C. K. Rhodes

Studies of multiphoton production of vacuum-ultraviolet radiation in the rare gases

Measurements of the vacuum-ultraviolet (<80-nm) radiation produced by intense ultraviolet (248-nm) irradiation (1015–1016 W/cm2) of rare gases have revealed the copious presence of both harmonic radiation and fluorescence from excited levels. The highest harmonic observed was the seventeenth (14.6 nm) in Ne, the shortest wavelength ever produced by that means. Strong fluorescence was seen from ions of Ar, Kr, and Xe, with the shortest wavelengths observed being below 12 nm. Furthermore, radiation from inner-shell excited configurations in Xe, specifically the 4d95s5p → 4d105s manifold of Xe7+ at ~17.7 nm, was detected. These experimental findings, in alliance with other studies concerning multielectron processes, give evidence for a role of electron correlations in a direct nonlinear process of inner-shell excitation.

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.

Evidence for coherent electron motions in multiphoton x-ray production from Kr and Xe clusters

Studies of multiphoton-induced X-ray generation in Kr and Xe clusters give direct information concerning (1) the atom-specific energy transfer rate, (2) the dependence of the X-ray yield on the strength of the intra-cluster inelastic electron scattering cross section, and (3) the threshold intensity for X-ray generation. Measurements of these three classes of observables with subpicosecond ( approximately 300 fs) ultraviolet (248 nm) radiation at a maximum intensity of approximately 1019 W cm-2 all indicate that the non-linear coupling to the cluster has an anomalous strength with respect to that derivable from conventional single-particle interactions. Five cases have been examined (Kr(M), Kr(L), Xe(N), Xe(M) and Xe(L)), spectrally spanning the range from approximately 80 eV to approximately 5 keV. In order to reconcile theoretical estimates with these experimental findings, three generalizations of the original formulation of the interaction were necessary. The most important involves an enhancement in the coupling arising from the coherent motion of the (Z) field-ionized electrons induced by the external driving field. The coherently energized electrons act like a quasi-particle possessing a charge Ze and a mass Zm, thereby presenting a sharply augmented coupling resembling that associated with energetic ion-atom collisions. The second involves the process of multiple electron ejection from an inner-shell, a mechanism that was found imperative to interpret the high level of ionization observed in the Xe M-shell. The third modification concerns multiple transits of the driven electrons in the cluster. The inclusion of these considerations consistently brings the theoretical analysis into agreement with the three measured properties. These results also indicate that energy deposition rates exceeding approximately 1 W/atom are feasible in appropriately designed molecules incorporating heavy atoms. The limiting magnitude of the excitation energy Delta Emax characteristic of the coherent coupling is estimated to be in the range Zamc2<or= Delta Emax<or=Z2amc2, a bound that can considerably exceed the K-shell binding energy of the heaviest atoms.

Stability analysis of relativistic and charge-displacement self-channelling of intense laser pulses in underdense plasmas

The stability against small azimuthal perturbations of confined modes of propagation of intense short-pulse radiation governed by relativistic and charge-displacement nonlinearities in underdense plasmas is examined theoretically. In the plane of the dimensionless parameters rho 0 identical to r0 omega p,0/c and eta identical to P0/Pcr, defined by the critical power (Pcr) and the initial conditions represented by the focal radius (r0) of the incident radiation, the unperturbed plasma frequency ( omega p,0), and the peak incident power (P0), zones corresponding to stable (single-channel) and unstable (strong filamentation) regimes of propagation are established. The general finding is that large regions of stable propagation exist. The results show that for values of rho 0 sufficiently close to the dimensionless radius of the zeroth eigenmode rho c,0, the self-channelling is stable for all values of eta >1, a condition of exceptional robustness. It is also found that for the region 1 >1 regime, these results demonstrate the crucial role of the ponderomotively driven charge displacement in stabilizing the propagation. Physically, the ponderomotive radial displacement of the electrons and the contrasting inertial confinement of the ions simultaneously produce the two chief characteristics of the channels. They are the refractive self-focusing of the propagating energy arising from the displaced electrons and the spatial stability of the channels produced by the immobile electrostatic spine formed by the ions.

Multiphoton Ionization of Atoms

Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a20), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.

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

A tunable, ultrahigh spectral brightness KrF* excimer laser source

An extremely high spectral‐brightness ArF* 193‐nm excimer source with the following output pulse characteristics is described: ∼30‐mJ pulse energy, ∼5‐nsec pulse duration, spectral width less than 260 MHz, absolute frequency control to within 1.8 GHz, ∼5×15‐μrad beam divergence, and repetition rate up to 10 Hz. Within the uncertainty of measurement, the spectral width of the output radiation is Fourier transform limited and the beam divergence corresponds to the diffraction limit of the radiating aperture.

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.