Search Results (34 total)

We report the results of theoretical and experimental studies on passive spatiotemporal shaping of cw mode-locked picosecond laser pulses for driving the photocathode of a high-brightness, high-current energy recovery linear accelerator. The temporal pulse shape is modified using birefringent crystals, while a refractive optical system is used to generate a flattop spatial beam profile. An optical transport system is designed and implemented to deliver the flattop pulse onto a photocathode sited 9 m away from the shapers. The alignment tolerances on the beam shaper and the temporal pulse stacker have been studied both theoretically and experimentally. The experimental results agree well with theoretical simulations.

We demonstrate that the intense electric field of a subpicosecond single-cycle terahertz pulse can control and manipulate the temporal, spectral, and spatial phase of a copropagating ultrashort laser pulse through the Pockels effect in an electro-optic crystal. In the temporal and spectral domains, the single-cycle THz pulse can impart either a positive or a negative quadratic phase modulation to the probe pulse, leading to a spectral shift, broadening, or pulse expansion or compression. While acting in the spatial domain, the THz-induced phase modulation induces a lenslike effect, providing focusing or defocusing of the copropagating probe beam. The experimental results are in good agreement with simulations. Our study gives a comprehensive picture of the nonlinear spatiotemporal dynamics in the high-field regime driven by the intense single-cycle THz pulse.

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100  μJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.

A Reply to the Comment by R. Bonifacio et al..

In this Letter we report the first experimental characterization of superradiance in a single-pass high-gain free-electron laser (FEL) seeded by a 150 femtosecond (FWHM) Ti:sapphire laser. The nonlinear energy gain after an exponential gain regime was observed. We also measured the evolution of the longitudinal phase space in both the exponential and superradiant regimes. The output FEL pulse duration was measured to be as short as 81 fs, a roughly 50% reduction compared to the input seed laser. The temporal distribution of the FEL radiation as predicted by a numerical simulation was experimentally verified for the first time.

A novel, single shot, nondestructive scheme to measure the bunch length of submillimeter relativistic electron bunches using the electro-optical method is described. In this scheme, the birefringence induced by the electric field of the electrons converts the temporal characteristics of the bunch to a spatial intensity distribution of an optical pulse. Electric field characteristics, induced birefringence, and retardation are calculated for a few typical electron beam parameters and criteria limiting the resolution are established.

We observe optical third- and fifth-harmonic generation to wavelengths of ultraviolet and vacuum ultraviolet at an unconventional vacuum-glass interface without plasma formation. We extend this harmonic-generation technique to various other interfaces and suggest that odd-multipole harmonic generation at an interface by a focused light beam is a general phenomenon. © 1996 The American Physical Society.

Optical third-harmonic generation (THG) is generally a weak process but is dipole allowed, therefore it occurs in all materials, including dielectric materials with inversion symmetry. We report that when using focused high-intensity ultrashort laser pulses, this normally weak THG process becomes highly operative at a simple air-dielectric interface and is much stronger than the bulk of most dielectric materials. We characterized this nonlinear optical response at interfaces as a phenomenological surface-enhanced THG in transmission and/or in reflection. This surface THG is further cascaded in transmission or reflection from layered composite dielectric materials of a high-low index of refraction, resulting in a marked increase in photon conversion efficiency than that of a single interface. Although the present THG efficiency is lower than that of a typical phase-matching harmonic crystal, it is important to note that the surface-enhanced optical THG is a fundamental physical process occurring at all interfaces and is relatively free from the constraint of a phase-matching condition and wavelength restriction. Using optical THG at an interface, it becomes possible to generate wavelengths at which harmonic crystals are unavailable. These findings may lead to a new development of surface-enhanced studies and prompt a reexamination of the processes of high-harmonic generation at interfaces using a focused beam.

We report a study of surface-plasmon-mediated multiphoton photoelectric emission from thin films of Ag, Au, Cu, and Al. The experiments were performed in the Kretchmann attenuated-total-internal reflection geometry while the excitation source was an unamplified femtosecond colliding-pulse mode-locked ring laser. Contrast to the electron emission obtained by irradiating the laser on a metal surface, electron yield increases by several orders of magnitude with fairly high quantum efficiency, is observed when photons are coupled to the surface-plasmon modes of these films. Although the photon absorption reaches its maximum when the reflectivity exhibits a deep minimum at the surface-plasmon resonance angle, it is found that the maximum electron yield occurs at a slightly different angle than the reflectivity dip. The results of these measurements favor the field-density calculations using the Fresnel equations. The width of the electron temporal profile, measured utilizing this nonlinear photoelectric effect, however, fails to show the narrowing commensurate with the higher-order nonlinearity.

The low-temperature heat capacity of electrotransport-purified scandium, yttrium, gadolinium, and lutetium have been measured from 1 to 20 K. The electronic specific-heat constant γ and the Debye temperature CTHETA_D are determined to be 10.334±0.011 mJ/g-at. K² and 354.3±1.0 K, respectively, for scandium, 7.878±0.004 mJ/g-at. K² and 244.4±0.5 K, respectively, for yttrium, 6.380±0.026 mJ/g-at. K² and 163.4±0.1 K, respectively, for gadolinium, and 8.194±0.016 mJ/g-at. K² and 183.2±0.3 K, respectively, for lutetium. We believe that the above electronic specific-heat constants and Debye temperatures represent the intrinsic values for these four rare-earth metals. The use of the low-temperature heat-capacity results for these metals to evaluate the various contributions to the heat capacities of the magnetic lanthanide metals is examined. The total many-body enhancement factor and its components (the electron-phonon, electron-paramagnon, and spin-wave) are calculated.

The low-temperature (1.3-20.0 K) high-magnetic-field (0-10 T) heat capacity and the magnetization and magnetic susceptibility (1.7-300 K) of the strongly Pauli paramagnetic RCo₂ (R=Sc, Y, or Lu) compounds with the MgCu₂-type structure were measured. The heat-capacity results for ScCo₂, YCo₂, and LuCo₂ show that the electronic specific-heat constant decreases with increasing magnetic fields (by 7%, 4%, and 10%, respectively, at 10 T). For YCo₂ the coefficient of the T³ term (β) in the heat capacity is found to increase by 18% at 10 T, but for ScCo₂ and LuCo₂ β remains constant within experimental error. Analyses based on several theoretical models of the quenching of spin fluctuations by high magnetic fields suggest that the characteristic spin-fluctuation temperature is ∼20 K for ScCo₂, ∼35 K for YCo₂, and ∼16 K for LuCo₂. The magnetization and the field dependence of the magnetic susceptibility of the same samples as used in the heat-capacity measurements indicate the presence of ferromagnetic impurities in the samples, but the estimated concentrations are sufficiently low that they probably have no effect on the observed heat capacities. Maxwell's thermodynamics relationship between the field dependence of the heat capacity and the temperature dependence of the magnetic susceptibility has been examined.

The low-temperature high-field heat capacity of a CeSn₃ single crystal was found to develop an anisotropy for H>2.50 T with the magnetic field parallel to the [100] and [110] directions. The magnetic susceptibility of CeSn₃ below 5 K was also anisotropic with χ₁₁₀>χ₁₀₀ by ∼ 5% at 1.8 K. For the first time ever an anisotropic quenching of spin fluctuations in any highly enhanced paramagnet has been observed.

We have studied the L₃MM Auger spectra of V and Cr compounds where V and Cr exist in their maximal-valent state, with a nominally 3d⁰ electronic configuration. Using argon-ion bombardment as a means of in situ reduction of the valence state of V and Cr in these compounds and hence increasing the 3d character in the valence band, we show that this change causes a pronounced enhancement of the L₃M₂₃M₄₅ and L₃M₄₅M₄₅ transitions relative to the L₃M₂₃M₂₃ transition. Thus the relative intensities of these Auger transitions may be used as a sensitive indicator of the 3d character in the valence band. Such enhancement may also be viewed as a relative shift of the L₃M₂₃M₄₅ and L₃M₄₅M₄₅ transitions from what may be thought of as an interatomic to an intra-atomic character.

The low-temperature heat capacity of nineteen Sc-Zr alloys with compositions ranging from pure Sc to 34-at.% Zr in Sc and five Sc-Mg alloys with magnesium concentrations less than 8 at.% were measured from 1 to 20 K. Zirconium additions up to 0.5 at.% raises the electronic specific-heat constant (γ); then it decreases monotonically with further zirconium additions up to 34-at.% Zr. The effect of magnesium is to decrease the electronic specific-heat constant slowly. The heat-capacity results were used to calculate an experimental density of states (DOS) curve for scandium. The theoretical DOS curves are significantly different from the experimental DOS curve. The various enhancement factors have been calculated using these DOS curves. A change in slope in γ at ∼ 2-at.% Mg and the peak in γ at 0.5-at.% Zr are probably associated with the second energy band crossing the Fermi energy at the points H and L in the Brillouin zone as the electron concentration is lower or raised, respectively. The effect of dilute amounts of zirconium and magnesium on the Debye temperature (Θ_D) of scandium is unusual, initially rising for zirconium additions and falling for magnesium additions. At high solute concentrations Θ_D rises and falls more or less in a "saw-tooth"-like fashion. It is suggested that this variation in Θ_D is due to the formation of short-range-order configurations of Sc impurity atoms in the solid solution alloys.

The two-dimensional structures of adsorbed argon layers have been described by a modified cell theory with the cell-size variations evaluated by a self-consistent condition. The calculated lattice parameters and the atomic-vibration amplitudes at various temperatures are in satisfactory agreement with neutron scattering data of argon adsorbed on Grafoil substrate. The liquid phase and the melting transition may be absent in the two-dimensional systems. The calculation also indicates a transition between commensurate and incommensurate lattices for argon on Grafoil near 50°K, in agreement with experimental observations.

It is proposed that regions of low and high mass density exist in liquid helium and moreover are spatially well-ordered in the superfluid state with the result that the ground-state wave function Ψ₀ is periodic. A ground-state energy 8.1% lower than that determined by Monte Carlo procedure for a pair-correlated wave function f(r_ij) by McMillan is obtained with Ψ₀=[Πi>j[f(r_ij)]][1+B Σi>jcos2k₀r_ij/k₀r_ij] with B=0.054π and k₀=2π/6.8 Å^-1. A mean-field theory of the λ transition has been formulated. The nature of the superfluid state is a consequence of the spatial periodicity of the wave function and a simple physical picture of the roton is presented as well as a rigorous roton wave function.

Starting from the Lennard-Jones interatomic potential, a modified cell theory has been used to describe the solid-liquid phase transition in argon. The cell-size variations may be evaluated by a self-consistent condition. With the inclusion of cell-size variations, the transition temperature, the solid and liquid densities, and the liquid-phase radial-distribution functions have been calculated. These ab initio results are in satisfactory agreement with molecular-dynamics calculations as well as experimental data on argon.

For any given velocity autocorrelation function, the incoherent scattering functions and the density self-correlation functions are uniquely determined in the Gaussian approximation. Since the natures of velocity autocorrelation functions have already been clarified, it is only necessary to evaluate the non-Gaussian correction terms. We have related these corrections to the deviations of partially-averaged mean-square displacements from the canonical average. Two time constants are involved: the collision time and the mixing time, the latter being much longer (by a factor of 50) than the former. The results are in qualitative agreement with molecular dynamics as well as neutron inelastic scattering data for liquid argon.

By a self-consistent procedure, the velocity autocorrelation functions of both liquid and gaseous argon have been calculated without introducing any arbitrary parameters. The results are in satisfactory agreement with computer experiments. The correlation functions are primarily determined by the nearest-neighbor coordination. Because of the strong hard-core repulsion, the behaviors are more vibratory and damp out quickly at high densities and are more diffusive at lower densities.

The L₃M₂₃M₂₃, L₃M₂₃M₄₅, and L₃M₄₅M₄₅ Auger spectra of metals with both open and filled 3d bands are presented and analyzed in detail. From these experimental data it is possible to derive two parameters, a hole localization parameter λ_d and an energy parameter Δε, which can completely characterize the Auger spectra of the entire 3d transition-metal series. λ_d gives the quantitative extent of the delocalization of the 3d electrons in metals with unfilled 3d bands. Through a knowledge of λ_d it becomes possible to quantitatively evaluate the contributions of (i) the hole-hole interaction and (ii) the relaxation, to the total energy difference Δε between single- and double-hole final states. Furthermore these parameters are correlated with the quasiatomic vs band characteristics of Auger electrons.

The heat capacity of a carefully characterized β-Ce sample was measured from 1 to 20 K. The present results are in excellent agreement with the earlier data from 1.5 to 12.8 and 15.8 to 20 K. Between 12.8 and 15.8 the present results are ∼4% higher than the earlier values and this is thought to be due to less lattice strains in the present sample. The magnetic heat capacity below 1.33 K exhibits an exponential behavior, yielding an energy gap of 1.41 ± 0.02 K.

X-ray photoelectron and Auger spectra involving N₁, N₂, and N₃ vacancy states of Pd, Ag, Cd, In, and Sn were measured and compared with results of free-atom calculations. As previously observed in Cu and Zn Auger spectra that involve 3d-band electrons, we now also find free-atom characteristics, with regard to widths and structure, in the Ag and Cd M₄-N_4,5N_4,5 and M₅-N_4,5N_4,5 Auger spectra that arise from transitions of 4d-band electrons. Theoretical N₁ widths computed with calculated free-atom Auger energies agree well with measurements. Theory, however, predicts wider N₂ than N₃ vacancy states (as observed for Xe), while the measured N₂ and N₃ widths are nearly equal to each other and to the average of the calculated N₂ and N₃ widths. The calculations are made difficult by the exceedingly short lifetime of some 4p vacancies and by the extreme sensitivity of super-Coster-Kronig rates, which dominate the deexcitation to the transition energy and to the fine details of the atomic potential.