Studies of soliton formation of longitudinal acoustic phonons in crystalline solids

We have used the picosecond ultrasonics technique to study the effects of dispersion and elastic non-linearity on the shape of longitudinal acoustic pulses propagating in a silicon crystal. In these experiments, a sub-picosecond light 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 pump pulse intensity is high, the non-linear effects become strong enough to balance the dispersion, and an acoustic soliton emerges from the original acoustic pulse. The form of the soliton is in good agreement with results obtained from a computer simulation based on a non-linear dispersive wave equation.