Large Acoustic Transients Induced by Nonthermal Melting of InSb

We have observed large-amplitude strain waves following a rapid change in density of InSb due to nonthermal melting. The strain has been measured in real time via time-resolved x-ray diffraction, with a temporal resolution better than 2 ps. The change from the solid to liquid density of the surface layer launches a high-amplitude strain wave into the crystalline material below. This induces an effective plane rotation in the asymmetrically cut crystal leading to deflection of the diffracted beam. The uniform strain in the layer below the molten layer is $2.0(\pm 0.2)\%$. A strain of this magnitude develops within 5 ps of the incident pulse showing that the liquid has reached the equilibrium density within this time frame. Both the strain amplitude and the depth of the strained material in the solid can be explained by assuming a reduction in the speed of sound in the nonequilibrium liquid compared to measured equilibrium values.

Figure 1

The left-hand figure shows the unperturbed lattice. The expansion gives rise to an effective lattice plane rotation, which can be seen as the greater exit angle, ${\alpha }^{\prime }$, of the reflected x rays in the right-hand figure, compared to the angle, $\alpha$, for the unperturbed lattice.

Figure 2

The surface strain gives rise to an angularly deflected x-ray beam. (a) X-ray signal at 2° angle of incidence with laser off. (b) X-ray signal at 2° angle of incidence with laser on (c). The diffraction signal as function of time at 0.9° angle of incidence. As the sample melts, the diffraction intensity drops. The temporal resolution is limited by streak camera jitter.

Figure 3

Melting depth (○) plotted together with the intensity of the deflected x rays (▵), both as a function of incident laser fluence. The inset shows the deflection angle as a function of laser fluence. The x-ray energy used was 2935 eV, 75 eV below the Bragg peak.

Figure 4

The intensity of the deflected x rays together with the displacement angle as a function of time after laser excitation.

Figure 5

Sketch showing the acoustic waves traveling through the sample. The difference in the speed of sound between liquid and solid is not illustrated for clarity.