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

Laser pulses, of a duration of the order of 100 femtosecondsare a veryunique tool to study the physical mechanisms of energy transfer from theelectron-hole (e-h) plasma to the lattice in semiconductors. The incidentphotons are absorbed by the electrons, creating a hot and denseelectron-hole plasma and breaking covalent bonds thus softening the lattice.After the pulse, the electron-hole pairs recombine, the plasma expands, andthrough electron-phonon interaction the energy is transferred to thelattice. Several experiments have recently been reported using femtosecondpulses to create a high density e-h plasma in silicon and study its timeevolution [1,2,3]. The use of such intense and short pulses raises thepossibility of breaking so many covalent bonds that the melting temperatureof the crystal can be lowered [4,5] significantly. In a first period, a newphase 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 severalelectron-phonon relaxation times (τe-p) the energy is transferred to theatoms and the normal liquid phase is obtained. The understanding of theexact nature of the melting induced by very short pulses relies on a goodknowledge of the energy transfer from the laser pulse to the sample. In thispaper, we report measurements of the total amount of energy of a 100 fs, 620nm and 310 nm of wavelengths, light pulse reflected by a silicon singlecrystal and its variation with pulse intensity (self reflectivity with notest beam). We also give measurement of the light scattered from the surfaceto see changes of the surface roughness. Finally, we give the result of apump-test experiment showing the formation of a "blackhole" in the center ofthe incident spot, as already reported by other authors [6].