Frequency-Domain Interferometry in the XUV with High-Order Harmonics

Abstract

We demonstrate that frequency-domain interferometry can be performed in the extreme ultraviolet range using high-order harmonics. We first show that two phase-locked harmonic sources delayed in time can be generated in the same medium despite ionization. This gives insight into the dynamics of the generation and ionization processes. We then apply the technique to the study of the temporal evolution of an ultrashort laser-produced plasma at the femtosecond time scale. PACS numbers: 42.65.Ky, 32.80.Rm Recently, high-order harmonic generation (HHG) has attracted considerable interest, not only as an intriguing and spectacular phenomenon, but also as a potential source presenting unique properties in the extreme ultraviolet (XUV) range. The two main characteristics of this radiation are the ultrashort pulse duration and the good coherence that are both unprecedented in this spectral region. The short pulse duration (down to a few tens of femtosecond) has been characterized [1-3] and is already used in atomic and molecular spectroscopy [4]. On the other hand, a number of experiments in the last few years have shown that good coherence properties could be obtained in some generating conditions [5][6][7][8] but no applications of this coherence have been performed yet. XUV interferometry, developed so far with x-ray lasers [9-12], would benefit a lot from the HHG properties. First, the tunability of the radiation allows one to adapt the wavelength to the probed medium, for example, far from resonances. In plasmas, different electron densities can be probed by changing the harmonic order, leading to a precise density mapping. Note that beside this coarse tunability, a fine adjustment of the wavelength can be obtained by generating the harmonics with a chirped laser [13] or with a laser mixed with an optical parametric amplifier [14]. Second, the ultrashort harmonic pulse duration is well adapted to the study of ultrafast processes, and could prevent, for example, the blurring of the fringes due to fast evolution of the density profile of a plasma close to the critical surface, as is the case when using x-ray lasers [9]. Finally, HHG is a tabletop XUV source, naturally synchronized with the generating laser at the same repetition rate (10 Hz to 1 kHz) allowing systematic experiments. In this work, we perform frequency-domain interferometry with high-order harmonics. We first demonstrate that the technique can be transposed to harmonics, and then apply it to probe the electron density of a laser-produced plasma in a high density gas jet with a femtosecond temporal resolution.Frequency-domain interferometry is now a widely used technique in the infrared for femtosecond time-resolved studies in solid-state and plasma physics [15][16][17]. It can be described as the temporal analog of the Young two-slit experiment [18]. In the latter, two spatially separated phase-locked sources lead to interferences in the far field after the beams have diffracted. In the former, two temporally separated phase-locked pulses interfere in ...

Spatially resolved Fourier transform spectroscopy in the extreme ultraviolet

“…This radiation is typically emitted in a train of attosecond pulses with excellent spatial and temporal coherence, as shown in various interferometric and spectroscopic measurements [4][5][6][7][8][9][10][11][12]. As a result, interferometry with high harmonics found important applications in e.g.…”

Nouvelles perspectives de diagnostics de plasmas denses par génération d'harmoniques d'ordre élevé

“…Si Ton utilise la partie basse de l'image comme reference, la position des franges dans le plasma ne donne alors qu'une valeur inferieure de la densite electronique. Nous en deduisons pres de la cible des densites d'au moins 2.5 10 20 electrons/cm 3 …”

“…Nous avons ensuite applique ce schema interferometrique a la mesure de densite d'un plasma d'aluminium [2]. Dans la seconde experience nous avons montre qu'il etait possible de generer deux sources harmoniques separees en temps et bloquees en phase en focalisant une double impulsion laser au meme endroit dans le milieu generateur [3]. Apres dispersion du rayonnement par un reseau on observe alors des franges d'interference dans le domaine des frequences.…”

Contrôle de la phase relative d'impulsions harmoniques à une échelle de temps attoseconde

“…Cette technique originale nous a permis de réaliser une cartographie 2-D de la densité électronique d'un plasma produit par laser [ !)]. L'analogue temporel de cette technique, qui consiste à produire deux impulsions harmoniques séparées en temps mais bloquées en phase a également été démontré [12,13], et utilisé pour le diagnostic de plasmas par interférométrie fréquentielle UVX [12,14]. Nous exposons dans cet article une extension de cette méthode à quatre impulsions.…”