T. S. Luk

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.

Picosecond, tunable ArF* excimer laser source

A 40‐mJ ArF* laser with pulse duration ∼10 ps and spatial and spectral properties close to the transform limits is described. Substantial extraction of the available energy from the final amplifier is demonstrated, a fact providing direct evidence against the presence of significant nonlinear losses in the amplifying medium up to an intensity of ∼1 GW/cm.2

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.

Interaction of atomic and molecular systems with high-intensity ultraviolet radiation

The interaction of atomic and molecular species with picosecond ArF* laser radiation is studied at intensities up to 1015 W/cm2. Anomalously strong, collision-free multiple ionization is observed. Standard theoretical models of stepwise ionization fail to describe the results. The experimental findings point to a collective response of the atom. At intensities of ~1013 W/cm2, selective multiquantum excitation of autoionizing states in Kr, followed by stimulated emission at wavelengths as short as 91.6 and 93 nm, is observed. The 93-nm radiation is tunable over a 600-cm−1 interval, whereas the 91.6-nm frequency is fixed. It appears that electron collisions redistribute energy among excited states.

High-brightness subpicosecond terawatt KrF* system driven with a frequency-converted self-mode-locked pulse-compressed Ti:Al 2 O 3 laser

The phase control achievable in a subpicosecond self-mode-locked pulse-compressed Ti:Al2O3 system permits the production of a seed beam for a KrF* (248-nm) system operating near the power limit determined by self-phase modulation in the optical window material used. These limits are ~14 and ~30 GW/cm2 for CaF2 and MgF2 optics, respectively. We show experimental results for CaF2, which at 248 nm demonstrate a pulse length of ~270 fs, a pulse energy of ~240 mJ, and a resulting brightness of ~2.9 × 1021 W cm−2 sr−1.

Third-harmonic generation using an ultrahigh-spectral-brightness ArF* source

Tunable coherent radiation in the vicinity of 64 nm has been produced by third-harmonic generation of the output of an ultrahigh-spectral-brightness ArF*source in various simple gaseous media. The following performance parameters of the ArF* system are used: pulse energy, ~300 mJ; pulse duration, ~7 nsec; spectral width, <260 MHz; beam divergence, ~5microradx15microrad; and repetition rate up to 10 Hz. Third-harmonic generation with peak powers up to 30 W has been observed in an apparatus that eliminates photoabsorption of the third harmonic in the differential pumping stages.

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.

Measurement of energy penetration depth of subpicosecond laser energy into solid density matter

The energy penetration depth characteristic of the interaction of intense subpicosecond (∼600 fs) ultraviolet (248 nm) laser radiation with solid density material has been experimentally determined. This was accomplished by using a series of ultraviolet transmitting targets consisting of a fused silica (SiO2) substrate coated with an 80–600 nm layer of MgF2. The measurement of He‐like and H‐like Si and Mg lines, as a function of MgF2 thickness, enabled the determination of the energy penetration depth. It was found that this depth falls in the range of 250–300 nm for a laser intensity of ∼3×1016 W/cm2. Based on numerical simulations, it is estimated that solid density material to a depth of ∼250 nm is heated to an electron temperature of ∼500 eV.

Subpicosecond ultraviolet multiphoton electron spectroscopy of rare gases

The line positions and linewidths of photoelectron spectra arising from multiphoton ionization are analyzed for the first reported time in the subpicosecond (0.5–1.0-psec) region. Measurements performed with 248-nm radiation at intensity level for which the quiver energy of the free-electron motion is comparable with the photon energy are an reported. The short-pulse property causes the photoelectron energy to become a dynamical variable associated with the process of ionization and significantly influences both the line positions and the linewidths of the observed electron spectra.