Picosecond Transient Grating Generation Of Tunable Ultrasonic Waves

A convenient method of optically exciting and monitoring coherent acoustic waves in transparent or light-absorbing liquids and solids is described. The acoustic frequency is easily and continuously tunable from ≈ 3 MHz to at least 30 GHz with our experimental apparatus and in principle over a considerably wider range. In anisotropic materials any propagation direction can be selected. The optically generated acoustic waves can be optically amplified, cancelled, or phase shifted.