Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

Attosecond Physics

The advent of ultraintense laser pulses generated by the technique of chirped pulse amplification (CPA) along with the development of high-fluence laser materials has opened up an entirely new field of optics. The electromagnetic field intensities produced by these techniques, in excess of ${10}^{18}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$, lead to relativistic electron motion in the laser field. The CPA method is reviewed and the future growth of laser technique is discussed, including the prospect of generating the ultimate power of a zettawatt. A number of consequences of relativistic-strength optical fields are surveyed. In contrast to the nonrelativistic regime, these laser fields are capable of moving matter more effectively, including motion in the direction of laser propagation. One of the consequences of this is wakefield generation, a relativistic version of optical rectification, in which longitudinal field effects could be as large as the transverse ones. In addition to this, other effects may occur, including relativistic focusing, relativistic transparency, nonlinear modulation and multiple harmonic generation, and strong coupling to matter and other fields (such as high-frequency radiation). A proper utilization of these phenomena and effects leads to the new technology of relativistic engineering, in which light-matter interactions in the relativistic regime drives the development of laser-driven accelerator science. A number of significant applications are reviewed, including the fast ignition of an inertially confined fusion target by short-pulsed laser energy and potential sources of energetic particles (electrons, protons, other ions, positrons, pions, etc.). The coupling of an intense laser field to matter also has implications for the study of the highest energies in astrophysics, such as ultrahigh-energy cosmic rays, with energies in excess of ${10}^{20}\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The laser fields can be so intense as to make the accelerating field large enough for general relativistic effects (via the equivalence principle) to be examined in the laboratory. It will also enable one to access the nonlinear regime of quantum electrodynamics, where the effects of radiative damping are no longer negligible. Furthermore, when the fields are close to the Schwinger value, the vacuum can behave like a nonlinear medium in much the same way as ordinary dielectric matter expanded to laser radiation in the early days of laser research.

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Optics in the relativistic regime

http://absimage.aps.org/image/DAMOP12/MWS_DAMOP12-2012-000066.pdf

Optics in the Relativistic Regime

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.

https://science.sciencemag.org/content/sci/264/5161/917.full.pdf

Terawatt to Petawatt Subpicosecond Lasers

Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent, and femtosecond radiation. In this article, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons are reviewed. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications.

https://hal.archives-ouvertes.fr/hal-01164031/document

Femtosecond x rays from laser-plasma accelerators

Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.

/pdf/ultrafast-x-ray-sources-4qk26uqwx3.pdf

Ultrafast x‐ray sources*

We report the first measurements of significant polarization of the He-like emission in a laser-produced plasma. From these measurements we infer a negative second-order (i.e., oblate) anisotropy of the electron velocity distribution function in the energy deposition region. This result is in agreement with our kinetic Fokker-Planck calculations which indicate non-Maxwell behavior and strong anisotropy due to nonlocal electron heat flow.

Electron distribution anisotropy in laser-produced plasmas from X-ray line polarization measurements

In this work, we show that x-ray-line polarization spectroscopy can be a powerful diagnostic to study laser-produced plasmas. Kinetic calculations are compared to experiments designed to probe the lowdensity plasma region (with a 1-ps laser pulse) and the overdense plasma (with a 400-fs laser pulse). We observe the transition from a "pancakelike" electron distribution function at low density to a "beamlike" electron distribution function in the overdense plasma. The results are in agreement with the calculations which indicate non-Maxwellian behavior and strong anisotropy due to nonlocal electron heat flow.

X-ray-line polarization spectroscopy in laser-produced plasmas

We report on the observation of high-electron-density (ne ≈ 15nc where nc is the critical density, 4 × 1021 cm−3 for λ = 0.53 μm) and high-temperature (Te ≈ 400 eV) plasma obtained when an intense high-contrast 400-fs laser pulse interacts with a solid Al target. After the laser intensity is increased from 1016 to 5 × 1017 W/cm2, the kiloelectron-volt x-ray emission is brighter by approximately an order of magnitude and is still produced from very dense plasma (6 × 1022 cm−3).

Production of a high-density and high-temperature plasma with an intense high-contrast subpicosecond laser

We describe a technique for spectrally and temporally resolving the kilo-electron-volt emission from ultrashort plasmas produced from solid targets with a tabletop terawatt 400-fs laser. The first time-resolved Al spectra (near 8 A) obtained with a 2-ps time resolution are presented. The results clearly demonstrate that the resonance emission width decreases as the plasma density increases. The ultrafast Kα emission component is also measured in our experimental conditions.

Y. Beaudoin

Papers

Ultrafast x‐ray sources*

Electron distribution anisotropy in laser-produced plasmas from X-ray line polarization measurements

X-ray-line polarization spectroscopy in laser-produced plasmas

Production of a high-density and high-temperature plasma with an intense high-contrast subpicosecond laser

Time-resolved kiloelectron-volt spectroscopy of ultrashort plasmas.