Coherent dynamics of plasma mirrors

Abstract

Coherent ultrashort X-ray pulses provide new ways to probe matter and its ultrafast dynamics 1-3 . One of the promising paths to generate these pulses consists of using a nonlinear interaction with a system to strongly and periodically distort the waveform of intense laser fields, and thus produce high-order harmonics. Such distortions have so far been induced by using the nonlinear polarizability of atoms, leading to the production of attosecond light bursts 4 , short enough to study the dynamics of electrons in matter 3 . Shorter and more intense attosecond pulses, together with higher harmonic orders, are expected 5,6 by reflecting ultraintense laser pulses on a plasma mirror-a dense (≈10 23 electrons cm −3 ) plasma with a steep interface. However, short-wavelength-light sources produced by such plasmas are known to generally be incoherent 7 .In contrast, we demonstrate that like in usual low-intensity reflection, the coherence of the light wave is preserved during harmonic generation on plasma mirrors. We then exploit this coherence for interferometric measurements and thus carry out a first study of the laser-driven coherent dynamics of the plasma electrons.One of the challenges of high-order harmonic generation (HHG), beyond the production of very high orders with good efficiencies, is to preserve the initial properties of the laser beam in this frequency conversion process. In particular, keeping a high degree of coherence is essential for many applications, such as coherent imaging 8 . This has already been shown to be possible for HHG in gases 9 , but remains an open question for dense plasmas. In this case, an intense laser pulse interacts with an initially solid target, and creates a dense reflective plasma at the surface. In an early study using picosecond laser pulses 10 , interference measurements in the far-field using Young slits suggested that the coherence of the light field in the source plane was lost in interaction with such an extremely dense and hot (a few 10 6 K) plasma, for instance through stochastic processes or plasma instabilities. This conclusion was consistent with the observation of an uncollimated harmonic emission. Here, we demonstrate that such deleterious effects can be avoided using well-controlled interaction conditions, that is, ultrashort (<100 fs) laser pulses with a high temporal contrast. In these conditions, the plasma hardly has time to expand during the interaction (density gradient scale length L l/10, with l being the laser wavelength), and thus behaves as a high-flatness mirror-a plasma mirror 11 . We then exploit the coherence of the produced harmonics to study the dynamics of plasma electrons on the attosecond timescale.Our experiment uses 60 fs pulses with a high temporal contrast (10 10 on the nanosecond timescale), to produce high-order harmonics on plasma mirrors through coherent wake emission 12 (CWE), at intensities from a few 10 16 W cm −2 to a few 10 17 W cm −2 .Groups of harmonics in the beam diverging from the plasma mirror are selected with different ...

Temporal characterization of attosecond pulses emitted from solid-density plasmas

“…This implies that the CWE mechanism generating the harmonics in our case scales, unlike harmonic generation from noble gases, nearly linearly with the incident laser intensity. This finding is supported by earlier results [12,22], which also report a near-linear dependence of the CWE harmonic signal on laser intensity.…”

“…In contrast to that, in ROM, the harmonics are generated by the electrons in the plasma surface oscillating at relativistic velocities, resulting in a cutoff that is target independent and scales only with the laser intensity [19]. In addition, recent experiments have demonstrated that the harmonic emission from a solid target is beamed in the direction of the specular reflection under high-contrast conditions [18,20,21], have shown the transition from CWE to ROM harmonics [13,18], addressed the issue of harmonic coherence [22] and have confirmed the theoretically predicted harmonic power-law scaling [23] and spectral cutoff [19] in the ultra-relativistic regime [11].…”

Attosecond control of collective electron motion in plasmas

“…First, due to the global temporal drift of the APT it induces, a change in the CE phase changes the intensities at which each individual attosecond XUV burst in the APT is generated. Second, a change in intensity modifies the subcycle 5 emission time of each XUV burst because it changes the time it takes for the returning Brunel electrons to reach the dense part of the plasma where they cross and trigger CWE [27,28]. For a few-cycle pulse envelope, the combination of these two effects leads to a dependence of the APT time structure on the CE phase of the pulse.…”

Émission cohérente de sillage et dynamique plasma

“…Les interférences entre trois sources de différentes intensités, peuvent être décrites par l'équation suivante [10]:…”

“…Dans le cas considéré ici, la symétrie de la configuration fait que le terme de phase affecte le contraste des franges de fréquence |k |, mais pas leurs positions. Ce déphasage peut être extrait des interférogrammes, en effectuant une analyse de Fourier [10]. La figure 4(a) montre le résultat de cette mesure, pour les harmoniques 11 à 13, et différents rapports d'éclairements 0 .…”