Showing papers by "Gerard Mourou published in 1984"

Time-Resolved Laser-Induced Phase Transformation in Aluminum

Abstract: Phase transformation in condensed matter is an important area of study in solid state physics since it relates to the genesis and evolution of new microstructure. The mechanisms responsible in such critical phenomena are still not fully understood. Previous experimental information has left unmeasured such important parameters as the minimum number of nuclei and their common critical radius for a transformation to occur as well as the interphase velocity. Several probe techniques have now been developed to time-resolve phase transformations in semiconductors during laser annealing. However, most of these probes (eg. electrical conductivity,1,2 optical reflection,3,4 optical transmission,5 Raman scattering,6 and time of flight mass spectrometry7) supply no direct information about the atomic structure of the material. Probing the structure can reveal when and to what degree a system melts as it is defined by degradation in the long range order of the lattice. True structural probes based on x-ray8 and low energy electron9 diffraction and EXAFS10 with nanosecond time resolution have been developed offering fresh insight into both the bulk and surface dynamics of material structure. Also, a subpicosecond probe based on structural dependent second harmonic generation11 has been demonstrated. But at present, only the technique of picosecond electron diffraction12 can produce an unambiguous picture of the structure on the picosecond timescale. In this letter we report on the results of using this probe to directly observe the laser-induced melting of aluminum.

Subpicosecond Electro-Optic Sampling Using Coplanar Strip Transmission Lines

Abstract: A sampling technique using the electro-optic effect capable of characterizing subpicosecond electrical signals has recently been demonstrated. In this system 100 fs pulses from a colliding pulse mode-locked laser were used to generate the electrical test signal via a Cr:GaAs photoconductive switch. A second beam of pulses was used to probe the birefringence induced by the electrical pulses as they propagate down a balanced stripline fabricated on LiTa03. Signal averaging allowed submillivolt signals to be recovered. The best temporal response of the system was achieved with the velocity matched geometry, which was obtained for a particular angle of incidence of the probe beam on the electro-optic crystal. In this configuration a risetime of 500 fs was obtained. During this work it was observed that strong dispersion effects were taking place when the wavelength of the electromagnetic signal approached the cross-sectional dimensions of the transmission line. Temporal resolution is improved and dispersion effects are reduced as the dimensions of the stripline are reduced. In order to further improve the temporal response of the sampler in this configuration, the thickness would have to be reduced to the order of 10 um, which presents severe fabrication difficulties.

Time-Resolved, Laser-Induced Phase Transformation in Aluminum

Abstract: The technique of picosecond electron diffraction is used to time resolve the laser-induced melting of thin aluminum films. It is observed that under rapid heating conditions, the long range order of the lattice subsists for lattice temperatures well above the equilibrium point, indicative of superheating. This superheating can be verified by directly measuring the lattice temperature. The collapse time of the long range order is measured and found to vary from 20 ps to several nanoseconds according to the degree of superheating. Two interpretations of the delayed melting are offered, based on the conventional nucleation and point defect theories. While the nucleation theory provides an initial nucleus size and concentration for melting to occur, the point defect theory offers a possible explanation for how the nuclei are originally formed.

Generation of 90 fs Pulses with a CPM Laser Pumped by a Frequency-Doubled CW Mode-Locked Nd:YAG

Abstract: We have demonstrated the operation of a dye laser which is synchronously pumped by frequency-doubled Nd:YAG and also mode-locked by colliding pulses in the saturable absorber DODCI.

A Stable Kilohertz Nd: YAG Regenerative Amplifier

Abstract: A recently developed Pockels ceil driver able to operate at any repetition rate up to 1 kHz makes it practical to build a regenerative amplifier from a continuously pumped Nd:YAG laser. The amplifier output is approximately 1 mJ in a 100 ps pulse with ± 1% energy stability. The seed pulse is from a CW modelocked Nd:YAG oscillator (Spectra Physics, series 3000). This combination provides an ideal source for a synchronously pumped dye laser and high repetition rate amplifier.1

Pulse shaping in dispersive transmission lines

Abstract: A technique for generating monotonically increasing electrical waveforms with high-voltage amplitude in the nanosecond and subnanosecond time scale has been developed. This method employs the dispersive properties of transmission lines near their cutoff frequency to generate useful pulse shapes. These properties have been predicted by Hasnain et al.1 and observed by Valdmanis et al.2 in the picosecond range. This works extends the concept to the nanosecond and subnanosecond time scale.