W.A. Schroeder

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.

Bifurcation mode of relativistic and charge-displacement self-channelling

Stable self-channeling of ultra-powerful (P{sub 0} - 1 TW -1 PW) laser pulses in dense plasmas is a key process for many applications requiring the controlled compression of power at high levels. Theoretical computations predict that the transition zone between the stable and highly unstable regimes of relativistic/charge-displacement self-channeling is well characterized by a form of weakly unstable behavior that involves bifurcation of the propagating energy into two powerful channels. Recent observations of channel instability with femtosecond 248 nm pulses reveal a mode of bifurcation that corresponds well to these theoretical predictions. It is further experimentally shown that the use of a suitable longitudinal gradient in the plasma density can eliminate this unstable behavior and restore the efficient formation of stable channels.

Dynamics of optimized stable channel formation of intense laser pulses with the relativistic/charge-displacement mechanism

Studies of the dynamics of stable relativistic/ponderomotive channel formation demonstrate that the use of an appropriate longitudinal gradient in the electron density can significantly enhance the efficiency of the power compression. A unidirectional stable zone locking rule, which allows the operating point of the system to enter the region of stable channelled propagation, but blocks departures from it, is established. These characteristics are extremely favourable for kilovolt x-ray amplification, charged-particle acceleration and the initiation of nuclear reactions.

A multi-kilohertz electro-optic switch for ultrafast laser systems

Summary form only given.We report on the use of a multi-kilohertz intracavity electro-optic switch to both efficiently cavity-dump a femtosecond Ti:sapphire oscillator and perform the switching for a high-repetition-rate Ti:sapphire regenerative amplifier. The polarization switch consists of a transverse-field KD*P Pockels cell and a novel near-Brewster sapphire Rochon polarizer. In contrast to more conventional longitudinal Pockels cells with dielectric thin-film polarizers, this switch is capable of 50 kHz operation and near 100% switching efficiency due to the Rochon polarizers high extinction ratio (>10/sup 4/:1) and low insertion loss (nominally 100% transmission for the undeviated p-polarization and 100 nm about the central design wavelength), and high damage threshold (>10 GW/cm/sup 2/) are attractive properties for ultrafast laser systems, especially regenerative amplifiers.

High-power, femtosecond, thermal-lens-shaped Yb:KGW Laser

Thermal lens shaping for astigmatism compensation is extended to a 65 MHz, diode-pumped Yb:KGW oscillator mode-locked with the aid of a saturable Bragg reflector to yield 250fs (347fs) pulses at an output power of 3.5 W (5W).

L-shell emission from high-Z solid targets by intense (10/sup 19/ W/cm/sup 2/) irradiation with a 248 nm laser

Efficient (1.2% yield) multikilovolt x-ray emission from Ba(L) (2.4--2.8{angstrom}) and Gd(L) (1.7--2.1{angstrom}) is produced by ultraviolet (248nm) laser-excited BaF{sub 2} and Gd solids. The high efficiency is attributed to an inner shell-selective collisional electron ejection. Much effort has been expended recently in attempts to develop an efficient coherent x-ray source suitable for high-resolution biological imaging. To this end, many experiments have been performed studying the x-ray emissions from high-Z materials under intense (>10{sup 18}W/cm{sup 2}) irradiation, with the most promising results coming from the irradiation of Xe clusters with a UV (248nm) laser at intensities of 10{sup 18}--10{sup 19}W/cm{sup 2}. In this paper the authors report the production of prompt x-rays with energies in excess of 5keV with efficiencies on the order of 1% as a result of intense irradiation of BaF{sub 2} and Gd targets with a terawatt 248nm laser. The efficiency is attributed to an inner shell-selective collisional electron ejection mechanism in which the previously photoionized electrons are ponderomotively driven into an ion while retaining a portion of their atomic phase and symmetry. This partial coherence of the laser-driven electrons has a pronounced effect on the collisional cross-section for the electron ion interaction.

Theoretical/experimental studies of ultraviolet high-power-density self-trapped channels

Experimental evidence indicates that the dynamics of channeled propagation arising from a relativistic/charge-displacement mechanism can produce exceptionally high intensities (>10{sup 20} W/cm{sup 2}) and power densities (>10{sup 19} W/cm{sup 3}) under spatially well-controlled conditions in the interior of the channel. This process opens up a new domain of high intensity interactions in which controllable field strengths in the 10-50(e/a{sub 0}{sup 2}) range can be applied to materials. Furthermore, these field strengths can be developed on very fast rise times ({approx}1-3 fs), since the dynamical response of the channel formation can be on the order of the plasma time {approx}2{pi}/{omega}{sub p}. Under these conditions, matter is expected to respond in a highly complex and nonlinear way. Of particular significance are (1) the detailed dynamics of the channeling mechanism, including the wavelength scaling, and (2) the development of advanced analytical tools to represent the interactions theoretically.

Multiterawatt Ultraviolet Lasers

The advances in ultrashort pulse generation and chirped pulse amplification (CPA) techniques have been the driving force in the development of laser systems with peak powers at and beyond the Terawatt (1012 W) level1–7. Most high power systems developed to date, work in the infrafed, and Terawatt and Petawatt (1015 W) class lasers have been demonstrated in Ti:Sapphire, Nd:glass and Cr:LISAF based systems. These systems are moving towards shorter pulse durations, now commonly of the order of tens of femtoseconds, and design goals are heading towards high repetition rate (kHz)8 multiterawatt systems. The progress in this field has certainly been inspired by the improvements in solid-state laser materials for ultrashort pulse generation. An analogous statement does not generally hold, however, for the ultraviolet wavelengths.