Inelastic light scattering by trains of ultrashort acoustic solitons in sapphire

We monitor the development of a picosecond strain wave packet into a train of ultrashort acoustic solitons along the c axis of sapphire using Brillouin scattering. One-dimensional propagation yields an intricate oscillation pattern of the scattered intensity against distance that is interpreted in terms of optical interference and Bragg resonances of light reflected from the moving soliton train. By exploring this pattern over a range of scattering angles, we derive quantitative information on the soliton parameters using only a simple analytical framework. Further more, the analogy between the wave packet and a diffraction grating is explored that provides a direct estimate of the amount of solitons in the train from the observed number of oscillation periods.