Femtosecond X-ray generation through relativistic electron beam–laser interaction

Methods for generating ultra-short X-rays using the interaction of intense laser pulses with relativistic electron beams, and their application to measuring ultra-fast phenomena in solid state materials, are reviewed. Two different methods that use a long electron bunch and short laser pulse are discussed: Thomson scattering and optical slicing which have been implemented on linac and storage ring beams, respectively. The possibility of generating ultrashort electrons bunches from laser-plasma injectors is discussed.     

Poincaré representation of polarization-shaped femtosecond laser pulses

The usage of Poincaré phase space for the representation of polarization-shaped femtosecond laser pulses is discussed. In these types of light fields the polarization state (i.e. ellipticity and orientation) changes as a function of time within a single laser pulse. Such deliberate variation can be achieved by frequency-domain femtosecond pulse shaping in which two polarization components are manipulated individually. Here it is shown how these light pulses can be represented as temporal trajectories through the ellipticity-orientation (Poincaré) phase space, whereas conventional light (either continuous-wave or pulsed) is determined by only one specific Poincaré location. General properties of parametric Poincaré trajectories are discussed, and their relation to experimentally accessible pulse-manipulation parameters (i.e. amplitudes and phases) determined. Specifically, it is shown how the maximum rate by which a given polarization state can be turned into a different one (at significant intensity levels) is limited by the spectral laser bandwidth. Apart from their direct usage in polarization-shaped pulse representation, Poincaré trajectories also form the basis for intuitive quasi-three-dimensional renderings of the electric field profile. There, the temporal evolution of polarization, intensity, and chirp is directly apparent in a single illustration. Received: 10 December 2002 / Published online: 24 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-931/888-4906, E-mail: brixner@physik.uni-wuerzburg.de     

X-ray diffraction from the ripple structures created by femtosecond laser pulses

In this paper, we present the investigation and characterization of the laser-induced surface structure on an asymmetrically cut InSb crystal. We describe diffraction from the ripple surface and present a theoretical model that can be used to simulate X-ray energy scans. The asymmetrically cut InSb sample was irradiated with short-pulse radiation centred at 800 nm, with fluences ranging from 10 to 80 mJ/cm². The irradiated sample surface profile was investigated using optical and atomic force microscopy. We have investigated how laser-induced ripples influence the possibility of studying repetitive melting of solids using X-ray diffraction. The main effects arise from variations in local asymmetry angles, which reduce the attenuation length and increase the X-ray diffraction efficiency.     

Nanoand microstructuring of materials’ surfaces using femtosecond laser pulses

Multimodal nanoand microscale surface textures are produced by scanning the surfaces of various structural materials using IR femtosecond laser radiation. The topographies of the modified surfaces and their wettabilities upon hydrophobization are studied.     

Multifilamentation of femtosecond laser pulses induced by?small-scale air turbulence

We report on experimental and numerical investigations of femtosecond pulse propagation locally disturbed by the turbulent flow field of a hot-air blower. The experiments show that turbulence may shorten the collapse/filamentation distance and induce the onset of multiple filaments. This is supported by numerical simulations indicating that the high spatial frequency part of the turbulence spectrum plays a significant role.     

Fast electron beam–plasma interaction in single-walled carbon nanotubes

A theoretical study on the plasmon-polariton modes coupled with a fast electron beam inside a metallic single-walled carbon nanotube is presented. The Maxwell’s equations coupled with a linearized hydrodynamic model for the nanotube’s charge oscillations are used. By considering the electron beam effects, general expression of dispersion relation of electromagnetic modes on nanotube’s surface is obtained. It is shown numerically that by considering the electron beam effects, the polariton frequency shifts to lower values.     

Generation of a hollow laser beam by a multimode fiber

A simple method to generate a hollow laser beam by multimode fiber is reported.A dark hollow laser beam is generated from a multimode fiber and the dependence of the output beam profile on the incident angle of laser beam is analyzed.The results show that this hollow laser beam can be used to trap and guide cold atoms.     

Proton beam generation by ultra-high intensity laser–solid interaction

We report on some recent experimental results on proton production from ultra-intense laser pulse interaction with thin aluminium and plastic foil targets. These results were obtained at Laboratoire d'Optique Appliquée with the 100 TW ‘salle jaune’ laser system, delivering 35 fs laser pulses at 0.8 μm, reaching a maximum intensity on target of a few 10¹⁹ W/cm².

In such extreme interaction conditions, an intense and collimated relativistic electron current is injected from the plasma created on the laser focal spot into the cold interior of the target. Its transport through dense matter, ruled by both collisions and self-induced (electro-magnetic) field effects, is the driving mechanism for proton acceleration from the rear side of thin foils: when reaching and leaving the foil rear-side, the fast electrons create a large charge separation and a huge electrostatic field with a maximum value of few TV/m, capable of accelerating protons.

A parametric study as a function of the laser driver and target parameters indicates an optimal value for target thickness, which strongly depends on the laser prepulse duration. In our experiments, we did irradiate targets of various materials (CH, Al, Au) changing the prepulse duration by using fast Pockels cells in the laser chain. CR-39 nuclear track detectors with Al filters of different thickness and a Thomson parabola were used to detect proton generation. The best results were obtained for 2 μm Al targets, leading to the generation of proton energies with energies up to 12 MeV.     

Axially magnetized electron–positron and electron plasma competition on the self focusing of intense laser beam

The spot-size evolution of circularly polarized intense laser beam propagating through the axially magnetized electron–positron (EP) and electron plasmas is discussed, in mildly relativistic and weakly non-linear (a² ? 1) regime. The non-linear current density source terms are obtained by making used of the perturbative technique. The variational principle approach method is applied to the solution of the non-linear Schrodinger wave equation. It is shown that the laser beam spot size decreases for the left and increases for the right handed polarized beams with increasing the external magnetic field, owing to the beam passages inside the electron plasma. Furthermore, it is revealed that the self focusing property strongly enhanced in the EP plasma in comparison to the electron plasma. Moreover, self focusing of linearly polarized laser beam is investigated for EP plasma by superposition of the right and left circularly polarized beams.     

Spin correlations in nonperturbative electron–positron pair creation by petawatt laser pulses colliding with a TeV proton beam

The influence of the electron spin degree of freedom on nonperturbative electron–positron pair production by high-energy proton impact on an intense laser field of circular polarization is analyzed. Predictions from the Dirac and Klein–Gordon theories are compared and a spin-resolved calculation is performed. We show that the various spin configurations possess very different production probabilities and discuss the transfer of helicity in this highly nonlinear process. Our predictions could be tested by combining the few-TeV proton beam at CERN-LHC with an intense laser pulse from a table-top petawatt laser source.     

High intensity laser–matter interaction and electron plasma acceleration

The basic principles of the electron acceleration in laser produced plasmas and the related secondary sources of energetic radiation with a particular attention to betatron radiation are presented.     

Modulation instability of an intense laser beam in an unmagnetized electron–positron–ion plasma

The modulation instability of an intense circularly polarized laser beam propagating in an unmagnetized, cold electron–positron–ion plasma is investigated. Adopting a generalized Karpman method, a three-dimensional nonlinear equation is shown to govern the laser field. Then the conditions for modulation instability and the temporal growth rate are obtained analytically. In order to compare with the usual electron–ion plasmas, the effect of positron concentration is considered. It is found that the increase in positron-to-electron density ratio shifts the instability region towards higher vertical wave numbers but does not cause displacement along the parallel wave number direction, and the growth rate increases as the positron-to-electron density ratio increases.     

Fabrication of micro/nano crystalline ITO structures by femtosecond laser pulses

A method is proposed for the fabrication of micro/nano crystalline indium tin oxide (c-ITO) structures using a Ti:Sapphire laser with a repetition rate of 1 kHz and a wavelength of 800 nm. In the proposed approach, an amorphous ITO (a-ITO) thin film is transformed into a c-ITO micro/nano structure over a predetermined area via laser beam irradiation, and the residual a-ITO thin film is then removed using an etchant solution. The fabricated c-ITO structures are observed using scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM). The observation results show that the use of a low repetition rate laser induces a high thermal cycling effect within the ITO film and therefore prompts the formation of micro-cracks in the c-ITO structure. In addition, it is shown that as the laser power approaches the ablation threshold of the a-ITO thin film, nanogratings and disordered nanostructures are formed along the center lines of the c-ITO patterns formed using linearly polarized and circularly polarized laser beam irradiation, respectively. The nanogratings are found to have a period of approximately 200 nm (i.e. one-quarter of the irradiation wavelength), while the nanostructures have an average diameter of approximately 100–160 nm.     

Multifilamentation of femtosecond laser pulses propagating in?turbulent air near the ground

The propagation of femtosecond laser pulses in turbulent air near the ground is analyzed. Confining to a power regime distinctly above the critical power for self-focusing, i.e. P≈100P _cr, and concentrating on initial peak intensities around 2.5×10¹¹W/cm², the onset and early evolution of multiple filaments are addressed. Making use of the turbulence phase-screen method, numerical simulations of the pulse propagation indicate that turbulence fields with spatial scales below 6 mm are able to induce the onset of multifilamentation. An analytical linear plane wave perturbation model of the underlying modulation instability of the pulse front is introduced in support of the computational results. By this means, insight into the amplification of an initial perturbation of the pulse front from the point of view of the spatial frequency domain is given.     

Formation of colorized silicon by femtosecond laser pulses in?different background gases

A single-crystal silicon(111) wafer surface fixed on an x–y translation stage is scanned with a focused femtosecond laser beam at a wavelength of 800 nm under different atmospheres (air, vacuum, and nitrogen). Different colors from different angles on the surface of the silicon then appear. From the result of the experiments, periodic ripple surface structures emerge on the surface of colorized silicon, and the phenomenon is more obvious in vacuum and nitrogen than in air. The periods of the surface structures on silicon are not the same in the different atmospheres. Under vacuum, the period is the longest and is closer to the wavelength of the laser irradiation. Different from metals, the range of energy density is smaller when the colorized silicon appears with femtosecond laser pulses. Through SEM, TEM, and AFM, we observe in detail the microstructures of colorized silicon that forms in air, vacuum, and nitrogen and analyze the possible physical mechanism. Finally, research into the optical reflection of the colorized silicon indicates that the reflectivity is not higher than 30% in the 250–800 nm range.     

Generation of coherent off-diagonal raman-active phonons by femtosecond laser pulses in high-temperature superconductor YBa2Cu3O7−x

The time-resolved optical response from various crystallographic planes of a YBa₂Cu₃O_7?x monocrystal is investigated. In such measurements of temporal behavior, the phonon system is driven by an ultrashort (subpicosecond) pulse into a coherent state and is probed by a second ultrashort pulse with a given temporal delay. A comparison of the Fourier transforms of the temporal responses with the spontaneous Raman scattering spectra shows that the contribution to a temporal response comes not only from fully symmetric phonons, but also from off-diagonal modes. The mechanism of generation of coherent phonons in high-temperature superconductors is discussed.     

Plasma-induced spatiotemporal modulation of propagating femtosecond pulses

The dynamics of the femtosecond pulse propagation in a plasma channel is investigated by the pump-probe longitudinal diffractometry and second harmonic generation frequency-resolved optical gating (SHG-FROG) technique. The spatial characteristics, corresponding to the electronic density and the size of the channel, can be observed by the recorded ring pattern, and the spectral and temporal characteristics are recorded by the SHG-FROG traces. The spatiotemporal characteristics will help us to better understand the dynamics of the plasma induced by the femtosecond pulse and the femtosecond pulse propagating in the plasma channel.     

Cleaning of artificially soiled paper using nanosecond, picosecond and femtosecond laser pulses

Cleaning of cultural assets, especially fragile organic materials like paper, is a part of the conservation process. Laser radiation as a non-contact tool offers prospects for that purpose. For the studies presented here, paper model samples were prepared using three different paper types (pure cellulose, rag paper, and wood-pulp paper). Pure cellulose serves as reference material. Rag and wood-pulp paper represent essential characteristics of the basic materials of real-world artworks. The papers were mechanically soiled employing pulverized charcoal. Pure and artificially soiled paper samples were treated with laser pulses of 28 fs (800 nm wavelength) and 8–12 ns (532 nm) duration in a multi pulse approach. Additionally, the cellulose reference material was processed with 30 ps (532 nm) laser pulses. Damage and cleaning thresholds of pure and soiled paper were determined for the different laser regimes. Laser working ranges allowing for removal of contamination and avoiding permanent modification to the substrate were found. The specimens prior and after laser illumination were characterized by light-optical microscopy (OM) and scanning electron microscopy (SEM) as well as multi spectral imaging analysis. The work extends previous nanosecond laser cleaning investigations on paper into the ultra-short pulse duration domain.     

Three-dimensional simulation of laser–plasma-based electron acceleration

A sequential three-dimensional (3D) particle-in-cell simulation code PICPSI-3D with a user friendly graphical user interface (GUI) has been developed and used to study the interaction of plasma with ultrahigh intensity laser radiation. A case study of laser–plasma-based electron acceleration has been carried out to assess the performance of this code. Simulations have been performed for a Gaussian laser beam of peak intensity 5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 1 × 10¹⁹ cm^− 3, and for a Gaussian laser beam of peak intensity 1.5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 3.5 × 10¹⁹ cm^− 3. The electron energy spectrum has been evaluated at different time-steps during the propagation of the laser beam. When the plasma density is 1 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~14 MeV, with an energy spread of ±7 MeV. On the other hand, when the plasma density is 3.5 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~23 MeV, with an energy spread of ±7.5 MeV.