Search Results (5 total)

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

We demonstrate that spectral interferometry can be used together with a well-characterized reference pulse to directly determine the spectral amplitude and spectral phase of coherent four-wave-mixing (FWM) emission from multiple quantum wells. The temporal amplitude and phase are then readily obtained by an inverse Fourier transformation. We illustrate that it is necessary to accurately determine the spectral phase, as well as the spectral amplitude, in order to correctly determine the temporal behavior. We subsequently use this technique to measure discontinuities in the temporal phase. Instabilities in the phase occur and phase jumps of ∼π are produced when both the real and imaginary parts of the FWM field are zero. These jumps occur at the minima in the quantum beats, and this condition requires the excitation of approximately equal numbers of heavy- and light-hole excitons.

We report on the polarization dependence of the ionization of helium and neon by 120-fs pulses at 614 nm. For linearly polarized pulses the data for He²⁺ and Ne²⁺ show enhancements over sequential tunneling. For circularly polarized pulses the data show no distinct enhancements. Dynamic resonances, which produce similar enhancements in the multiphoton regime, do not explain the data in the tunneling regime. Two direct-ionization models are considered: a shake-off model, and a semiclassical model for rescattering of an electron by the ion core. The exhibited polarization dependence of the ion yield enhancement is consistent with the semiclassical rescattering mechanism. Definitive isolation of a direct, nonsequential ionization mechanism in the optical tunneling regime requires continued scaling experiments on the laser wavelength, pulse width and ellipticity, and the target species.

We derive in closed form the solutions to the rate equations for sequential field ionization of gases by a focused laser beam. We obtain the low-intensity and the high-intensity limits of the ion yield for each charge state produced by the laser pulse. Furthermore we derive the scaling laws for the appearance intensity and the saturation intensity of the ion yield. We find that the appearance intensity depends not only on the binding energy of the ionized electron but also on the quantum numbers of the shell from which the electron ionizes. We interpret this dependence on the quantum number to be the species dependence in the appearance intensities discussed by Meyerhofer and co-workers [Phys. Rev. Lett. 63, 2212 (1989)]. Formulas for five ionization models are presented.

We have measured the ion yields for helium ionized by 120 fs, 614 nm laser pulses for intensities near 10¹⁶ W/cm². We have found that for these ultrashort pulses the He⁺² data exhibit a feature which saturates in parallel with the He⁺ signal indicating that the ionization may proceed nonsequentially. We propose a new mechanism, which can exist only in the tunneling regime, for such nonsequential ionization.