The effect of laser illumination on the propagation of ultrasonic waves in single crystalline lithium niobate

Abstract

By means of ultrasonic experiments we have observed in Fe-doped LiNbO₃-crystals elastic inhomogeneities and an ultrasonic absorption which stronglydecrease under laser illumination. The elastic inhomogeneities before illumination lead to a non-exponential decay of the ultrasonic echo pattern. The corresponding variations of the acoustic velocity and absorption across the sound beam have been displayed directly by an ultrasonic “topography”. — In contrast to the photorefractive behaviour of LiNbO₃ the elastic inhomogeneities and the ultrasonic absorption are not enhanced but disappear as soon as the illumination is switched on: Under illumination the echo pattern becomes well exponential as expected for an ideal crystal. The physical nature of the elastic inhomogeneities and the acoustic absorption inside the sample which can be “bleached out” by optical radiation is not yet well understood. But there is evidence that Fe⁺⁺-impurities play an important role.