Thèse de doctorat en Physique
Sous la direction de Stéphane Sebban.
Soutenue en 2015
Développement d'un laser XUV collisionnel à plasma OFI ultrabref et intense
Collisionally-pumped “OFI” plasma-based soft X-ray lasers are achieved by focusing an ultra-intense infrared laser pulse into a gas. The resulting laser-plasma interaction allows the generation of a plasma column in population inversion, made of multi-charged ions and energetic electrons. We are interested in the emission from the 3d94dJ=0 → 3d94pJ=1 atomic transition of krypton IX (Nickel-like) at 32. 8 nm. When this plasma is seeded by an external high-harmonic source, the resulting emission exhibits excellent spatial properties, while demonstrating a significantly higher photon yield at the relevant wavelength. Although being compact and exhibiting numerous attractive characteristics, collisional plasma-based X-ray lasers face limitations intrinsic to their pumping scheme. Indeed, they used to deliver quite long pulses (a few picosecond), thus limiting the scope of applications. The main focus of this thesis has been associated with the implementation of an original technique aimed at achieving 100 fs-range duration of emission by quenching the plasma amplifier gain lifetime through collisional over-ionization (Collisional Ionization Gating). This required operating at very high electron densities (about 1020 cm-3), which involved the implementation of optical waveguiding techniques. The “seeded regime” has been used to sample the ultrafast gain lifetime of such a plasma amplifier. A time-dependent Maxwell-Bloch code allowed describing the ultrashort amplification dynamics and deriving a final soft X-ray pulse duration. The method additionally allows a larger photon yield per shot (14 µJ), thus promising a nearly three orders of magnitude surge in soft X-ray pulse intensity compared to previous performances. Another important focus of this thesis dealt with the implementation of a circularly polarized plasma-based X-ray laser. Such source allows the study of dichroism, magnetization dynamics in matter or chiral domains in biology. The source has been demonstrated by seeding a krypton IX plasma amplifier with a resonant circularly polarized high-harmonic signal. In agreement with experimental measurements, our Maxwell-Bloch numerical model confirms the conservation of the high-harmonic polarization over amplification in the plasma and the efficiency of the scheme, which paves the way for prospective single-shot measurements.
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