}}^{-1\mathrm{}}$ was measured at 6328 $\mathrm{A}$ in samples doped with Ni and Mo when exposed to ruby laser pulses with peak intensities of 160 MW / ${\mathrm{cm}}^2$ and about 15-nsec duration. Calculated and previously reported experimental values of the two-photon absorption coefficient for SrTi${\mathrm{O}}_3$ are in good agreement with these results. A model which assumes a coloration rate proportional to the square of the intensity and a simultaneous first-order bleaching process was found to give a good fit to the experimental data on coloration versus intensity.