Showing papers by "Gerard Mourou published in 1994"

Laser‐induced breakdown by impact ionization in SiO2 with pulse widths from 7 ns to 150 fs

Abstract: Results of laser‐induced breakdown experiments in fused silica (SiO2) employing 150 fs–7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse‐width range. Fluence breakdown threshold does not follow the scaling of Fth∼ √τp, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to ∼3×108 V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.

Terawatt to Petawatt Subpicosecond Lasers

Abstract: The application of the chirped-pulse amplification technique to solid-state lasers combined with the availability of broad-bandwidth materials has made possible the development of small-scale terawatt and now even petawatt (1000-terawatt) laser systems. The laser technology used to produce these intense pulses and examples of new phenomena resulting from the application of these systems to atomic and plasma physics are described.

Hybrid grating - prism stretcher - compressor system with cubic phase and wavelength tunability and decreased alignment sensitivity.

Abstract: A novel stretcher–compressor design for chirped-pulse amplification systems is presented. The stretcher, which incorporates high-groove-density gratings and an intragrating prism pair, is shown to be highly cubic-phase tunable. Using a hybrid stretcher with 2400-line/mm gratings and off-the-shelf prisms, we have demonstrated amplification of sub-100-fs pulses. In addition, this cubic-phase tunability combined with the broadband efficiency of high-groove-density (2000 lines/mm) gratings allows for the possibility of a system that is tunable in wavelength.

Photoconductive element and method for measuring high frequency signals

Abstract: A photoconductive method or apparatus for measuring high frequency signals using a relatively inexpensive photoconductive material with a relatively long duration or recombination time. The photoconductive method or apparatus utilizes a photoconductive element of photoconductive material such that the duration or recombination time of the photoconductive material is longer than the pulse width of the signal to be measured. As such, the photoconductive element produces a step function response to the signal to be measured rather than a rectangular response with respect to the signal to be measured. The photoconductive element avoids the need to sacrifice measurement sensitivity by introducing defects in the photoconductive material to shorten the recombination time or duration. The measurement bandwidth of the photoconductive element is not limited by the recombination time of the photoconductive material which is mostly dominated by the carrier lifetime. The measurement bandwidth for the photoconductive element is limited by the pulse width of the optical signal driving the photoconductive element which can be very short duration thereby providing improved resolution. As such, the photoconductive element can comprise a cheaper photoconductive material which has a longer duration or recombination time but with a higher sensitivity and improved measurement resolution.

Spectroscopic analysis of short-pulse laser-produced plasmas

Abstract: Experimental spectra of hot dense plasmas of aluminium produced by the interaction of a subpicosecond laser with solid targets at 1016 and 5 × 1017 W/cm2 are analyzed and discussed. A detailed analysis of the K-shell spectra is given through time-dependent calculations of atomic physics postprocessed to Fokker-Planck calculations of the laser-matter interaction. The non-Maxwellian character of the electron distribution function is shown. An evaluation of the electronic density and of the ion temperature 7i will be presented through Stark line broadening calculations. An X-ray spectrum from a Tantalum target also will be presented along with a preliminary interpretation.

Photoconductive step-function sampling

Abstract: Measurement of picosecond electrical signals using a photoconductive step-function gate is demonstrated analytically and experimentally. The time resolution of our step-function technique is limited only by the rise time of the step-function, which is approximately the same as the laser pulse width. Also, a regular, undoped semiconductor material, which is essential for the realization of a short-duration gate, can be used instead of the highly defected material. The use of undoped material gives 10 to 100 times higher sensitivity in the measurement than the impulse-function technique because of the high mobility of the undoped material. >

High-efficiency frequency doubling of ultrahigh-intensity Nd:glass laser pulses

Abstract: Short-pulse, high-peak-power lasers are important tools for high-intensity laser- matter interaction experiments. Solid-state table-top terawatt lasers utilize the concept of chirped pulse amplification1,2 to generate near-field intensities of up to two orders of magnitude higher (after compression) than in conventional fusion lasers. Solid target interaction experiments with intensities above 1018 W/cm2 require a very high intensity contrast (~1010:1) to ensure that the laser pulse is interacting with the solid rather than with a pre-formed plasma. Second harmonic generation (SHG) in nonlinear crystals is important because it extends the available wavelength range and significantly improves the contrast ratio.

Self-channeling of intense femtosecond laser pulses in air

Abstract: For the first time, the propagation properties of intense, ultrashort pulses through air are being studied. The propagation of these pulses have interesting applications.1 When the pulsewidths are as short as 100 fs, laser pulses with less than 1 mJ of energy have peak powers exceeding the critical power for self-focusing. Therefore in high peak-power, femtosecond laser systems, self-focusing is a key consideration.

Self-action of ultrashort intense laser pulses in dense gases: self-focusing, optical breakdown, supercontinuum generation

Abstract: Self-action during the propagation of ultrashort intense laser pulses through dense gases is practically unavoidable and leads to a dramatic change of pulse characteristics such as the beam size and frequency spectrum. The authors report results of experiments on propagation of ultrashort intense laser pulses in high pressure (1-40 atm) gas media. Self-focusing (SF) was observed under various conditions of gas pressure and laser power, accompanied by such phenomena as supercontinuum (SC) generation, conical emission (CE) and optical breakdown (OB). Supercontinuum generation (i.e. a nearly white spectrum generated upon propagation of intense short laser pulses through nonlinear media) has been observed in gases only recently, with appearance a new generation of powerful ultrashort lasers. The authors measured for the first time the spectral evolution of the supercontinuum radiation as a function of incident laser power and gas pressure. The single shot SC spectrum was deeply modulated, with this modulation changing with gas pressure and laser power. Beyond the SC generation threshold the authors also observed an intense conical emission with sharp spectral and spatial maxima.

Optical breakdown with femtosecond laser pulses

Abstract: The phenomena of optical breakdown (damage) of normally transparent optical materials such as fused silica involve ionization of valence band electrons by the intense incident laser field. Subsequent electron heating, acceleration, and impact ionization of secondary electrons in the laser field result in an exponential multiplication of free electrons, which causes the material breakdown. Laser-induced breakdown has been studied for three decades, and two major theories, the avalanche model and the photoionization model, exist for the breakdown process.1,2 In an avalanche- dominated process, a few initial electrons are generated through photoionization or thermal excitation from defects. These free electrons are accelerated by the intense laser field and create secondary electrons via collisions with atoms. The process repeats itself and eventually results in breakdown. In the photoionization theory, on the other hand, free electrons are created by multiphoton ionization or tunneling ionization only, and electron avalanche is negligible. In both models, the breakdown threshold is dependent on the laser pulse width.

Long-range, high-resolution laser radar

Abstract: A high energy, chirped pulse laser was used to demonstrate long range, high resolution radar. A broadband nanosecond chirped pulse propagated to a target, and upon compression sub-millimeter surface resolution was obtained. This technique avoided the nonlinear effects in air that were seen when a picosecond pulse propagated to the target.

Laser plasma interactions at electron-relativistic laser intensities

Abstract: A review is given (intended for the non-expert) of the field of ultrashort laser pulses at ultra high intensities and their interactions with plasmas. The review covers progress and basic concepts for theory, modelling and experiments, with emphasis on the background aspects, the basic experimental considerations and some possible applications.