It has recently been demonstrated that crystals of LiTaO3 and LiNbO3 can be made more resistant to optically induced refractive-index inhomogeneities caused by laser irradiation by annealing the crystals in the present of an electric field. The explanation given for the improvement was that some impurity entered the crystal during the annealing cycle, modifying the conductivity of the material. Additional data presented here suggest that the impurity is hydrogen. The exact mechanism by which the susceptibility to index inhomogeneity is reduced is not understood at this time.
Investigations on cylindrical rods of yttrium iron garnet (YIG) have shown that the magnetostrictive effect at microwave frequencies makes them effective microwave acoustic transducers, and that a lower bound of the acoustic Q is of the order of 2×105 at 1 kMc/sec. This paper describes measurements of acoustic Q's of nearly this same magnitude obtained with the nonmagnetic yttrium gallium garnet (YGaG) and yttrium aluminum garnet (YAlG). A table is given of the longitudinal and shear acoustic wave velocities, elastic stiffness constants, and elastic isotropy for yttrium gallium garnet and yttrium aluminum garnet. Similar data for yttrium iron garnet by Clark and Strakna are included for reference.
Light intensity modulators have been developed using single-domain lithium tantalate as the electro-optic material. A broadband transistor amplifier which can develop 0.2-W output power drives the modulator sample which presents a capacitive load of 5 pF. Approximately 80% modulation is achieved from dc to 220 Mc/sec, when the light is made to traverse the sample twice. The modulation bandwidth is limited by the transistor amplifier. Very little acoustic ringing is observed when the modulator is used as a fast light switch.
The temperature dependence of the acoustic losses in single crystal yttrium iron garnet has been obtained at 500 and 1000 Mc/sec. The method of measurement involves propagation of plane shear waves in a cylindrical rod, the direction of propagation being along a [100] crystallographic direction. At both frequencies acoustic Q's of 2X10 5 have been obtained and these values are relatively insensitive to temperature for r>100°K. Below 100°K, the acoustic losses are dominated by an internal friction, ^(T), peak near 15°K.
It has been established that LiTaO3 can be made resistant to laser-induced inhomogeneities in the index of refraction at power levels as high as 500 W/cm2. This is accomplished by annealing LiTaO3 in an electric field of 250 V/cm at a temperature of 700°C for ½ h and then cooling the crystal to room temperature with the field on at a rate of 100°C/h. The susceptibility to laser-induced index changes in LiNbO3 is reduced by this treatment but not to the same extent as in LiTaO3. A mechanism is proposed to explain the observed reduction in susceptibility to damage.
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