Mixing periodic topographies and structural patterns on silicon surfaces mediated by ultrafast photoexcited charge carriers

Ultrafast laser irradiation of silicon can significantly modify charge densities and optical indices, impacting the formation and the development of nanoscale-arranged periodic structures. Photoexcitation degree as well as thermodynamic, hydrodynamic, and structural aspects are reported for crossed orientation of laser-induced periodic surface structures generated on single-crystal silicon after multiple-pulse femtosecond laser irradiation. The periodic topography and microstructure generated by light scattering on surface nanoroughness were characterized to gain insights into the regime of photoexcitation, subsequent thermodynamic conditions, and inhomogeneous energy deposition related to periodic nanostructure formation and growth. A generated free-carrier density around (3 +/- 2) x 10(21) cm(-3) is estimated from time-resolved ellipsometry and supported by time-dependent density-functional theory calculations. A finite-difference time-domain solution of the far-field interference of the surface scattered light and the refracted laser wave confirms the periodically crossed energy deposition for this excitation degree. The interference process does not necessarily involve surface plasmon polaritons, and quasicylindrical evanescent waves are identified as plausible scattered waves requiring less restrictive conditions of photoexcitation. Ab initio calculations are also performed to evaluate the transient state of silicon under strong electron-phonon nonequilibrium upon fs laser irradiation. For the reached excitation degree, an electron temperature up to 8000 K is deduced, supporting local amorphization processes observed as a result of high mechanical stress and quenching rates. Ab initio combined with electromagnetic calculations agree well with the results of topography and structural characterization.