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Ultrahigh-Intensity Plasma Optics

Final Report Summary - PLASMOPT (Ultrahigh-Intensity Plasma Optics)

Laser technology now makes it possible to produce femtosecond (1fs=10-15 s) light pulse with extreme peak powers, exceeding 100 TW. By tightly focusing these pulses, ultrahigh laser intensities can be obtained, such that when a target is placed at focus, it is unavoidably converted into a highly ionized plasma, where the motion of electrons in the laser field gets relativistic. The overall idea of the PLASMOPT project is to study the laws of optics coming into play when such ultraintense femtosecond laser pulses interact with a plasma.

There are two motivations behind the study of these processes. The first one is fundamental in nature, and aims at a detailed understanding of this extreme regime of laser-matter interaction. The second goal is to exploit such optical processes to develop new light sources with remarkable properties, which would be difficult or impossible to obtain by other means. To achieve these goals, PLASMOPT has combined experiments, numerical simulations on massively-parallel computers, as well as simple physical modeling and analytical calculations.

A large part of the project focused on plasma mirrors, which are dense plasmas generated by intense femtosecond pulses at the surface of initially solid targets. Plasma mirrors have been raising a growing interest in the last decade, especially because they can be exploited to generate, in the reflected beam, harmonic frequencies of the incident beam, with very high orders. As a result, such a process has the potential of producing coherent beams of pulsed short-wavelength radiation (up to the X-ray range) from an initially near-visible laser beam. Moreover, these harmonic spectra are associated in the time domain to trains of attosecond (1 as=10-18s ) pulses. Such pulses are short enough to be subsequently used to temporally resolve the dynamics of electrons in matter. For the last ten years, these attosecond pulses -the shortest coherent light pulses even produced- have been generated using the interaction of lasers of moderate intensities with gases: it is hoped that in the near future, plasma mirrors used at extreme intensities will provide attosecond sources with considerably improved characteristics (e.g. in terms of pulse energy or photon energy). Making this happens was one of the goals of PLASMOPT, and indeed it has achieved major steps in this direction.

With the PLASMOPT project, the understanding of the physics of plasma mirrors driven by ultraintense laser pulses has been significantly improved, and simple models have been validated by comparison with experiments. For instance, we can now predict analytically the spatial properties of the generated high-harmonic beams, which are determined by the curvature of the plasma mirror surface under the action of the considerable radiation pressure exerted by the incident laser pulse. A technique to generate optically-controlled transient plasma gratings has been invented and demonstrated, and these gratings have been used to measure for the first time the spatial properties of the harmonic source in both amplitude and phase. Finally, the main achievement of PLASMOPT is related to a challenging and crucial problem in Attosecond Science, which is the generation of single attosecond light pulses, instead of the trains naturally produced in these experiments. We have identified a totally new approach to achieve this goal, based on the so-called “attosecond lighthouse effect”, which makes it possible to imprint different propagation directions to the successive attosecond pulses of the generated train, and thus to isolate each of them by simple spatial filtering of the light beam. In collaboration with Laboratoire d’Optique Appliquée, this effect has been demonstrated experimentally, leading to the first generation of isolated attosecond pulses from plasma mirrors. This work will have a broad impact in ultrafast science, since we have demonstrated that this effect is very general and equally applies to other generation processes.