Propagation and Diffraction of Picosecond Acoustic Wave Packets in the Soliton Regime

Abstract

Recent experiments on propagation of picosecond acoustic wave packets in condensed matter have opened up a new, exciting area of soliton physics. Single cycle strain pulses as short as several picoseconds can be generated in a thin metallic film, yielding local strain fields of the order of 10⁻⁴. The combination of phonon dispersion and anharmonicity of the atomic interaction potential may give rise to strongly nonlinear, but stable propagation of the wave packets over a distance of the order of several millimeters in a single crystalline material. We present new results on nonlinear propagation of acoustic wave packets created by nJ femtosecond optical pulses in a lead molybdate single crystal, employing the Brillouin Scattering technique as a local probe of acoustic strain. Studies of diffraction of narrow discs of acoustic strain show anomalous diffraction of the various Fourier components constituting the wave packet. Propagation of virtually one-dimensional nature is studied by exciting the metal film over a large area using an amplified femtosecond laser. We show that these data can be interpreted by means of the Korteweg-de Vries equation and strongly suggest the development of acoustic solitons.