Figure 3: Nonlinear reshaping of giant acoustic strain pulses in gold films.

Measurements are made as a function of excitation fluence in 120 nm (a,b) and 220 nm (c,d) thick gold films deposited on a 35-nm thin cobalt transducer. (a,c) High-frequency components in the numerical solutions of the KdV equation (dashed line) are smeared out by SR (KdV+SR, red solid line). (b,d) Experimental curves (blue solid lines) demonstrate the nonlinear steepening of giant (up to 1 per cent) strain pulses, which becomes more pronounced for high excitation fluences and for thicker gold layers. The quantitative agreement with the KdV solutions (KdV+SR, red solid lines) is obtained by adjusting a single fit parameter, namely the 20 nm heat penetration depth in cobalt at 800 nm pump wavelength. No fit parameters were used to derive the experimental strain pulses. Curves for three different values of excitation fluence (10, 20 and 30 mJ cm⁻²) are vertically displaced for clarity.