Abstract
Experiments are described in which the picosecond ultrasonic technique is used to study the effects of dispersion and elastic nonlinearity on the shape of longitudinal acoustic pulses propagating in crystalline solids. In these experiments, a subpicosecond light pulse (pump pulse) is used to generate an acoustic pulse at one surface of the sample. After propagation through the sample, the shape of this acoustic pulse is modified. When the amplitude of the pump pulse is sufficiently low, nonlinear effects can be neglected and a measurement of the acoustic pulse shape can be used to determine the phonon dispersion. This measured phonon dispersion is compared with various lattice dynamical models. When the pump pulse intensity is high, the nonlinear effects become strong enough to balance the dispersion, and acoustic solitons emerge from the original acoustic pulse. Measurements of the characteristics of these solitons are in agreement with the results of computer simulations based on the Korteweg-de Vries equation.
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