All optical electron injector using an intense ultrashort pulse laser and a solid wire target

Energetic electron bunches were generated by irradiating a solid tungsten wire 13 μm wide with 50 femtosecond pulses at an intensity of ∼3×10¹⁸ W/cm². The electron yield, energy spectrum and angular distribution were measured. These energetic electron bunches are suitable for injection into a laser driven plasma accelerator. An all-optical electron injector based on this approach could simplify timing and alignment in future laser-plasma accelerator experiments. PACS 41.75.Ht; 41.75.Lx; 52.38.Kd; 52.38.Ph     

Study of forward accelerated fast electrons in ultrashort Ti <Emphasis Type="BoldItalic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> sources

The results of an experiment aimed at studying hot electrons emerging from a target rear side in ultrashort laser-based [ _]Kα sources are described. In particular, forward accelerated fast electrons propagating through a Ti foil are found to be emitted in a cone perpendicular to the target surface. The energy of these electrons is estimated as well as their divergence. A comparison of the experimental findings with the results of a PIC simulation is also reported, aimed at identifying the physical processes responsible for the production of this forward propagating electron population. PACS 52.38.-r; 52.38.kd; 52.38.Ph     

Development of a two-color interferometer for observing wide range electron density profiles with a femtosecond time resolution

A two-color interferometer for preformed plasma characterization is developed. We observe the electron density distribution of preformed plasmas on a 5 μm-thick copper target irradiated with a high-intensity Ti:sapphire laser. The two-color interferometer extended the observable electron density region using a fundamental (800 nm) probe beam to cover the lower density region and a second harmonic (400 nm) probe beam to cover the higher density region, simultaneously. This characterization of the electron density distribution of preformed plasmas with femtosecond time resolution significantly contributes to the understanding of high-intensity laser–thin-foil interactions during high-energy electron, ion, and X-ray generation. PACS 52.38.-r; 52.50.Jm; 52.70.-m     

Two-dimensional organization of a large number of stationary optical filaments by adaptive wave front control

We present an adaptive technique for the formation of multiple co-propagating and stationary filaments in a gaseous medium. Wavefront shaping of the initial beam is performed using a deformable mirror to achieve a complete two-dimensional control of the multi-spot intensity pattern in the laser focus. The spatial organization of these intensity spots yields reliable formation of up to five stable and stationary filaments providing a test bed for fundamental studies on multiple filamentation. PACS 42.65.Jx; 52.38.Hb; 42.65.Sf     

Energy scaling of quasi-monoenergetic electron beams from laser wakefields driven by 40-TW ultra-short pulses

By focusing 40-TW, 30-fs laser pulses to the peak intensity of 10¹⁹ W/cm² onto a supersonic He gas jet, we generate quasi-monoenergetic electron beams for plasma density in the specific range 1.5×10¹⁹ cm^-3≤n_e≤3.5×10¹⁹ cm^-3. We show that the energy, charge, divergence and pointing stability of the beam can be controlled by changing n_e, and that higher electron energies and more stable beams are produced for lower densities. The observed variations are explained physically by the interplay among pump depletion and dephasing between accelerated electrons and plasma wave. Two-dimensional particle-in-cell simulations support the explanation by showing the evolution of the laser pulse in plasma and the specifics of electron injection and acceleration. An optimized quasi-monoenergetic beam of over 300 MeV and 10 mrad angular divergence is demonstrated at a plasma density of n_e≃1.5×10¹⁹ cm^-3. PACS 52.35.-g; 52.38.Hb; 52.38.Kd; 52.65.-y     

High-intensity lasers and controlled fusion

The use of high-intensity lasers to cause ignition in inertial confinement fusion is presented, with emphasis on current experimental programs and physical concepts that are at the forefront of the field. In particular, we highlight the issues of fast electron transport through dense materials, an essential element of the Fast Ignitor concept.Received: 22 November 2002, Published online: 5 August 2003PACS: 52.20.Fs Electron collisions - 52.38.Kd Laser-plasma acceleration of electrons and ions     

High order harmonic generation by non-linear reflection of a pedestal-free intense laser pulse on a plasma

We demonstrate the influence of the prepulses and ASE of ultrashort pulses interacting with a solid target by addressing the direct comparison of the harmonic spectra generated by reflection onto a solid target with and without the introduction of a plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free from any plasma expansion. PACS 42.65. Ky; 52.38.-r; 52.38.Ph; 52.50.Jm     

Sub-picosecond ultraviolet laser filamentation-induced bulk modifications in fused silica

We present experiments with sub-picosecond ultraviolet laser pulses (248 nm, 450 fs) tightly focused in the bulk of fused-silica samples. The high laser intensities attained generate plasma through multi-photon absorption and electron avalanche processes in the bulk of the material. Depending on the initial experimental conditions three distinct types of structural changes in the material are observed, from small changes of the refractive index to birefringence, and even cracks and voids. We also observe the creation of micro-channels, up to 115 m in length, inside the material due to self-guiding and filamentation of the laser pulses in the transparent material. The selective change of the refractive index is a promising method for the fabrication of photonic structures such as waveguides and three-dimensional integrated optical devices. PACS  52.38.Hb; 42.65.-k; 42.70.-a     

Controlling coherent and incoherent plasma emissions: the role of electron plasma waves

We present results of our two-pulse time resolved measurements of second harmonic and hard X-ray generation in the interaction of an intense (10¹⁶ Wcm^-2, 100 fs, 800 nm) laser with a preplasma generated on a solid surface. The time-resolved study as a function of both plasma scale length and laser polarization brings out interesting features of electron plasma wave dynamics. The harmonic and X-ray emission show contrary behavior and we interpret the results in terms of Resonance Absorption and Wave-Breaking mechanisms. Simple optimization of the scale length of the preplasma and the polarization parameters of the main pulse results in significant enhancements, up to a factor of 100 for X-rays and 10 for the second harmonic respectively. These results can help us understand the governing mechanisms for higher harmonic generation and for fast particle generation, aiding the development of more efficient sources. PACS 52.35.Fp; 42.65.Ky; 52.38.Ph     

UV–Supercontinuum generated by femtosecond pulse filamentation in air: Meter-range experiments versus numerical simulations

We report new experimental and numerical results on supercontinuum generation at ultraviolet/visible wavelengths produced by the propagation of infrared femtosecond laser pulses in air. Spectral broadening is shown to similarly affect single filaments over laboratory distance scales, as well as broad beams over long-range propagation distances. Numerical simulations display evidence of the crucial role of third harmonic generation in the build-up of UV–visible wavelengths, by comparison with current single-envelope models including chromatic dispersion and self-steepening. PACS 52.38.Hb; 42.65.Tg; 42.65.Jx; 42.68.Ay     

Shape of ion energy spectra in ultra-short and intense laser–matter interaction

Structured proton spectra and a two-temperature electron distribution were simulated using a 1D3V particle-in-cell code for a plasma created by 35-fs laser pulses with an intensity of I10¹⁹ W/cm². Such a two-temperature electron distribution was used as a prerequisite for a fluid dynamical model allowing us to describe the experimentally measured deep dips in the proton energy spectra of laser-produced plasmas from water-droplet targets. PACS 52.50.Jm; 52.38.Kd; 41.75.Jv     

Numerical modeling of transient progression of plasma formation in biological tissues induced by short laser pulses

A theoretical model based on the rate equation for free electron density is proposed to investigate transient progression of plasma formation in soft biological tissues during laser shock processing. The laser focusing region around the focus point is considered to be one-dimensional along the direction of the incident beam, and is discretized into numerous thin control volumes. In simulation of the transient plasma progression, the laser intensity distribution and the temporal evolution of the free electron density are calculated sequentially for each control volume using a fourth-order Runge–Kutta method with adaptive time step control. The rate-equation formalism is first validated with previously published theoretical and experimental results. Simulation of the dynamics of plasma formation is then performed. The results include temporal evolution and spatial distribution of the free electron density as well as the growth of the plasma. It is shown that the threshold laser intensity for optical breakdown in water and the maximum length of the resulting plasma obtained from the present model are in good agreement with existing experimental data. PACS 42.65.-k; 52.38.-r; 87.80.-y     

Electronic mechanism of ion expulsion under UV nanosecond laser excitation of silicon: Experiment and modeling

We present experimental and modeling studies of UV nanosecond pulsed laser desorption and ablation of (111) bulk silicon. The results involve a new approach to the analysis of plume formation dynamics under high-energy photon irradiation of the semiconductor surface. Non-thermal, photo-induced desorption has been observed at low laser fluence, well below the melting threshold. Under ablation conditions, the non-thermal ions also have a high concentration. The origin of these ions is discussed on the basis of electronic excitation of Si surface states associated with the Coulomb explosion mechanism. We present a model describing dynamics of silicon target excitation, heating and charge-carrier transport. PACS 52.38.Mf; 68.34.Tj; 68.35.Rh; 79.20.Ds     

Design considerations for table-top, laser-based VUV and X-ray free electron lasers

A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV-electrons. With future Petawatt lasers ultra-high beam currents of ∼100 kA in ∼10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may also open a brilliant light source for new methods in clinical diagnostics. PACS 41.60.Cr; 52.38.Kd     

Additional acceleration and collimation of relativistic electron beams by magnetic field resonance at very high intensity laser interaction

For the interpretation of experiments for acceleration of electrons at interaction up to nearly GeV energy in laser produced plasmas, we present a new model using interaction magnetic fields. In addition to the ponderomotive acceleration of highly relativistic electrons at the interaction of very short and very intense laser pulses, a further acceleration is derived from the interaction of these electron beams with the spontaneous magnetic fields of about 100 MG. This additional acceleration is the result of a laser-magnetic resonance acceleration (LMRA) around the peak of the azimuthal magnetic field. This causes the electrons to gain energy within a laser period. Using a Gaussian laser pulse, the LMRA acceleration of the electrons depends on the laser polarization. Since this is in the resonance regime, the strong magnetic fields affect the electron acceleration considerably. The mechanism results in good collimated high energetic electrons propagating along the center axis of the laser beam as has been observed by experiments and is reproduced by our numerical simulations. PACS 41.75.Jv; 52.38.Kd; 52.65.Cc     

Emission characteristics of debris from CO<Subscript>2</Subscript> and Nd:YAG laser-produced tin plasmas for extreme ultraviolet lithography light source

We describe a comparative study of the emission characteristics of debris from CO₂ and Nd:YAG laser-produced tin plasmas for developing an extreme-ultraviolet (EUV) lithography light source. Tin (Sn) ions and droplets emitted from a Sn plasma produced by a CO₂ laser or an Nd:YAG laser were detected using Faraday cups and quartz crystal microbalance (QCM) detectors, respectively. The droplets were also monitored by using silicon substrates as witness plates. The results showed higher ion kinetic energy and lower particle emission for the CO₂ laser than the Nd:YAG laser for the same laser energy (50 mJ). The average ion energy was 2.2 keV for the CO₂ laser-produced plasma (LPP), and 0.6 keV for the Nd:YAG LPP. The debris accumulation of the CO₂ LPP detected by the QCM detectors, however, was less than one fourth of that of the Nd:YAG LPP for the same laser energy. Using ion energy data, the mirror lifetime is estimated for the CO₂ and Nd:YAG lasers. In both cases, the upper limit of the number of shots was of the order of 10⁶. PACS  52.38.DX; 52.38.Ph; 52.38.Mf     

Water-assisted drilling of microfluidic chambers inside silica glass with femtosecond laser pulses

Microfluidic chambers embedded in silica glass are drilled by water-assisted ablation with a femtosecond laser. The continuous scanning ablation increases the processing speed up to 50 μm/s. Not only may microchambers or microtrenches be obtained at high speed and in one step, but also combined structures consisting of cascaded microchambers and microtrenches may be fabricated. The inner-wall morphology of the microchambers is analyzed by a scanning electron microscope. PACS 87.80.Mj; 52.38.Mf; 82.50.Pt; 42.62.-b; 42.70.Ce     

Pulsed laser ablation TOF-MS analysis of planets and small bodies

We are developing miniature time-of-flight mass spectrometers, based on nanosecond pulsed Nd:YAG IR and UV laser ablation, for robotic landed missions to Mars, asteroids, comets, and planetary moons. The goal is to obtain grain-scale and bulk major, minor, and trace composition (both atomic and molecular) from solids, with minimal sample manipulation and preparation. We present here a summary of this effort, focusing on the variety of analyses that could be enabled by pulsed laser ablation for sampling and ionization. We show that a number of geochemical and astrobiological objectives on small bodies or Mars could be addressed with highly simplified mass-spectrometer instrumentation, provided a thorough analysis of the yields of elements and organics to the laser-ablation system is conducted. PACS 07.87.+v; 52.38.Mf; 82.80.Rt     

Ablation studies using a diode-pumped Nd : YVO4 micro-laser

We report an investigation of ablating several materials using a nanosecond pulse duration Nd:YVO₄ micro-laser operating at wavelengths of 1064 and 532 nm and high pulse-repetition rate (20 kHz). Probe beam deflection, ballistic pendulum measurements and scanning electron microscopy are used to characterise the interaction. It is shown that good-quality micro-scale features can be produced in polyimide, gold foils and silicon targets by ablation using this laser. PACS 42.55.Xi; 52.38.Mf; 87.80.Mj     

Optodynamic description of a linear momentum transfer from a laser induced ultrasonic wave to a rod

We present a new optodynamic experimental technique to measure the linear momentum obtained by a rod during a nanosecond laser pulse ablation of the rod’s front face on the basis of the displacement due to an ultrasonic wave reflection at its rear end. With the help of a simple theory, we explained the step-like motion of the rod’s free end. This theory conforms well with the general shape of the measured displacement history curve. The acquired momentum can be directly estimated by measuring the height of a step from the step-like motion of the rod’s end. Measurements based on an arm-compensated Michelson interferometer also enabled us to follow the attenuation of an ultrasonic wave and so to determine the characteristic attenuation time. This quantity plays a major role in the transfer of linear momentum from within the initial ultrasonic wave to the final net uniform motion of the specimen. PACS  52.38.Mf; 43.35.Yb; 43.35.Cg