J. P. Palastro

Excitation of terahertz radiation by laser pulses in nonuniform plasma channels

The excitation of terahertz radiation by laser pulses propagating in miniature plasma channels is considered. Generation of radiation by laser pulses in uniform plasmas is generally minimal. However, if one considers propagation in corrugated plasma channels, conditions for radiation generation can be met due to the inhomogeneity of the channel and the presence of guided waves with subluminal phase velocities. It is found that for channels and laser pulses with parameters that can be realized today, energy conversion rates of a fraction of a joule per centimeter can be achieved. Miniature corrugated channels can also be used for creation of THz radiation by bunched electron beams.

Propagation of Bessel and Airy beams through atmospheric turbulence

We investigate, through simulation, the modifications to Bessel and Airy beams during propagation through atmospheric turbulence. We find that atmospheric turbulence disrupts the quasi-non-diffracting nature of Bessel and Airy beams when the transverse coherence length (Fried parameter) nears the initial aperture diameter or diagonal, respectively. The turbulence-induced transverse phase distortion limits the effectiveness of Bessel and Airy beams for applications requiring propagation over long distances in the turbulent atmosphere.

Resonant heating of a cluster plasma by intense laser light

Cluster heating by a strong laser field is studied using a particle-in-cell code (PIC) for a range of intensities and cluster sizes. Above a threshold intensity, heating is dominated by a nonlinear resonant absorption process.

The extreme nonlinear optics of gases and femtosecond optical filamentationa)

Under certain conditions, powerful ultrashort laser pulses can form greatly extended, propagating filaments of concentrated high intensity in gases, leaving behind a very long trail of plasma. Such filaments can be much longer than the longitudinal scale over which a laser beam typically diverges by diffraction, with possible applications ranging from laser-guided electrical discharges to high power laser propagation in the atmosphere. Understanding in detail the microscopic processes leading to filamentation requires ultrafast measurements of the strong field nonlinear response of gas phase atoms and molecules, including absolute measurements of nonlinear laser-induced polarization and high field ionization. Such measurements enable the assessment of filamentation models and make possible the design of experiments pursuing applications. In this paper, we review filamentation in gases and some applications, and discuss results from diagnostics developed at Maryland for ultrafast measurements of laser-gas ...

Effect of two-beam coupling in strong-field optical pump-probe experiments

Nonlinear optics experiments measuring phase shifts induced in a weak probe pulse by a strong pump pulse must account for coherent effects that only occur when the pump and probe pulses are temporally overlapped. It is well known that a weak probe beam experiences a greater phase shift from a strong pump beam than the pump beam induces on itself. The physical mechanism behind the enhanced phase shift is diffraction of pump light into the probe direction by a nonlinear refractive index grating produced by interference between the two beams. For an instantaneous third-order response, the effect of the grating is to simply double the probe phase shift, but when delayed nonlinearities are considered, the effect is more complex. A comprehensive treatment is given for both degenerate and nondegenerate pump-probe experiments in noble and diatomic gases. Results of numerical calculations are compared to a recent transient birefringence measurement [Loriot et al., Opt. Express 17, 13429 (2009)] and a recent spectral interferometry experiment [Wahlstrand et al., Phys. Rev. A85, 043820 (2012)]. We also present results from two new experiments using spectrally resolved transient birefringence with 800 nm pulses in Ar and air and degenerate chirped pulse spectral interferometry in Ar. Both experiments support the interpretation of the negative birefringence at high intensity as arising from a plasma grating.

Pulsed mid-infrared radiation from spectral broadening in laser wakefield simulations

Spectral red-shifting of high power laser pulses propagating through underdense plasma can be a source of ultrashort mid-infrared (MIR) radiation. During propagation, a high power laser pulse drives large amplitude plasma waves, depleting the pulse energy. At the same time, the large amplitude plasma wave provides a dynamic dielectric response that leads to spectral shifting. The loss of laser pulse energy and the approximate conservation of laser pulse action imply that spectral red-shifts accompany the depletion. In this paper, we investigate, through simulation, the parametric dependence of MIR generation on pulse energy, initial pulse duration, and plasma density.

THz generation by optical Cherenkov emission from ionizing two-color laser pulses

Two-color photoionization produces a cycle-averaged current driving broadband, conically emitted THz radiation. We investigate, through simulation, the processes determining the angle of conical emission. We find that the emission angle is determined by an optical Cherenkov effect, where the front velocity of the current source is faster than the THz phase velocity.

Studies of spectral modification and limitations of the modified paraxial equation in laser wakefield simulations

Laser pulses propagating through plasma undergo spectral broadening through local energy exchange with driven plasma waves. For propagation distances on the order of the energy depletion length, spectral shifts can be comparable to the laser central frequency and wavenumber, a result of approximate action conservation. Here, we examine the local spectral shift, energy depletion, and action conservation of nonlinear laser pulses using the modified paraxial simulation code WAKE. Breakdown of the modified paraxial equation (MPE), which is based on the assumption of slow temporal variation of the pulse envelope, is monitored via consideration of the wave action. Although action is theoretically conserved for the continuous MPE, we observe that for large red shifts, action decays for the discrete implementation. Numerical analysis of the propagation algorithm verified the observed behavior. Increasing resolution improves action conservation up to a time, which is identified to be the validity limit for the MPE...

Thomson-scattering measurements in the collective and noncollective regimes in laser produced plasmas (invited).

We present simultaneous Thomson-scattering measurements of light scattered from ion-acoustic and electron-plasma fluctuations in a N(2) gas jet plasma. By varying the plasma density from 1.5×10(18) to 4.0×10(19) cm(-3) and the temperature from 100 to 600 eV, we observe the transition from the collective regime to the noncollective regime in the high-frequency Thomson-scattering spectrum. These measurements allow an accurate local measurement of fundamental plasma parameters: electron temperature, density, and ion temperature. Furthermore, experiments performed in the high densities typically found in laser produced plasmas result in scattering from electrons moving near the phase velocity of the relativistic plasma waves. Therefore, it is shown that even at low temperatures relativistic corrections to the scattered power must be included.

Efficient simulation of electron trapping in laser and plasma wakefield acceleration

The two-dimensional quasistatic simulation code WAKE [P. Mora and T. Antonsen, Phys. Plasmas 4, 217 (1997)] used to model laser pulse propagation in tenuous plasma is modified to describe the dynamics of energetic particles. In the original code, all particles were assumed to satisfy the quasistatic approximation, which assumes that the driver and its wakefields are undisturbed during the transit time of plasma electrons through the pulse. Here, WAKE is modified to include the effects of electron trapping and beam loading by introducing a population of beam electrons, which are no longer subject to the quasistatic approximation. Algorithms for populating the group of beam particles are considered and are benchmarked to the full particle-in-cell simulations and experimental results. These modifications to WAKE provide a tool for simulating GeV laser or plasma wakefield acceleration on desktop computers.