We develop and implement an experimental strategy for the generation of high-energy high-order harmonics (HHG) in gases for studies of nonlinear processes in the soft x-ray region. We generate high-order harmonics by focusing a high energy Ti:Sapphire laser into a gas cell filled with argon or neon. The energy per pulse is optimized by an automated control of the multiple parameters that influence the generation process. This optimization procedure allows us to obtain energies per pulse and harmonic order as high as 200 nJ in argon and 20 nJ in neon, with good spatial properties, using a loose focusing geometry (\documentclass[12pt]{minimal}\begin{document}$f_\# \approx 400$\end{document}f#≈400) and a 20 mm long medium. We also theoretically examine the macroscopic conditions for absorption-limited conversion efficiency and optimization of the HHG pulse energy for high-energy laser systems.
We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed.
Spatial and spectral properties of the high-order harmonic emission in argon for seeding applicationsHe, Xinkui; Miranda, Miguel; Schwenke, Jörg; Guilbaud, Olivier; Ruchon, Thierry; Heyl, C.; Georgiadou, Elisavet; Rakowski, Rafal; Persson, Anders; Gaarde, Mette; L'Huillier, Anne General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. We characterize and control the harmonic emission in the spectral and spatial domain in order to define in which conditions the harmonic radiation can be a high-quality seed for soft X-ray and X-ray free electron lasers. The length of the gas cell where harmonics are generated was optimized and the energy per pulse was determined in absolute value with a calibrated X-ray photodiode. The beam spatial profile was measured and in some conditions a very collimated beam with a half angle divergence below one mrad could be obtained. We also show that increasing the intensity of the fundamental laser field leads to a considerable broadening of the bandwidth of the harmonic radiation, allowing us to cover a large spectral range. This effect is due to fundamental reshaping leading to efficient phase matching of both short and long trajectory contributions.
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