We have employed pump-and-probe techniques coupled to ellipsometry to measure the transient complex index of refraction at 1.064 μm of higly oriented pyrolitic graphite (HOPG) interacting with 20 ps, .532 μm laser pulses. When the laser pump fluence exceeds the threshold value for melting, measurements indicate a substantial decrease of both real and imaginary parts of the index of refraction, thus confirming that molten graphite becomes less metallic. Measurements provide also direct evidence of the insensitivity of our picosecond results to evaporation from the irradiated surface.

We have employed the pump-and-probe technique to perform picosecond time resolved measurements of the reflectivity changes in two archetypal layered compounds, IT-TiS2 and lT-TiSe2 probed at 1.064 μm after pumping by 20 ps, .532 μm laser pulses. At the threshold fluence, ∼ 40 mJ/cm2, the reflectivity drops sharply, marking the occurrence of a phase transformation on the surface of the sample. Above threshold, the reflectivity reaches a value as low as ∼0.1 at high fluences, strongly suggesting that, like in graphite, the high temperature phase is not metallic.

A Raman microprobe and TEM have been used to analyze the resolidified region of liquid carbon generated by picosecond pulse laser radiation. From the relative intensities of the zone center Raman-allowed mode for graphite at 1582cm−1 and the disorder-induced mode at 1360cm−1 , the average graphite crystallite size in the resolidified region is determined as a function of incident pulse energy density. By comparing with Rutherford backscattering spectra and Raman spectra from nanosecond pulsed laser melting experiments, additional information about the disorder depth in picosecond pulsed laser melted graphite is obtained. Comparisons of TEM micrographs for nanosecond and picosecond pulsed laser melting experiments show that the structure of the laser disordered regions in graphite are similar and exhibit similar behavior with increasing laser pulse fluence. The similarities in the resolidified regions under both irradiation schemes discourages a determination of the properties of liquid carbon after the liquid has resolidified.

The dynamics of the electron-hole plasma in silicon and germanium samples irradiated by 20 ps, 532 nm laser pulses has been investigated in the near infrared by time-resolved picosecond optical spectroscopy. The experimental reflectivities and transmissions are compared with the redictions of the thermal model for degenerate carrier distributions through the Drue iformalism. Above a certain fluence, a significant deviation between measured and calculated values indicates a strong increase of the recombination rate as soon as the plasma resonances become comparable with the band gaps. These new plasmon-aided recombination channels are particularly pronounced in germanium.

The pump-and-probe technique is employed to perform picosecond timeresolved measurements of the reflectivity changes in highly oriented pyrolitic graphite excited by 0.532-μm pump pulses. At low pump fluences, the presence of a short-lived plasma and a high-temperature gradient gives rise to an increase in the reflectivity probed at 1.9 μm but causes a decrease at 1.064 μm. At the threshold fluence, 140 mJ/cm2, the reflectivity drops abruptly, marking a phase transformation. Above the threshold, the reflectivity drops to -0.2 from its original value of 0.42 at 1.064 μm and to -0.4 from its ambient value of 0.50 at 1.9 μm. This new phase persists only for a few nanoseconds.

Three different photoelectric regimes are observed in the interaction of 15 ps, 266 nm laser pulses with crystalline silicon samples versus light fluence. A superposition of linear and quadratic photoionization is followed by a space charge limited regime up to the critical fluence F4ωth for the surface amorphization where highly nonlinear ion emission is observed. Ion and electron emissions become equal in magnitude at a fluence - ∼ 2F4ωth The absence of observable thermionic effects indicates that thermal equilibrium of the electronhole plasma and the lattice is reached during the laser pulse duration.