A. M. Malvezzi

Second harmonic generation in reflection from crystalline GaAs under intense picosecond laser irradiation

The emission of second harmonic radiation in reflection from crystalline GaAs irradiated with 20‐ps, 530‐nm laser pulses has been measured for incident laser fluences far exceeding the threshold fluence Fth for permanent reflectivity changes. The results are consistent with the occurrence of surface melting during the laser pulse. Detailed analysis of the second harmonic signals reveals an upper limit of 2 ps for the structural transition associated with the melting of the surface.

Nonlinear photoemission from picosecond irradiated silicon

Three distinctly different regimes of photoelectric emission are observed over a wide fluence range of uv-laser pulses irradiating single-crystal silicon samples. The role of the electron-hole plasma in the nonlinear photoemission is demonstrated by temporal correlation measurements. The diffusion properties of hot carriers are analyzed by investigating the influence of energy transport by hot carrier diffusion on the fluence threshold for melting with uv photons.

Picosecond laser melting and evaporation of GaAs surfaces

Picosecond laser‐induced melting and evaporation of GaAs surfaces are studied. The high reflectivities of molten GaAs observed at fluences above the melting threshold have a wavelength dependence inconsistent with a simple Drude model for a metallic molten GaAs surface. The observations at high laser fluences suggest that the liquid‐vapor phase transition is initiated by a fast‐expanding, high‐density fluid.

Plasmon‐phonon‐assisted electron‐hole recombination in silicon at high laser fluence

We present both theoretical and experimental results in silicon which clearly demonstrate that at time scales of 20 to 40 ps, after the pump laser pulse and at fluences greater than 100 mJ/cm2, the carrier density of the electron‐hole plasma drops for increasing fluence; this is not explained by Auger recombination. We show that this drop is specific to plasmon‐phonon‐assisted recombination, which naturally explains this behavior.

Time-resolved spectroscopy of plasma resonances in highly excited silicon and germanium

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 the time-resolved picosecond optical spectroscopy. The experimental reflectivities and transmission are compared with the predictions of the thermal model for degenerate carrier distributions through the Drude formalism. 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. 15 refs., 8 figs.

Interaction Of Picosecond Laser Pulses With Solid Surfaces

The interaction of picosecond laser pulses with solid surfaces is characterized by ultrafast energy transfer from the electronic system to the lattice. Heating and melting occur in the picosecond time scale. In semiconductors, the transfer is mediated by the electron hole plasma. Its kinetics is dominated by nonlinear recombination mechanisms that reduce considerably the maximum carrier concentration at high irradiation levels. Experiments with time resolved optical and photoelectrical techniques have established firm evidence for ultrafast thermal surface heating and rapid carrier recombination.

Ultrafast Phase Transition of GaAs Surfaces Irradiated by Picosecond Laser Pulses

Detailed analysis of the second harmonic signals in reflection from crystalline GaAs surfaces reveals that the structural transition associated with surface melting occurs in less than 2 ps. Preliminary results of picosecond time-resolved reflectivity measurements are also presented.

Optical Properties of Picosecond Laser Irradiated Graphite

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.

Invited Paper Fundamentals Of High Excitation Phenomena In Laser Processing

The interaction of picosecond laser pulses with solid surfaces is characterized by ultra-fast energy transfer from the electronic system to the lattice. The present understanding of the elementary mechanisms responsible for carrier thermalization and cooling is briefly reviewed. In semiconductors, the electron hole plasma acts as intermediate energy storage. Recombination limits the maximum density and determines lattice heating in degenerate conditions. Experiments with femtosecond and picosecond laser sources have unraveled the salient features of the energy dissipation routes from photons to lattice.

Time-resolved picosecond reflectivity study of laser-excited layered compounds

We have employed the pump-and-probe technique to perform picosecond time resolved measurements of the reflectivity changes in two archetypal layered compounds, 1T-TiS/sub 2/ and 1T-TiSe/sub 2/ probed at 1.064 ..mu..m after pumping by 20 ps, 0.532 ..mu..m laser pulses. At the threshold fluence, approx.40 mJ/cm/sup 2/, 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 approx.0.1 at high fluences, strongly suggesting that, like in graphite, the high temperature phase is not metallic.