Counter-propagating Laser Pulses Research Articles

Plasma-based generation and control of a single few-cycle high-energy ultrahigh-intensity laser pulse.

A laser-boosted relativistic solid-density paraboloidal foil is known to efficiently reflect and focus a counterpropagating laser pulse. Here we show that in the case of an ultrarelativistic counterpropagating pulse, a high-energy and ultrahigh-intensity reflected pulse can be more effectively generated by a relatively slow and heavy foil than by a fast and light one. This counterintuitive result is explained with the larger reflectivity of a heavy foil, which compensates for its lower relativistic Doppler factor. Moreover, since the counterpropagating pulse is ultrarelativistic, the foil is abruptly dispersed and only the first few cycles of the counterpropagating pulse are reflected. Our multidimensional particle-in-cell simulations show that even few-cycle counterpropagating laser pulses can be further shortened (both temporally and in the number of laser cycles) with pulse amplification. A single few-cycle, multipetawatt laser pulse with several joules of energy and with a peak intensity exceeding 10(23) W/cm(2) can be generated already employing next-generation high-power laser systems. In addition, the carrier-envelope phase of the generated few-cycle pulse can be tuned provided that the carrier-envelope phase of the initial counterpropagating pulse is controlled.

Collision of counterpropagating laser-excited wake bubbles.

The collision of wake bubbles behind two counterpropagating laser pulses in rarefied plasma is investigated using particle-in-cell simulation. Special attention is paid to the highly nonlinear dynamics of the electrons in the interaction region. It is found that, as the two bubbles approach each other and collide, the electrons in the interaction region first oscillate in a periodic fashion, forming a quasistationary dense electron density ripple with fairly regular spatial structure. At longer times, the electron motion becomes chaotic, and the density grating is gradually smeared. The electrons escape in the transverse direction, and eventually the two bubbles merge to form a single one. The transition of the electron motion from regular to chaotic is confirmed by analytical modeling using test electrons moving in counterpropagating planar electromagnetic waves. The findings shed light on the dynamics of wake-bubble collisions and the complex behavior induced by multiple laser pulses in plasmas.

Coherence-based axial point-spread function engineering for two-photon excited polymerization

We show in proof-of-principle experiments an axial resolution improvement in two-photon-excited fluorescence and polymerization enabled by the short coherence length of two counter-propagating ultrashort laser pulses. The interference of these pulses generate a spatial interference fringe pattern, whose width scales inversely to the spectral bandwidth and is therefore considerably shorter than the effective focal length of a laser beam conventionally focused from only one side.

Enhanced radiation pressure-assisted acceleration by temporally tuned counter-propagating pulses

Within the last decade, laser-ion acceleration has become a field of broad interest. The possibility to generate short proton- or heavy ion bunches with an energy of a few tens of MeV by table-top laser systems could open new opportunities for medical or technical applications. Nevertheless, today's laser-acceleration schemes lead mainly to a temperature-like energy distribution of the accelerated ions, a big disadvantage compared to mono-energetic beams from conventional accelerators. Recent results [1] of laser-ion acceleration using radiation-pressure appear promising to overcome this drawback. In this paper, we demonstrate the influence of a second counter-propagating laser pulse interacting with a nm-thick target, creating a well defined pre-plasma.

Relativistic electron mirrors from nanoscale foils for coherent frequency upshift to the extreme ultraviolet

Reflecting light from a mirror moving close to the speed of light has been envisioned as a route towards producing bright X-ray pulses since Einstein’s seminal work on special relativity. For an ideal relativistic mirror, the peak power of the reflected radiation can substantially exceed that of the incident radiation due to the increase in photon energy and accompanying temporal compression. Here we demonstrate for the first time that dense relativistic electron mirrors can be created from the interaction of a high-intensity laser pulse with a freestanding, nanometre-scale thin foil. The mirror structures are shown to shift the frequency of a counter-propagating laser pulse coherently from the infrared to the extreme ultraviolet with an efficiency >104 times higher than in the case of incoherent scattering. Our results elucidate the reflection process of laser-generated electron mirrors and give clear guidance for future developments of a relativistic mirror structure.

Operation of a picosecond narrow-bandwidth Laser–Thomson-backscattering X-ray source

A tunable source of intense ultra-short hard X-ray pulses represents a novel tool for the structural analysis of complex systems with unprecedented temporal and spatial resolution. With the simultaneous availability of a high power short-pulse laser system this provides unique opportunities at the forefront of relativistic light–matter interactions. At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) we demonstrated the principle of such a light source (PHOENIX – Photon Electron collider for Narrow bandwidth Intense X-Rays) by colliding picosecond electron bunches from the ELBE linear accelerator with counter-propagating femtosecond laser pulses from the 150TW Draco Ti:Sapphire laser system. The generated narrowband X-rays are highly collimated and can be reliably adjusted from 12keV to 20keV by tuning the electron energy (24–30MeV). Ensuring the spatial–temporal overlap at the interaction point and suppressing the Bremsstrahlung background a signal to noise ratio of greater than 300 was reached.

Acceleration with self-injection for an all-optical radiation source at LNF

We discuss a new compact γ-ray source aiming at high spectral density, up to two orders of magnitude higher than currently available bremsstrahlung sources, and conceptually similar to Compton sources based on conventional linear accelerators. This new source exploits electron bunches from laser-driven electron acceleration in the so-called self-injection scheme and uses a counter-propagating laser pulse to obtain X and γ-ray emission via Thomson/Compton scattering. The proposed experimental configuration inherently provides a unique test-bed for studies of fundamental open issues of electrodynamics. In view of this, a preliminary discussion of recent results on self-injection with the FLAME laser is also given.

Scattering of relativistic electron beam by two counter-propagating laser pulses: A new approach to Raman X-ray amplification

We present a detailed study of the properties of electron beam injected and trapped in an high intensity optical lattice. By using the hydrodynamic and kinetic approaches, we identified the beam trapping conditions, the high-frequency longitudinal beam eigenmodes and their dependence on the electron angular and energy spread. The coupling of these beam eigenmodes to the laser waves is also considered. This corresponds to the convective parametric instability: a stimulated scattering of two laser beams creating the optical lattice on the trapped electron beam mode. The amplification coefficients for the up-scattered Raman modes propagating parallel to the electron beam are calculated and their dependence on the beam characteristics is analyzed. The study of relativistic electron beams collective behaviour in a guiding and wiggling field has become a common interest for accelerator physics, plasma physics, nonlinear optics and laser science. By wiggling relativistic charged particles one may produce a scattered electromagnetic wave with a wavelength much shorter than the spatial period of the wigglers. This process lays at the basis of a number of techniques of XUV, X-ray and -ray generation, which find numerous applications in the diagnostics from medical physics to fusion science. Here we present a new approach to XUV/X-ray generation, based on the scattering of the bunch of relativistic electrons by a specific electromagnetic structure, called high intensity optical lattice. Such a field is created by the interference of two coherent laser pulses of equal frequencies and intensities, and it represents a standing wave pattern, where charged particles can be trapped. The electron oscillations in such a ponderomotive potential can be coupled to the laser wave field and emitted as high- and low- frequency electromagnetic waves. This process can be employed for generation of coherent radiation in a broad wavelength range extending from the infrared to X-ray domain (1). One of the promising applications of this technique is Raman XUV/X-ray amplification using a relativistic electron beam, propagating through an optical lattice (2, 3). The conventional schemes of backscattering amplification in stationary wigglers impose unrealistically severe limitations on the quality (angular and velocity spread) of the electron beam (4, 7). Raman amplifiers with optical lattices even for moderate laser intensities open a possibility to reduce strongly these limitations and to use laser accelerated electron bunches thus paving a way for compact all-optical systems.

Geometrical constraints on plasma couplers for Raman compression

Backward Raman compression in plasma is based on a 3-wave resonant interaction, which includes two counter-propagating laser pulses (pump and seed pulses) and an electron plasma wave (Langmuir wave). The resonant interaction can be ensured in nearly homogeneous plasmas. However, for high-power, large-aperture experiments, the homogeneous region becomes pancake-shaped and would likely be surrounded by thicker regions of inhomogeneous plasma. When these inhomogeneous plasma regions are extensive, significant inverse bremsstrahlung and seed dispersion may impede the compression effect. These deleterious effects may, however, be mitigated by chirping the seed and pump pulses.

Multimodal hard X-ray imaging of a mammography phantom at a compact synchrotron light source

The Compact Light Source is a miniature synchrotron producing X-rays at the interaction point of a counter-propagating laser pulse and electron bunch through the process of inverse Compton scattering. The small transverse size of the luminous region yields a highly coherent beam with an angular divergence of a few milliradians. The intrinsic monochromaticity and coherence of the produced X-rays can be exploited in high-sensitivity differential phase-contrast imaging with a grating-based interferometer. Here, the first multimodal X-ray imaging experiments at the Compact Light Source at a clinically compatible X-ray energy of 21 keV are reported. Dose-compatible measurements of a mammography phantom clearly demonstrate an increase in contrast attainable through differential phase and dark-field imaging over conventional attenuation-based projections.

Two-Photon Excited Fluorescence in Rare-Earth Doped Optical Fiber for Applications in Distributed Sensing of Temperature

Distributed temperature sensing based on time-correlated two-photon excited fluorescence (TPF) in doped optical fiber is described. Counter-propagating laser pulses generate a TPF flash at the position of their overlap which is scanned along the fiber by a variable relative time delay. The flash is transmitted to one end where it is detected and analyzed to yield the temperature from its thermal dependence. To identify suitable dopants, the two-photon excitation spectra of glass doped with various rare-earths were recorded. Preliminary results on TPF in praseodymium doped single-mode fiber are presented.

Production of fast ion and nuclei bunches of solid densities from nanolayers by ultrarelativistic laser pulses

A method for generation of quasimonoenergetic, low emittance fast ion/nuclei bunches of solid densities from nanotargets by two counterpropagating laser pulses of ultrarelativistic intensities is proposed, based on the threshold phenomenon of particles reflection due to induced nonlinear Compton scattering. Particularly, a setup is considered which provides generation of ion bunches with parameters that are required in hadron therapy.

Scaling laws for electron cold injection in the narrow collision pulse approximation

The latest advances in laser wakefield electron acceleration show a better beam quality, but much progress is still needed concerning the control and tunability of the electron beam. The recently proposed cold injection scheme offers a solution to this problem. It involves the use of two counter-propagating laser pulses to dephase a certain number of electrons into the wakefield of the main pulse, so that they are accelerated to high energies. As circular polarization is used, there is no stochastic heating and the injection process becomes much easier to model. We show that cold injection can be reduced to a one-dimensional problem in the case of a narrow collision pulse. The dephasing process can then be seen as competition between the longitudinal ponderomotive force of the main pulse and the stationary beatwave force arising from collision of the two circularly polarized lasers. This analysis leads to scaling laws for cold injection in the narrow collision pulse approximation and to a condition for its realization. Three-dimensional particle-in-cell simulations support both these scaling laws and the condition for cold injection to occur.

Nonlinear interaction of charged particles with nonplane counterpropagating laser pulses of relativistic intensities

The nonlinear threshold phenomena of particle reflection and capture of electrons in the induced Compton process that have previously been revealed in the case of plane monochromatic counterpropagating waves, take place also with the actual nonplane laser pulses of ultrashort duration and lead to particle acceleration. In contrast to analogous phenomena in the induced Cherenkov and undulator processes, the Compton reflection-capture mechanism with laser pulses of relativistic intensities practically may be realized for arbitrary initial energies of particles. The acceleration effect for particles initially in rest is explored numerically, taking into account the significance of this case connected with the relativistic electron bunches of high densities, which currently may be realized by relativistic lasers on the ultrathin solid foils where the electrons initially are almost in rest.

Laser singular Theta-pinch

The interaction of the two counter-propagating ultrashort laser pulses with singular wavefronts in the thin slice of the underdense plasma is considered. It is shown that ion-acoustic wave is excited via Brillouin three-wave resonance by corkscrew interference pattern of paraxial singular laser beams. The orbital angular momentum carried by light is transferred to plasma ion-acoustic vortex. The rotation of the density perturbations of electron fluid is the cause of helical current which produces the kilogauss axial quasi-static magnetic field. The exact analytical configurations are presented for an ion-acoustic current field and magnetic induction. The range of experimentally accessible parameters is evaluated.

Cold injection for electron wakefield acceleration

The present paper elaborates on the cold injection scheme, which was recently proposed in the context of laser wakefield acceleration (Davoine et al 2009 Phys. Rev. Lett.102 065001). This scheme allows one to inject a bunch of electrons into a laser wakefield, which is possible thanks to the collision between the main and a counter-propagating laser pulse. Unlike in the conventional colliding pulse schemes, in this process, a beatwave is created during the collision, which allows the injection of electrons with negligible heating. In this paper, we show that the injection of on-axis electrons observed in simulations is well described by a one-dimensional (1D) model, as long as conditions given here are satisfied. Injection of off-axis electrons is also influenced by transverse effects, but the basic mechanisms remain the same. Then, a comparison with the conventional colliding pulse schemes shows that each scheme can occur in different regimes. In particular, cold injection proves to be more interesting regarding the energy spread issue. Indeed, the simulations demonstrate that electron bunches with sub-MeV absolute energy spreads can be injected, leading, after acceleration, to electrons at several GeV and relative energy spread below 1%.

Swarm of ultra-high intensity attosecond pulses from laser-plasma interaction

We report on the realistic scheme of intense X-rays and γ-radiation generation in a laser interaction with thin foils. It is based on the relativistic mirror concept, i.e., a flying thin plasma slab interacts with a counterpropagating laser pulse, reflecting part of it in the form of an intense ultra-short electromagnetic pulse having an up-shifted frequency. A series of relativistic mirrors is generated in the interaction of the intense laser with a thin foil target as the pulse tears off and accelerates thin electron layers. A counterpropagating pulse is reflected by these flying layers in the form of a swarm of ultra-short pulses resulting in a significant energy gain of the reflected radiation due to the momentum transfer from flying layers.

Moving electron density gratings induced in the beat-wave field of two counterpropagating laser pulses

The production and physical characteristics of moving electron density gratings (MEDGs) are investigated by analytical model and particle-in-cell (PIC) simulations. When the frequency difference of two counterpropagating pump pulses is equal to the electron plasma frequency, large amplitude electron plasma wave is resonantly excited in the plasma. As a result, electrons are excited to form a moving, gratinglike structure in the interfering region of pump pulses, while the ions keep at rest. The spatial period and velocity of the MEDG are obtained by solving the electron density modulation equation, which agrees well with one-dimensional PIC simulation results. When a short signal laser pulse is scattered on the MEDG, the frequency of the backscattering wave shifts due to the stimulated Raman scattering.

Fast electron propagation in high-density plasmas created by 1D shock wave compression: Experiments and simulations

We present results from an experimental characterization of fast electron transport in high density plasmas created by 1D shock wave compression. The Kα fluorescence from a Cu layer embedded in Al or CH foil targets is measured. We use long laser pulses (LP) with 180 J, 1.5 ns, 0.53μm to compress the foils by shock wave propagation to 2-3 times their solid density and heat them to ∼ 4eV (close to the Fermi temperature). A counter-propagating high-intensity short laser pulse (SP), with 40 J, 1 ps, 57×1019 Wcm−2, generates intense currents of fast electrons which propagate through the deep regions of the target just before shock breakthrough. The results are compared to the uncompressed, solid density case (without the LP beam). The complete set of measurements is compared to numerical results, including a 2D hydrodynamic description of the compression and pre-pulse effects, 2D PIC simulations of the SP beam interaction and both hybrid and PIC simulations of the electron transport in the target depth and sheaths. In the case of the non-compressed targets we need to take fast electron refluxing into account to reproduce the experimental results. By exploring the domain of warm temperatures, we identify a regime for the incident fast electron current density, 1010 < jh < 1012 Acm−2, for which the collective mechanisms of electron transport differs appreciably between solid density and compressed matter.

Nonperturbative multiphoton electron-positron–pair creation in laser fields

A detailed analysis of electron-positron--pair creation from vacuum in counterpropagating laser pulses is presented. Our particular interest lies in the nonperturbative multiphoton regime of interaction in different frequency domains. The pair creation may proceed both resonantly and nonresonantly where characteristic Rabi oscillations are shown for the near-resonance. By means of a quasiclassical method we obtain intuitive physical insight into the process. An independent numerical approach confirms and extends the analytical results by providing rich information on momentum spectra, resonance widths, and the time dependence of the creation probability, both on and off the resonance.