High Intensity Short Pulse Research Articles

Controlled ultrasonic tissue erosion

Controlled ultrasonic tissue erosion has many clinical applications, including the placement of very precise sharply defined perforations in biological interfaces and membranes with focused ultrasound. With carefully chosen acoustic parameters, tissue can be rapidly eroded away at a constant etching rate. The maximum erosion rate for minimal propagated energy is obtained by using very short high intensity pulses. The etching rate is higher with shorter pulse durations. For short pulses less than 10 cycles of the drive frequency, an optimum pulse repetition rate exists which maximizes the etching rate. Higher gas saturation in the surrounding medium reduces the etching rate and reduces the spatial sharpness of the holes produced. Most of the erosion appears to be produced in the first several cycles of the therapy pulse. For example, a series of short (about 3 cycles) focused pulses of 2100 W/cm2 (Isppa) at 788 kHz can erode a very well defined 2 mm diameter hole in a 1 mm thick sample of fresh pork atrial posterior wall in about 1 min at the optimum pulse repetition rate (about 18 kHz). Controlled ultrasonic tissue erosion may provide an effective image guided noninvasive tool in treatment of neonatal patients with hypoplastic left heart syndrome. Without the mixing of oxygenated blood across perforations placed in the atrial septum, these infants do not survive.

A compact electron spectrometer for hot electron measurement in pulsed laser solid interaction

Ultraintense laser-matter interactions provide a unique source of temporally short, broad spectrum electrons, which may be utilized in many varied applications. One such, which we are pursuing, is as part of a diagnostic to trace magnetic field lines in a magnetically confined fusion device. An essential aspect of this scheme is to have a detailed characterization of the electron angular and energy distribution. To this effect we designed and constructed a compact electron spectrometer that uses permanent magnets for electron energy dispersion and over 100 scintillating fibers coupled to a 1024×1024 pixel charge coupled device as the detection system. This spectrometer has electron energy coverage from 10 keV to 60 MeV. We tested the spectrometer on a high intensity (1017–1021 W/cm2) short pulse (<100 fs) laser, JanUSP, at Lawrence Livermore National Laboratory using various solid targets. The details of the spectrometer and the experimental results will be reported.

Designing and building an integrated target chamber system for high intensity short-pulse laser-target interaction experiments

We have recently designed and are building a plasma experiment chamber system for a high intensity short pulse Ti:sapphire 100 TW laser (λ=800 nm, 2 J, 20 fs) at the Advanced Photon Research Center, JAERI. The expected primary use of the chamber is for high intensity (up to 1020 W cm−2) laser solid target experiments for hot electron, ion, and x-ray production. In order to achieve these intensity levels, the focal spot size on the target needs to be much smaller (⩽φ10 μm) than those used in relatively high laser energy experiments using ⩾100 fs laser pulse widths. The system consists of a target monitor system, a target insertion device (manipulator shuttle), a target stage assembly, and an on-target beam profiler all integrated together. We used calibrated apertures, hereafter called “zero pinholes,” placed at the focal point to assess the on-target energy accurately within a defined spot area. It is intended for general experiments rather than specifically configured ones, e.g., time-diffraction kind of experiments. However, great care was taken to have the capability to accommodate these types of experiments in the future.

High-order harmonics from an ultraintense laser pulse propagating inside a fiber.

A strong effect of high harmonic radiation during the propagation of a high intensity short laser pulse in a thin wall hollow channel ("fiber") is found and studied via relativistic particle-in-cell simulations. The fiber has finite width walls comprised of an overdense plasma. Only the harmonic radiation with the harmonic number above critical value, for which the fiber walls are transparent, propagates outwards in the form of a coherent ultrashort pulse with very short wavelength.

Laser influence on magnetoresistance

We present the model of interaction between the electrons inside a ferromagnetic-normal metal-ferromagnetic trilayer sample and the high intensity short pulse laser light. The incident electromagnetic field of laser on sample surface perturbs the Boltzmann distribution function determined by the electron–electron and electron–photon collision terms. The numerical results confirm that the laser pulse can influence significantly on the electron distribution function and as a consequence, the giant magnetoresistance.

Song pattern recognition in the grasshopper Chorthippus biguttulus: the mechanism of syllable onset and offset detection.

The male song of the duetting grasshopper Chorthippus biguttulus consists of syllables alternating with noisy pauses. The syllable-pause structure is important for song recognition by the female. Using playback experiments we investigated the mechanism by which intensity modulations within the song pattern are used to detect syllable onsets and offsets. We varied the relative onset level (level of the syllable beginning relative to the noisy pause) and the relative offset level (level of the noisy pause relative to the syllable end) independently in different experiments. For all females, an increase in intensity defining the syllable onset was necessary to evoke responses. Syllable offset cues were not always necessary: some females responded to continuous noise stimuli wherein only syllable onsets were marked by short pulses of high intensity. Those females that did not require syllable offset cues did not, however, lack a functional pause detection mechanism, since their responses to model songs containing silent pauses were restricted to a given range of pause durations. We propose that syllable-pause detection involves two independent processes: (1) syllable onset detection by a phasic neuronal unit that can be re-activated only after a short pause, and (2) the rejection of unacceptably long pauses by a second unit.

Role of collisions in particle-in-cell modeling of high-density short-pulse laser-plasma interaction

One-and-one-half-dimensional particle-in-cell (PIC) modeling with restored short-encounter collisional behavior is used to model the interaction of high-intensity short laser pulses with plasmas. The role of Coulomb collisions in expanding thin plasma targets at solid density is particularly investigated. It is shown that collisions play an important role for plasma expansion and ion acceleration mechanisms, even at high laser intensities.

Numerical studies on the properties of femtosecond laser plasma Kα sources

One of the features of ultra high intensity (UHI) short pulse laser–matter interactions is the prospect of creating a cheap, compact source of hard X rays with femtosecond pulse duration. The properties of such Kα sources are studied using analytical and numerical models of hot electron generation and subsequent transport in a range of materials (Reich et al., 2000). First, we find that there is an optimum laser intensity for Kα generation from bulk targets, which scales as Z4.4. Second, we show that efficient hard X-ray pulses with durations below 100 fs can be generated at intensities of ∼1016 W/cm2.

Fixed blueshift of high intensity short pulse lasers propagating in gas chambers

An explanation is given for the fixed blueshift observed of an intense short pulse laser propagating in a gas chamber filled with helium. The observed shift is found to be independent of the laser power and gas pressure above some critical power. The results are explained by the formation of filaments, which occur due to nonlinear gas polarization effects. The blueshift is in qualitative agreement with a one dimensional particle-in-cell simulation including ionization of the gas.

High-intensity short KrF laser-pulse generation by saturated amplification of truncated leading-edge pulse

A new method for the generation of high-intensity short KrF laser pulses was developed. The method efficiently uses truncated leading-edge seed-pulse generation by stimulated Brillouin scattering and pulse-shape steepening by saturated amplification, which is mainly based on the Frantz–Nodvik solution. Short KrF pulses of 7 ps were obtained from 20 ns seed pulses without using mode-locked lasers. The experimental results agreed well with the numerical calculations.

Laser frequency upshift, self-defocusing, and ring formation in tunnel ionizing gases and plasmas

The combined effects of tunnel ionization of gases on laser frequency upshift, defocusing, and ring formation are considered self-consistently. A high-intensity short pulse laser causes rapid tunnel ionization of a gas. The increasing plasma density leads to a decreasing refractive index, modulating the phase of the laser as it propagates and causing frequency upshift and supercontinuum generation. For laser intensity profile peaking on axis, the tunnel ionization produces a minimum of refractive index on axis, thus defocusing the laser. The defocusing reduces the ionization rate and frequency upshift. As the laser propagates over a Rayleigh length its trailing portion develops a ring shape distribution due to stronger defocusing of rays on axis than the off-axis rays, as seen in a recent experiment.

Generation of vacuum ultraviolet radiation by sum frequency mixing of a femtosecond and a nanosecond laser

The interaction of high-intensity short pulse (fs) lasers with atomic or molecular vapors permits the conversion of visible light to intense coherent radiation in the vacuum ultraviolet spectral region. We demonstrate sum frequency mixing of a high-intensity short pulse (fs) laser with a conventional long pulse (ns) laser to extend the accessible wavelength range of harmonic generation. The mixing process does not rely on meeting resonances and can in principle be implemented with any intense short pulse (fs) laser system in any nonlinear medium. The efficiency of the sum frequency mixing process is comparable to the efficiency of harmonic generation of a similar wavelength with the high-intensity short pulse (fs) laser alone. We propose the application of conventional tunable long pulse (ns) lasers to permit the generation of broadly tunable picosecond radiation in the vacuum ultraviolet.

The ISOLDE facility

The CERN on-line isotope separator facility ISOLDE is a unique tool for studying problems in nuclear, atomic, surface and solid state physics, nuclear medicine and astrophysics. Protons in very short high intensity pulses from the PS Booster are directed on either one of the targets of the two on-line mass separators. The 60 keV ions from them are fed into a common beam transport system to which most of the experiments are connected. The layout of the facility is presented and a detailed description of the two mass separators and the beam distribution system is given. Also the problems of radiation shielding and of the handling of radioactivity are discussed.

Propagation of high-intensity short resonant pulses in inhomogeneously broadened media

We investigate the areas of extremely short, intense (area ⩾π) resonant optical pulses on propagation through an inhomogeneously broadened atomic medium. Experimentally, there is no apparent change in the shape of such pulses on propagation, in contradiction to the area theorem. Here we show that, although the main part of the pulse is unchanged for short propagation distances (αl∼1), it is followed by a long weak tail formed by free induction decay from the excited atoms. The tail lengthens and oscillates on propagation through the medium and permits the area theorem to be obeyed. These oscillations, which depend critically on the Doppler broadening, are reflected in the spectral analysis of the propagated pulse. We also introduce another mechanism for pulse reshaping, which is a generalization of the McCall–Hahn pulse breakup and operates at long propagation lengths (αl≫1), where the number of absorbing atoms encountered by the pulse is comparable with the number of photons in the pulse.

Magnetic field in short-pulse high-intensity laser-solid experiments

High-intensity short laser pulses focused on the surface of a solid target produce large numbers of energetic electrons which in turn produce strong electric and magnetic fields as they penetrate the target. Layers of x-ray-emitting materials buried inside the target are often used to diagnose fast electron transport. We show that the magnetic field grows at the surface of the buried layers, and that this field may in some cases possibly inhibit fast electron penetration.

Optical field ionization effects on the generation of wakefields with short pulse lasers

In recent experiments involving high intensity short pulse lasers, large wakefields have been observed to occur even when the pulse is much longer than the length necessary to generate wakefields. It was observed in the experiments that the plasma wave starts near the position of the ionization front. The whole process of wake amplification from optical field ionization to Raman scattering is investigated by using a one-dimensional particle-in-cell simulation code which includes the effect of optical field ionization.

High–intensity lasers: interactions with atoms, molecules and clusters

The development of high–intensity short–pulse lasers by chirped pulse amplification is described and the use of short intense pulses for the study of atoms and molecules is discussed. As a case study, the dynamics of the ionization and dissociation of clusters of noble gas atoms is reviewed with special emphasis on the generation of energetic electrons and highly charged ions.

X-ray yields from Xe clusters heated by short pulse high intensity lasers

We have examined the absolute yield of keV x-rays emitted from gaseous plasmas created by the intense irradiation of large Xe clusters. We find that >10 μJ of x rays with photon energies above 1 keV are produced from clustering Xe gas targets when heated by 250 mJ, 2 ps laser pulses at an intensity of ∼1017 W/cm2. The yields show strong laser intensity dependence and variation with Xe cluster size.

Plasma Channel Formation and Guiding during High Intensity Short Pulse Laser Plasma Experiments

A plasma channel is formed behind a self-guided, subpicosecond, 2 TW laser pulse in a hydrogen gas jet plasma. The channel is produced from the radial expulsion of plasma ions due to charge separation created in the displacement (or cavitation) of plasma electrons by the large ponderomotive force of the laser. Using Thomson scattering diagnostics and mode structure measurements, an intense trailing laser pulse $(I\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{16}\mathrm{W}/{\mathrm{cm}}^{2})$ is observed to be guided throughout the length of this channel for about 20 Rayleigh lengths, approximately equal to the propagation length of the self-guided pump laser pulse.

Resonant hot-electron production in above-threshold ionization

We have observed surprising detailed structure in the photoelectron kinetic-energy spectrum of xenon under high-intensity short pulse conditions. We show that most of the photoelectrons with kinetic energies from 0--50 eV result from resonant processes at intensities up to 1.9\ifmmode\times\else\texttimes\fi{}${10}^{14}$nW/${\mathrm{cm}}^{2}$. In particular, we find that the high-energy photoelectron structure in above-threshold ionization is actually composed of narrow individual peaks whose energy positions do not shift with intensity. The amplitudes of the structures change rapidly with intensity and turn on at different specific intensities. While those structures appear to be due to resonances, they cannot be attributed to traditional Rydberg transient resonances.