M. B. Mason

Explosion of atomic clusters heated by high-intensity femtosecond laser pulses

We have experimentally and theoretically studied the high-intensity ( .10 W cm), femtosecond photoionization of inertially confined noble-gas clusters. We have examined the energies of electrons and ions ejected during these interactions and found that particles with substantial kinetic energy are generated. Electrons with energies up to 3 keV and ions with energies of up to 1 MeV have been observed. These experimental observations are well explained by a theoretical model of the cluster as a small plasma sphere that explodes following rapid electron collisional heating by the intense laser pulse. @S1050-2947 ~97!02912-0#

INVESTIGATION OF HIGH-HARMONIC GENERATION FROM XENON ATOM CLUSTERS

We report on the generation of harmonic radiation (in the 70 - 90 nm range) from clusters of Xe atoms formed in a gas jet. We find that the harmonic yield from the clusters exhibits an anomalous cubic scaling with backing pressure to the gas jet. This scaling is consistent with a cluster dipole moment resulting from collective oscillations of electrons around the central ions of the cluster. Using a nanosecond ultraviolet prepulse to dissociate the clusters, we have also attempted to compare harmonic yields from clusters with those produced from monatomic Xe, under otherwise identical conditions. Our results suggest that yields from clusters might exceed those from monomers by up to a factor of five.

High-order harmonic generation efficiency increased by controlled dissociation of molecular iodine

We investigate the role of dissociation in high-order harmonic generation (HHG) by a vapour of molecular iodine. By controlling dissociation independently of harmonic generation we have shown for the first time that dissociation can lead to an increase in the harmonic yield. In I2, the intensity of the ninth harmonic of a 70 fs duration, 798 nm laser pulse was increased by a factor of two to three when the molecules were first dissociated by a 300 ps duration, 798 nm pulse with intensity ~1011 W cm-2. These results are relevant to the interpretation of previous HHG experiments in molecules and demonstrate a straightforward technique to enhance and modulate high-order harmonic radiation.

Control of laser heating in clusters through variation in temporal pulse shape.

We present results of experiments and simulations of the interaction between a high intensity, femtosecond laser pulse and an atomic cluster that show the temporal profile of the laser pulse plays a crucial role in the expansion dynamics of the cluster. Experiments were conducted in rare gas clusters of Xe, Kr and Ar of radius 30 - 80 A with a laser pulse of duration 70 - 240 fs and a peak intensity of ~1016 Wcm-2. The shape of the laser pulse was a Gaussian with a shoulder of intensity 0.02 times the peak pulse intensity appearing on either the rising or falling edge of the main pulse depending on the sign of chirp applied to the laser pulse. Significant differences (up to a factor of 2) in the energies of the ions obtained from the laser-cluster interaction were measured when the shape of the laser pulse was varied.

Explosion of irradiated with a high-intensity femtosecond laser pulse

We have studied the ion kinetic energy spectra produced by fragmentation of into single ions in high-intensity femtosecond laser pulses at 780 nm. was observed to dissociate completely in the intensity range to producing ions from to with kinetic energies up to 1 keV. For peak intensities the explosion was found to be isotropic and the major features were reproduced by a model based on Coulomb explosion. At higher intensities, however, the kinetic energy distribution was found to be significantly anisotropic.

High-intensity lasers: Interactions with atoms, molecules and clusters

The development of high–intensity short–pulse lasers by chirped pulse amplification is described and the use of short intense pulses for the study of atoms and molecules is discussed. As a case study, the dynamics of the ionization and dissociation of clusters of noble gas atoms is reviewed with special emphasis on the generation of energetic electrons and highly charged ions.

Implementation of a novel reflective doublet based stretcher design for a sub-50-fs chirped pulse amplification laser system

Summary form only given. The design of pulse stretchers for chirped pulse amplification laser systems becomes more complicated as the pulse duration decreases. For durations below 50 fs, high-order phases (up to fifth and sixth order) become important due to the large associated bandwidth (e.g., 23 nm for a 30-fs sech/sup 2/ pulse centered at 800 nm). Optical components in the stretcher and compressor have to be large to accommodate this bandwidth; therefore off-axis aberrations must be minimised and imperfect surface figures must be taken into account. To facilitate this difficult design process and to compare existing stretcher designs, a full three-dimensional ray-tracing model has been developed. The model traces a bundle of beams representing a finite beam profile, each beam containing thousands of spectral component rays. The rays are traced from surface to surface using ray-transfer equations and, where appropriate, the grating equation.

Three-dimensional ray tracing model for the design of femtosecond chirped pulse amplification laser systems

We have used the model to guide us while upgrading our chriped pulse amplification (CPA) laser system to reduce the compressed pulse duration to -40 fs. It has allowed us to choose an initial design for the stretcher and compressor pair and enabled us to optimize the final system configuration.

Design considerations for the stretcher of a 35-fs chirped pulse amplification laser system

We have developed a code which will model different configurations and designs of stretcher and compressor systems. The code allows us to compare the performance of systems in terms of pulse duration, pulse contrast and alignment sensitivity. The results of the code have enabled us to design a chirped pulse amplification system that will stretch and recompress 35 fs pulses.

Explosion of C/sub 60/ irradiated with a high-intensity femtosecond laser pulse

Summary form only given. We have studied the ions generated by the interaction of neutral C/sub 60/ molecules with high intensity femtosecond laser pulses using an ion time of flight spectrometer. The ion fragment distributions were examined over a range of focused intensities using both the fundamental and second-harmonic laser wavelengths of our Ti:sapphire laser system.