Sujet de stage / Master 2 Internship

Coherent diffractive imaging is a lensless imaging technique that makes use of spatial coherence and Fourier optics to reach spatial resolutions of the order of 10s to few nanometers. The aim of this internship will be to implement ptychography, a scanning coherent diffractive imaging technique, on a new laser high harmonic source working at 20 nm.

Coherent diffractive imaging (CDI) is a lensless imaging technique that uses spatial coherence and Fourier optics to remove the traditionally used objective optics in order to reach wavelength limited spatial resolution in X-ray microscopy. First developed for synchrotron facilities, its implementation on laser based short wavelength sources has allowed for the rise of nanoscale femtosecond imaging. For more than 15 years the CEA-LIDYL has developed new CDI techniques on laboratory sources, such as Fourier Transform Holography with Extended Reference [1], Stereo 3D imaging [2], or single shot CDI for time resolved experiments [3,4].

Those small scale sources are based on an ultra nonlinear phenomenon called laser high order harmonic generation (HHG). When focusing a femtosecond infrared laser beam on a gas jet, one can generate ultrashort (femtosecond to attosecond) coherent radiation in the extreme ultraviolet domain (100-10 nm). This process is now well understood, and is currently undergoing a rapid democratization for ultrafast spectroscopy experiments. However, the low photon flux is a strong limitation for many applications.

At CEA-LIDYL we recently developed a new high harmonic source based on a high repetition rate femtosecond laser. The aim of the internship is to implement on this beamline a ptychography setup. Ptychography is a specific CDI technique that relies on scanning, enabling to reach nanometre scale resolutions on large-scale samples [5]. In a second step, the technique will be extending to very large spectral bandwidths, in order to increase the usable photon flux and reach sub-femtosecond time resolutions.

1. Gauthier et al., Single-shot Femtosecond X-Ray Holography Using Extended References, PRL 105, 093901 (2010)
2. Duarte et al., Computed stereo lensless X-ray imaging, Nat. Photonics 13, 449 (2019)
3. Ravasio et al., Single-Shot Diffractive Imaging with a Table-Top Femtosecond Soft X-Ray Laser-Harmonics Source, PRL 103, 028104 (2008)
4. Boutu et al., Impact of noise in holography with extended references in the low signal regime, Opt. Express 24, 6318 (2016)
5. Thibault et al., High-Resolution Scanning X-ray Diffraction Microscopy, Science 321, 379 (2008)