X-Ray Diffuse Scattering Measurements of Nucleation Dynamics at Femtosecond Resolution

Femtosecond time-resolved small and wide angle x-ray diffuse scattering techniques are applied to investigate the ultrafast nucleation processes that occur during the ablation process in semiconducting materials. Following intense optical excitation, a transient liquid state of high compressibility characterized by large-amplitude density fluctuations is observed and the buildup of these fluctuations is measured in real time. Small-angle scattering measurements reveal snapshots of the spontaneous nucleation of nanoscale voids within a metastable liquid and support theoretical predictions of the ablation process.

Figure 1

Raw snapshots of the liquid structure factor on time scales from picoseconds to microseconds, as recorded on a CCD camera placed just behind the sample. The top left image shows lines of constant $Q$, corresponding to collection of a fraction of the diffuse scattering ring.

Figure 2

(a) The structure factor at times $t=500\mathrm{fs}$, 10 ps, and 20 ns, and compared to MD simulations [23] of the equilibrium liquid. (b) Comparison of the short time structure factor at $t=3\mathrm{ps}$ to a hard-sphere model with parameters corresponding to the initial crystalline state. Also shown is a comparison to the equilibrium liquid. (c) Small-angle scattering measurements down to $0.2{\AA }^{-1}$ at times $t=10\mathrm{ps}$, 100 ps, and 10 ns.

Figure 3

Time scans recording the development of the diffuse scattering at both high and low $Q$. (left) Integrated diffuse scattering over $1.0–3.0{\AA }^{-1}$. Also shown is the time-dependence recorded on the (111) Bragg peak under identical excitation conditions. (right) Integrated diffuse scattering time-dependence over a range $0.4–1.6{\AA }^{-1}$. The solid line is a guide to the eye.

Figure 4

Calculated structure factors for the case of Silicon just below the ablation threshold. (insets) Snapshots of the near-surface region at various times after excitation. The spontaneous development of voids (in green) below the surface (in red) is observed, leading to a quasidivergence in the low-$Q$ scattering.