Trains of Ultrashort Acoustic Solitons in Sapphire

Jaap I. Dijkhuis, LANL and the Department of Physics and Astronomy and the Debye Institute, University of Utrecht, The Netherlands

A soliton is a stable waveform that results from a dynamical balance between linear and nonlinear phase accumulations and was discovered in 1834 by John Scott Russell in a water canal. The phenomenon, however, appeared to be quite general. In optics, self-induced wave guides, solitons in optical fibers, the Kerr-mode-locked laser, and the existence of light bullets are prime examples. For matter waves, trains of solitons are recently observed in Bose-Einstein condensates.

In this talk, I focus on experiments relying on picosecond ultrasonics, which conclusively demonstrate that trains of ultrashort solitons can develop in sapphire at low temperatures. To this end, high- amplitude coherent phonon pulses are injected in the crystal by a metallic transducer (a thin chromium film) that was excited by an intense ultrashort laser pulse. The development of this acoustic wave packet, as it traverses the crystal, is monitored by Brillouin light scattering, enabling us to observe the formation of a so-called N-wave at room temperature and, at low temperatures, a supersonic soliton train. Simulations based on the Burgers equation and the Korteweg-de-Vries equation, respectively, account for all our experiments without a fitting parameter and predict soliton pulses as short as a few hundred femtoseconds. The ultrafast character of the solitons is experimentally proved by studying the interaction of the soliton train with a quantum mechanical transition at 870 GHz of a chromium impurity in optically pumped ruby. It appears that this transition is impulsively excited by the soliton train to a degree that can be accounted for by coherent acoustic Bloch equations. Experiments are under way to detect the solitons directly in the time domain.

O.L. Muskens and J.I. Dijkhuis, PRL 89, 285504 (2002)
O.L. Muskens, A.V. Akimov, and J.I. Dijkhuis, PRL 92, 035503 (2004)

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