Coherent light beams composed of ultrashort pulses are now increasingly used in different fields of Science, from time-resolved spectroscopy to plasma physics. Under the effect of even simple optical components, the spatial properties of these beams can vary over the duration of the light pulse [1]. In this letter, we show how such spatio-temporally coupled electromagnetic fields can be exploited to produce an attosecond lighthouse, i.e. a source emitting a collection of isolated attosecond pulses, propagating in angularly well-separated light beams. This very general effect not only opens the way to a new generation of attosecond light sources, particularly suitable for pump-probe experiments, but also provides a powerful new tool for ultrafast metrology, for instance giving direct access to fluctuations in the phase of the laser field oscillations with respect to the pulse envelop, right at the focus of even the most intense ultrashort laser beams. PACS numbers:Ultrashort light beams are said to exhibit spatiotemporal couplings (STC) when their spatial properties depend on time, and conversely [1] -i.e. their electric field E(x, y, z = z 0 , t) = E 1 (t)E 2 (x, y). The importance of STC has been largely overlooked in most laser-matter interaction experiments, until recently [2]. In strong-field science, STC are even considered as highly detrimental, because they systematically decrease the peak intensity at focus [3]. In this letter, we show that on the opposite, moderate and controlled STC provide a powerful means of controlling high-intensity laser-matter interactions, and pave the way to a whole range of new experimental capabilities.To demonstrate this idea, we consider a particular application of STC to attosecond pulse generation (1 as=10 −18 s), which has been the key issue in the development of attosecond Science [4]. All attosecond light sources demonstrated so far are based on high-order harmonic generation (HHG) of intense femtosecond laser pulses in different media [4,5]. Since many-cycle long pulses naturally produce trains of attosecond pulses, considerable efforts had to be deployed in the last fifteen years for the development of 'temporal gating' techniques, to isolate single attosecond pulses, more readily usable for time-resolved measurements of electron dynamics in matter. A variety of experimentally-challenging techniques have now been demonstrated for HHG in gases [6,7]. In contrast, the problem is still unsolved experimentally for HHG on plasma mirrors [8][9][10], one of the promising processes to obtain the next generation of attosecond light sources [11,12]. We describe here how STC provide a new approach to this problem, of unprecedented simplicity, generality and potential : one of the most basic types of STC, wavefront rotation [1] (WFR), can be exploited to generate a collection of single attosecond pulses in angularly well-separated light beams -an attosecond lighthouse-even with relatively long laser pulses.Let us first briefly summarize the concept of WFR at the focus of a femto...

The advent of ultrahigh-power femtosecond lasers creates a need for an entirely new class of optical components based on plasmas. The most promising of these are known as plasma mirrors, formed when an intense femtosecond laser ionizes a solid surface. These mirrors specularly reflect the main part of a laser pulse and can be used as active optical elements to manipulate its temporal and spatial properties. Unfortunately, the considerable pressures exerted by the laser can deform the mirror surface, unfavourably affecting the reflected beam and complicating, or even preventing, the use of plasma mirrors at ultrahigh intensities. Here we derive a simple analytical model of the basic physics involved in laser-induced deformation of a plasma mirror. We validate this model numerically and experimentally, and use it to show how such deformation might be mitigated by appropriate control of the laser phase.

High-order harmonics and attosecond pulses of light can be generated when ultraintense, ultrashort laser pulses reflect off a solid-density plasma with a sharp vacuum interface, i.e., a plasma mirror. We demonstrate experimentally the key influence of the steepness of the plasma-vacuum interface on the interaction, by measuring the spectral and spatial properties of harmonics generated on a plasma mirror whose initial density gradient scale length L is continuously varied. Time-resolved interferometry is used to separately measure this scale length.

Accelerating particles to relativistic energies over very short distances using lasers has been a long standing goal in physics. Among the various schemes proposed for electrons, vacuum laser acceleration has attracted considerable interest and has been extensively studied theoretically because of its appealing simplicity: electrons interact with an intense laser field in vacuum and can be continuously accelerated, provided they remain at a given phase of the field until they escape the laser beam. But demonstrating this effect experimentally has proved extremely challenging, as it imposes stringent requirements on the conditions of injection of electrons in the laser field. Here, we solve this long-standing experimental problem for the first time by using a plasma mirror to inject electrons in an ultraintense laser field, and obtain clear evidence of vacuum laser acceleration.With the advent of PetaWatt class lasers, this scheme could provide a competitive source of very high charge (nC) and ultrashort relativistic electron beams.1 arXiv:1511.05936v1 [physics.plasm-ph] 18 Nov 2015Femtosecond lasers currently achieve light intensities at focus that far exceed 10 18 W/cm 2 at near infrared wavelengths [1]. One of the great prospects of these extreme intensities is the laser-driven acceleration of electrons to relativistic energies within very short distances.At present, the most advanced scheme consists of using ultraintense laser pulses to excite large amplitude wakefields in underdense plasmas, providing extremely high accelerating gradients in the order of 100 GV/m [2]. However, over the past decades, the direct acceleration of electrons by light in vacuum has also attracted considerable interest and has been extensively studied theoretically [3][4][5][6][7][8][9][10][11]. These investigations have been driven by the fundamental interest of this most elementary interaction, and by its potential for extreme electron acceleration through electric fields of > 10's TV/m that ultraintense laser pulses provide.The underlying idea is to inject free electrons into an ultraintense laser field so that they always remain within a given half optical cycle of the field, where they constantly gain energy until they leave the focal volume. 1D Analytical calculations [3] show that for relativistic electrons, the maximum energy gain from this process is ∆E ∝ mc 2 γ 0 a 2 0 , where γ 0 is the electron initial Lorentz factor, and a 0 is the normalized laser vector potential, m the electron mass, and c the vacuum light velocity. Reaching high energy gains thus requires high initial energies γ 0 1 and/or ultrahigh laser amplitudes (a 0 1).In contrast with the large body of theoretical work published on this vacuum laser acceleration (VLA) of electrons to relativistic energies, experimental observations have largely remained elusive [12][13][14][15][16][17] -sometimes even controversial [18,19]-and have so far not demonstrated significant energy gains. This is because VLA occurs efficiently only for electrons injected in t...

International audienceThe nonlinear interaction of an intense femtosecond laser pulse with matter can lead to the emission of a train of sub-laser-cycle--attosecond--bursts of short-wavelength radiation1, 2. Much effort has been devoted to producing isolated attosecond pulses, as these are better suited to real-time imaging of fundamental electronic processes3, 4, 5, 6. Successful methods developed so far rely on confining the nonlinear interaction to a single sub-cycle event7, 8, 9. Here, we demonstrate for the first time a simpler and more universal approach to this problem10, applied to nonlinear laser-plasma interactions. By rotating the instantaneous wavefront direction of an intense few-cycle laser field11, 12 as it interacts with a solid-density plasma, we separate the nonlinearly generated attosecond pulse train into multiple beams of isolated attosecond pulses propagating in different and controlled directions away from the plasma surface. This unique method produces a manifold of isolated attosecond pulses, ideally synchronized for initiating and probing ultrafast electron motion in matter

The interaction of intense laser beams with plasmas created on solid targets involves a rich nonlinear physics. Because such dense plasmas are reflective for laser light, the coupling with the incident beam occurs within a thin layer at the interface between plasma and vacuum. One of the main paradigms used to understand this coupling, known as Brunel mechanism, is expected to be valid only for very steep plasma surfaces. Despite innumerable studies, its validity range remains uncertain, and the physics involved for smoother plasma-vacuum interfaces is unclear, especially for ultrahigh laser intensities. We report the first comprehensive experimental and numerical study of the laser-plasma coupling mechanisms as a function of the plasma interface steepness, in the relativistic interaction regime. Our results reveal a clear transition from the temporally-periodic Brunel mechanism to a chaotic dynamic associated to stochastic heating. By revealing the key signatures of these two distinct regimes on experimental observables, we provide an important landmark for the interpretation of future experiments.

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In this letter, cutting-edge 3D Particle-In-Cell simulations are used to demonstrate that so-called relativistic plasma mirrors irradiated by PetaWatt (PW) lasers and naturally curved by laser radiation pressure can be used to tightly focus Doppler-generated harmonics to extreme intensities between 10 25 − 10 26 W.cm −2 . Those simulations are then employed to develop and validate a general 3D model of harmonic focusing by a curved relativistic plasma mirror. Finally, the insight gained from this model is used to propose novel all-optical techniques that would further increase the plasma mirror curvature with the ultimate goal of approaching the Schwinger limit. PACS numbers: Valid PACS appear hereOne of the main goals of Ultra-High-Intensity (UHI) physics, which investigates light-matter interactions at ultra-high light intensities, has been to constantly push forward the maximum attainable light intensities for accessing novel physical regimes. With intensities now approaching I ≈ 10 22 W.cm −2 for PW-class lasers, UHI physics already offered remarkable opportunities to understand and model the complex laws governing plasma dynamics in the ultra-relativistic regime.A major challenge is now to push forward these intensities above 10 25 W.cm −2 to access nonlinear Quantum Electrodynamics (QED) regimes barely explored so far in the lab [1,2]. Above this limit, QED effects start playing a major role on the dynamics of electrons initially at rest: laser-accelerated electrons can produce γ-photons exerting a recoil comparable to electron momentum. Interaction of these γ-photons with laser photons can then produce stimulated e-/e+ pair cascades via the non-linear Breit-Wheeler process [3,4]. Approaching 10 29 W.cm −2 corresponding to the so-called Schwinger field E = 10 18 V.m −1 , light starts generating e-/e+ pairs out of vacuum [5][6][7]. Near this limit, vacuum would act as a non-linear medium whose refractive index depends on light intensity. Consequently, high intensity lasers could induce refraction of other light beams, causing breakdown of Maxwell's superposition principle that predicts that two light beams in vacuum simply add up and cannot interact with each other.Reaching intensities I > 10 25 W.cm −2 should thus have a considerable impact as it would give access to a totally new type of experiments thanks to which we could validate theories of non-linear QED/extreme laser physics developed so far [8,9]. It would also provide insight into complex astrophysical phenomena where such non-linear QED processes occur [10,11]. Yet, the light intensities required to unlock those exotic regimes are more than three orders of magnitude higher than the ones delivered by current optical laser technology, hence calling for the design of novel solutions.One of the most promising idea to break this intensity barrier consists in inducing a Doppler frequency up-shift of a laser of wavelength λ and then focusing the up-shifted radiation of wavelength λ u ≪ λ down to a focal spot size σ ≈ λ u . To implement this idea, a propi...