A. McPherson

Subpicosecond KrF* excimer-laser source.

A subpicosecond KrF* laser system capable of producing 20 +/- 2-mJ pulses has been developed. The means of producing ultrashort seed pulses for the KrF* amplifier system and characteristics of the full system are described. It is shown that efficient subpicosecond energy extraction is possible.

Stable self-channeling of intense ultraviolet pulses in underdense plasma, producing channels exceeding 100 Rayleigh lengths

Spatially confined propagation of high-power subpicosecond (~270-fs) ultraviolet (248-nm) pulses has been experimentally studied in cold underdense plasma. The observed channels were longitudinally uniform, were approximately 1.4 μm in diameter, and persisted for a length of 3–4 mm, a distance exceeding 100 Rayleigh ranges. X rays with a quantum energy > 0.5 keV were also detected from the zone of propagation in coincidence with the channel formation. The occurrence of self-channeling with the rapid formation of a stable, extended, and longitudinally homogeneous filament is in qualitative agreement with a theoretical picture involving relativistic and charge-displacement nonlinearities.

Dynamical orbital collapse drives super x-ray emission

Experimental studies of the characteristics of Xe(M) emission ( - 19 A) produced by multiphoton excitation of Xe clusters indicate that the nonlinear interaction automatically acts as a template leading to the preparation of the maximally radiating configurations for that spectral range. The new mechanism deduced is a general multiphoton multi-electron process which dynamically combines rapid multiphoton ionization, 4f-orbital collapse, and correlated electron motion.

Ultrahigh-intensity KrF* laser system

The operational characteristics of an ultrahigh-intensity subpicosecond large-aperture KrF* laser system are described. Measurements show the achievement of a focal spot diameter of less than 1.7 microm. Combined with measurements of the pulse width and pulse energy, this yields an average intensity of ~2 x 10(19) W/cm(2), a value corresponding to a peak electric field of ~24 (e/a(0)(2)). Light sources of this nature will find application in a broad range of studies of the nonlinear properties of matter in the strong-field regime.

Multiphoton-induced X-ray emission and amplification from clusters

The development of a unified picture of short-pulse high-intensity multiphoton processes, embracing atoms, molecules, and solids, appears possible through the study of clusters. Of particular significance are possible intra-cluster processes that can influence the mechanism of ionization and lead to the production of inner-shell vacancies. Inner-shell excitation leading to prompt X-ray emission is specifically considered and the treatment leads to the definition of a critical cluster size nc representing the achievement of maximal X-ray emission from the ensemble. These results suggest the possibility of designing a new class of molecular materials optimized for the efficient production and amplification of X-rays.

Wavelength dependence of multiphoton-induced Xe(M) and Xe(L) emissions from Xe clusters

A direct comparative measurement of the dependence on the wavelength of irradiation of the kilovolt x-ray yields ( and ) multiphoton-induced from Xe clusters by excitation with intense femtosecond pulses at 248 and 800 nm has been made. The spectroscopic findings demonstrate that both the Xe(M) and Xe(L) emissions are strongly reduced with excitation at the longer wavelength (800 nm). The peak strengths of the Xe(M) and Xe(L) emissions are diminished by factors of and , respectively. Significant spectral differences are also observed. This sharp reduction in the amplitude of the excitation is in conflict with a thermal model for the production of kilovolt x-rays (Xe(M) and Xe(L)) from multiphoton 248 nm excited Xe clusters. These results are consistent with a dynamical mechanism of enhanced coupling which involves ordered many-electron motions in which a dephasing of the electrons can appreciably influence both the amplitude of excitation and the threshold intensity for inner-shell vacancy production. Within the framework of this picture, these experimental findings indicate an effective dephasing time for Xe clusters of - 2 fs, a range that is consistent with the measured k-space scattering dynamics of carriers in GaAs.

Z - imaging of Xe(M) and Xe(L) emissions from channelled propagation of intense femtosecond 248 nm pulses in a Xe cluster target

The spatial image of the spectrum of Xe(M) and Xe(L) emissions produced by the development of stable channelled propagation of intense femtosecond ( fs) 248 nm pulses propagating through a gas-phase Xe cluster target has been obtained. Analysis (i) shows that the observed channel length exceeds 20 Rayleigh ranges, (ii) reveals enhanced emission from the channel by very highly excited species such as Xe ions having double 2p-vacancies A), (iii) gives spectral evidence for a substantial departure of the excitation from conditions of thermal equilibrium in the channel, (iv) confirms earlier studies which indicated the development of intensities of in the channel, and (v) provides additional evidence for the role of an anomolously strong multiphoton coupling in the generation of the observed X-ray emission.

Pump laser wavelength-dependent control of the efficiency of kilovolt x-ray emission from atomic clusters

An explanation is presented for the recently reported striking differences in the kilovolt Xe L-shell x-ray emission from Xe cluster targets excited by comparable terawatt ultraviolet (248 nm) and infrared (800 nm) femtosecond laser pulses under nearly identical experimental conditions (Kondo K et al 1997 J. Phys. B.: At. Mol. Opt. Phys. 30 2707-16). A classical analysis of these results, within the framework of the first Born approximation for electron-atom collisions producing inner-shell ionization, strongly suggests that both the times stronger Xe(L) emission under ultraviolet laser excitation and the observed differences in the x-ray spectra are caused primarily by the different ultraviolet and infrared pump laser wavelengths. The kinematics of photoionized electrons in the intense laser fields (-) and the Coulomb-driven expansion of the electron distribution photoionized from the atomic cluster both indicate that the strong pump-laser wavelength scaling in the production of kilovolt x-rays from Xe clusters results from the more localized and controlled electron-cluster interactions afforded by a shorter optical period.

Intensity dependence of the multiphoton-induced Xe(L) spectrum produced by subpicosecond 248 nm excitation of Xe clusters

Examination of the intensity dependence in the range of the multiphoton-induced Xe(L) spectrum from Xe clusters has demonstrated (i) a very high sensitivity for the production of ions having multiple vacancies and and (ii) a reduction of the average charge state of the emitting ions over the range of intensities in which the multiple-vacancy excitations are diminished. These results give evidence for a dynamical enhancement of the coupling strength arising from ordered many-electron motions induced by the incident 248 nm wave and indicate that the enhanced interaction can lead to a regime of strong coupling in which multiple electron ejection from inner shells can occur with substantial probability. Furthermore, the results are found to be in agreement with previous analyses of multiple electron processes, which involved comparisons with experimental data stemming from both electron (EBIT) and ion - atom collisional studies. The specific comparison with known properties of ion - atom ( + Ne) charge exchange collisions indicates that the multi-electron channels can dominate the multiphoton interaction if a threshold intensity is exceeded. The experimental results give evidence that this threshold falls in the 0.4 - range at 248 nm for the production of 2p vacancies in Xe clusters. These findings provide an experimental basis for the general expectation that a strengthened multi-electron coupling will govern the production of ions with deeply bound (>10 - 20 keV) multiple-vacancy states in clusters of heavy atoms irradiated at 248 nm with intensities above .

Dynamic absorption effects in KrF* amplifiers

The results of transient loss measurements performed in a self-sustained discharge KrF* amplifier are reported. Analysis of these results gives a minimum value of 20 for the effective gain to loss ratiog0/αeff, indicating that efficient extraction of energy in subpicosecond KrF* amplifiers in the ∼1 J range should be achievable.