PHOTOELASTIC EFFECT IN TI-3+-DOPED SAPPHIRE

We have used photothermal-beam-deflection spectroscopy to measure the change in the index of refraction of Ti3+-doped sapphire after pulsed excitation at 532 nm with a neodymium-doped yttrium aluminum garnet laser. The beam-deflection signal arises from both thermal and nonthermal sources. The nonthermal part of the signal decays with the fluorescence lifetime of Ti3+ in sapphire. The shape of the nonthermal part depends on whether the probe beam is polarized perpendicular (sigma) or parallel (pi) to the c axis of the crystal. This behavior can be explained by a photoelastic law based upon the anisotropic volume expansion caused by a strong Jahn-Teller effect of the TiO6 complex in the excited state. The experimental values of the change in the index of refraction for each ion per unit volume are DELTA-n/N = 3.3 angstrom 3 and DELTA-n/N = 1.2 angstrom 3 for sigma and pi polarization, respectively, compared with the calculated values of DELTA-n/N = 3.73 angstrom 3 (for sigma) and DELTA-n/N = 1.50 angstrom 3 (for pi) for a model that includes the photoelastic law. The bulk expansion per TiO6 complex, derived from the beam-deflection signal, is almost-equal-to 11.8 angstrom 3, while the calculated value based on the configuration-coordinate model is almost-equal-to 9.9 angstrom 3.