Overview

Light constitutes a unique probe to trigger and address processes at play in matter. In particular, in the nineties, with the advent of femtosecond light sources two avenues opened: i) pump-probe techniques could be applied at the natural timescale of molecular processes, giving birth to the nowadays flourishing femtochemistry field and ii) due to the high peak value of the light field obtained, the realm of NonLinear Optics (NLO) dramatically widened, setting up new investigation techniques such as Z-scan, confocal microscopy... Xstase will use these two qualities to investigate the interaction of matter with XUV light beams carrying either type of Angular Momentum, Spin (SAM) or Orbital (OAM) through linear and nonlinear processes. We will in particular try to synthesize XUV attosecond pulses through High Harmonic Generation (HHG) with angular momenta and precisely characterize them (1 as= 10-18 s). A typical HHG setup is sketched below.

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In a typical experiment, a femtosecond laser (10-50 fs) is focused on a gas jet where it produces high energy photons through extreme non linear processes. These photons, which are in the XUV range are analysed on a photon spectrometer (inset) or used to study photoionization processes with charged particles detectors (intermediate gas inlet, e.g. Time of Flight (TOF) Magnetic Bottle Electron Spectrometer (MBES)). In Xstase, we will try to impart angular momenta on these photons.

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HHG - Microscopic response

The process of HHG is best described by quantum mechanical methods. A long term collaboration with Richard Taïeb, Jérémie Caillat and Alfred Maquet from LCPMR provide us the theoretical support on the resolution of the Time-Dependant Shrödinger Equation (TDSE). Locally, Thierry Auguste is implementing the Strong Field Approximation (SFA) version of TDSE, but includes full 3D propagation of the incoming field and XUV field in his computations. In a pictorial model, the process may be split in three steps. The first one, occurring close to an extremum of the field, is the tunnel ionization of the outermost electron of the target gas. In a second step, this electron is driven by the laser field during about half of the period of the laser field, which is 1.3 fs with our most used Ti:Sapphire 800 nm wavelength lasers. This excursion in the continuum may drive the electron back to the ionic core where the collision (3rd step) may result in the emission of its kinetic energy as an XUV photon.

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Illustration of the three step model. The attosecond structure of the XUV radiation comes from the very narrow time window, close to the field extremum, during which tunnel ionization is significant.
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HHG - Macroscopic response

Like all nonlinear conversion phenomena, phase matching plays a crucial role in HHG. Here we upconvert IR photons to XUV in a gas cell with a laser beam intense enough to partially ionize the medium. Since the ions, the electrons and neutral do not have the same complex index, phase matching will depend on both time and space, as the medium gets progressively ionized at one point in the gas target. This makes the result of HHG very much laser dependent, even on a daily basis. We studied temporal aspects of this phenomenon (see T. Ruchon et al., NJP 2008 and X. He et al., Phys. Rev. A 2009). As a benefit, it also yields the good directionality of the XUV beam, which is emitted perpendicular to the IR wavefront, it may be used to cut on or off the emission during a pulse (ionization gating) and may be used to even split the emission from successive half cycles in the attosecond light house configuration (see e.g. Kim et al.).

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Illustration of the phase matching for HHG. The progressive ionization of the medium while the laser goes through leads to a varying index in time, both for its absorption and refractive parts (right graph). The same happens in space.