F. Vidal

Modeling the triggering of streamers in air by ultrashort laser pulses

The physical processes involved in the triggering of ionization waves (streamers) by ultrashort laser pulses, focused in air at 350 Torr and in a uniform electric field, are investigated by means of a one-dimensional (1-D) numerical model. The model describes the interaction of the laser pulse with air and takes into account many of the reactions in the laser-created plasma as well as the radial expansion of the plasma. Consequences of the model are that the threshold electric field for the appearance of streamers is an increasing function of the delay between the laser pulse and the electric field pulse and a decreasing function of the laser energy. Also, it appears that the electron temperature, the plasma density and radius, and the conduction of heat across the plasma boundaries play major roles in the capacity of the laser-created plasma to trigger streamers. The results of the model are compared with the available experimental data.

Generation of Intense Terahertz Radiation via Optical Methods

The development of new sources in the terahertz (THz) spectral region has attracted much attention over the past 20 years. In particular, the last three years have seen a surge of new laser-based techniques for generating intense, few-cycle THz pulses in the microjoule energy range, thus paving the way to the study of the nonlinear optical properties of various materials at THz frequencies. Simultaneously, innovative solutions for broad-band THz detection were found, allowing one to sense matter in the THz range with an unprecedented time resolution. In this paper, we will attempt to give a review of the properties and characteristics of the recently developed intense THz sources, with a particular eye on their potential application in ultrafast THz nonlinear spectroscopy.

The influence of electron density on the formation of streamers in electrical discharges triggered with ultrashort laser pulses

In an ongoing program using ultrashort laser pulses to provoke discharges in air over considerable distances at electric fields below breakdown threshold, we have studied the conditions for the onset of streamers in such laser-produced plasmas, both experimentally and through numerical simulations. The results demonstrate the importance of the electron density and of its gradient on the generation of streamers. Also, a significant reduction of the breakdown voltage for a 30 cm plane-plane gap in air was observed with a laser pulse energy of 15 mJ. Finally, a direct comparison of laser-induced breakdown in air and in nitrogen shows the influence of electron attachment to oxygen on the discharge process.

Influence of the laser pulse duration on spectrochemical analysis of solids by laser-induced plasma spectroscopy.

Quantitative analysis of aluminum and copper alloys by means of laser-induced plasma spectroscopy (LIPS) has been investigated for three representative laser pulse durations (80 fs, 2 ps, and 270 ps). The experiments were carried out in air at atmospheric pressure with a constant energy density of 20 J/cm2. Because the decay rate of the spectral emission depends on the laser pulse duration, the optimum detection requires an optimization of the temporal gating acquisition parameters. LIPS calibration (sensitivity and nonlinearity) and the limit of detection (LOD) are discussed in detail. While the LOD of minor elements embedded in alloy samples obtained by sub-picosecond or sub-nanosecond laser pulses are both time and element dependent, provided an appropriate temporal window is chosen, the optimum LODs (several parts per million (ppm)) prove to be independent of the laser pulse duration. Finally, it is found that for elements such as those detected here, gated LIPS spectra using picosecond or sub-picosecond laser pulses provide much better LOD values than non-gated spectra.

Study of laser-induced breakdown in a 30-cm air gap under a uniform field

In an ongoing effort to understand how ultrashort pulse lasers can be used to trigger spark discharges in air over considerable distances at electric fields much below that of self-breakdown, we have studied the influence of the laser pulse energy on the development of an electrical discharge over a 30-cm air gap in a uniform field geometry. In addition to the purely electrostatic effects associated with the production of free electrons as the laser beam is focused in the gap, the results highlight the importance of thermal effects to trigger the discharge when the plasma is produced several tens of microseconds before the electric field is applied. One of the most important mechanisms leading to breakdown of the gap is the production of a space leader from the laser-created plasma. Results showing the propagation of such a space leader in a 30-cm gap are presented for the first time.

Relativistic Eulerian Vlasov simulations of the amplification of seed pulses by Brillouin backscattering in plasmas

We apply an Eulerian Vlasov code to study the amplification by Brillouin scattering of a short seed laser pulse by a long pump laser pulse in an underdense plasma. The stimulated Brillouin backscattering interaction is the coupling of the pump and seed electromagnetic waves propagating in opposite directions, and the ion plasma wave. The code solves the one-dimensional relativistic Vlasov-Maxwell set of equations. Large amplitude ion waves are generated. In the simulations we present, the density plateau of the plasma is ne=0.3u2009nc (nc is the critical density), which excludes spurious stimulated Raman scattering amplification (which can occur only if ne<nc/4). We also varied the duration and/or amplitude of the short input seed pulse to study how these influence its subsequent behaviour. An initially broad pulse grows more rapidly than an initially narrow pulse. Furthermore, for an initially broader seed pulse, towards the end of the simulation, it is seen to become narrower and to gradually detach from th...

Complete energy conversion by autoresonant three-wave mixing in nonuniform media

Resonant three-wave interactions appear in many fields of physics e.g. nonlinear optics, plasma physics, acoustics and hydrodynamics. A general theory of autoresonant three-wave mixing in a nonuniform media is derived analytically and demonstrated numerically. It is shown that due to the medium nonuniformity, a stable phase-locked evolution is automatically established. For a weak nonuniformity, the efficiency of the energy conversion between the interacting waves can reach almost 100%. One of the potential applications of our theory is the design of highly-efficient optical parametric amplifiers.

Complete pump depletion by autoresonant second harmonic generation in a nonuniform medium

In this paper, we develop for the first time to our knowledge an analytical theory of second harmonic generation (SHG) in a generic nonuniform χ(2) medium. It is shown that by varying the properties of the medium gradually enough, the system can enter an autoresonant state in which the phases of the fundamental pump and of the generated second harmonic wave are locked. The effect of autoresonance allows efficient transfer of energy between the waves and, due to the continuous phase-locking in the system, all the energy of the pump could be converted to the second harmonic. Simple closed-form expressions for the waves amplitudes as a function of the longitudinal coordinate are derived, and an explicit criterion for the stability of the autoresonant state is obtained. Our analytical theory is compared to the numerical solution of the coupled mode equations, which are found to be in excellent agreement with each other. The analytical closed-form expressions that we derive could be very useful for practical design of SHG devices with increased performances, such as highly efficient, wideband frequency converters.

Autoresonant Harmonic Generation in Nonuniform Crystals

An experiment of second harmonic generation in a nonuniform crystal is presented, and interpreted in terms of an autoresonant wave-mixing theory. A good agreement is found between numerical simulations, analytical solutions and experimental data.

Temporal evolution of the state of local thermodynamic equilibrium of a laser produced plasma at low fluence

Summary form only given, as follows. Of the few studies that have been devoted to examining departure from LTE conditions, several suggest that laser produced plasmas at low fluences could be far from LTE at early times (before typically 1 /spl mu/s). The present work addresses this issue by investigating the departure from LTE conditions in Al plasmas created by a 10 ns excimer laser pulse at a wavelength of 308 nm with fluences of about 10 J/cm/sup 2/. The Boltzmann diagram method is applied to a few lines emitted by iron impurities contained in an Al matrix. Time evolution of the LTE departure is investigated by probing the plume at various delays after the laser pulse interaction with the target.