The total energy reflected from a silicon single crystal irradiated by a 100 femtosecond laser pulse is measured. We observe a plasma resonance at wavelengths of 620 nm and 310 nm indicating electron-hole densities higher than 1022 cm. The result are interpreted using a highly non linear theory. Very short relaxation times are observed and attributed to electron-hole collisions. The study of the light scattered by the silicon surface shows a sharp decrease at high fluences that we interprete by a possible screening of irregularities by emitted electrons .A pump-test experiment is also reported showing the emission of Si particles. A possible mechanism for the extraction of these particles is proposed.

Laser pulses, of a duration of the order of 100 femtosecondsare a very unique tool to study the physical mechanisms of energy transfer from the electron-hole (e-h) plasma to the lattice in semiconductors. The incident photons are absorbed by the electrons, creating a hot and dense electron-hole plasma and breaking covalent bonds thus softening the lattice. After the pulse, the electron-hole pairs recombine, the plasma expands, and through electron-phonon interaction the energy is transferred to the lattice. Several experiments have recently been reported using femtosecond pulses to create a high density e-h plasma in silicon and study its time evolution [1,2,3]. The use of such intense and short pulses raises the possibility of breaking so many covalent bonds that the melting temperature of the crystal can be lowered [4,5] significantly. In a first period, a new phase is obtained, with atoms almost immobile (having a low kinetic energy) but imbedded in a dense hot plasma. In a time of the order of several electron-phonon relaxation times (τe-p) the energy is transferred to the atoms and the normal liquid phase is obtained. The understanding of the exact nature of the melting induced by very short pulses relies on a good knowledge of the energy transfer from the laser pulse to the sample. In this paper, we report measurements of the total amount of energy of a 100 fs, 620 nm and 310 nm of wavelengths, light pulse reflected by a silicon single crystal and its variation with pulse intensity (self reflectivity with no test beam). We also give measurement of the light scattered from the surface to see changes of the surface roughness. Finally, we give the result of a pump-test experiment showing the formation of a "blackhole" in the center of the incident spot, as already reported by other authors [6].