Time-resolved interferometric detection of ultrashort strain solitons in sapphire

We study one-dimensional nonlinear propagation of high-amplitude acoustic waves in sapphire, for various sample temperatures, sample thicknesses, and pump fluences. Strain waves are generated in a 100-nm-thick chromium film and launched into the sapphire. For temperatures <60 K, damping can be neglected and propagation is dominated by the nonlinear and dispersive properties of the sapphire substrate. An interferometric technique is used to detect the wave on an epitaxially grown similar to 20-nm-thick Cr film at the opposite side of the sample. At the lowest temperature of 18 K, a train of up to seven solitons is detected in sapphire for a pump fluence of 11 mJ/cm(2). From the soliton amplitudes and velocities, we infer soliton temporal and spatial widths as short as 200 fs and 2 nm. A theoretical analysis based on numerical solution of the Korteweg-de Vries-Burgers equation yields excellent agreement to all experiments presented. Deviations to the direct theoretical result can be explained by pump intensity variations, affecting the (nonlinear) propagation properties.