Ultrafast reflectivity dynamics of highly excited Si surfaces below the melting transition

Solid Si under intense femtosecond irradiation is investigated over a wide range of fluences below the melting transition by supercontinuum light (400–800 nm) transient reflectivity. By solving a system of time-dependent equations, the fast and slow components of the energy and carriers diffusion can be disentangled from the reflectivity data, providing considerable insight into the nonequilibrium phase-change dynamics. The study of the fluence values immediately preceding the dramatic melting transition can be useful for discriminating between the thermal and electronic origins of the disordering of the structure by looking at the modification of the solid-state properties of the Si surface.

Figure 1

First stages of reflectivity changes, showing the buildup of state filling, consisting of a wavelength dependence that grows with increasing photon fluence, from 13 (bottom) to $25{\mathrm{mJ}/\mathrm{cm}}^2$ (top).

Figure 2

Numerical estimation of one-photon (${\alpha }_L$) and two-photon ($I_0{\beta }_{TPA}$) absorption in Si. Also reported is the free-carrier absorption contribution ${\alpha }_{FC}$.

Figure 3

From bottom to the top the complete visualization of three selected experiments at 13, 18, and $25{\mathrm{mJ}/\mathrm{cm}}^2$ fluences are reported, respectively. The color scale reports the experimental transient reflectivity change, while the gray pattern represents the modelization obtained with the diffusion rate equations.

Figure 4

From bottom to top, the temperature values of the lattice, optical phonons, and hot electrons resulting from the application of the model (as discussed in the main text) are reported at various fluences from 13 to $25{\mathrm{mJ}/\mathrm{cm}}^2$. The temperatures are obtained after an average over the probed volume extending 5–20 nm from the surface, corresponding to a volume homogeneously heated by the pump pulse (absorbed within a depth of 80 nm).