Spatial and spectral properties of the high-order harmonic emission in argon for seeding applications

Abstract

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.

Femtosecond time-resolved photoelectron spectroscopy with a vacuum-ultraviolet photon source based on laser high-order harmonic generation

“…Besides the astigmatism, the short distance between the HHG source and the grating also prevents us from optimizing the so called "loose focusing conditions" of the driving laser for HHG in gas jets or cells (8,9,10,11,14) for optimal HHG flux as with a longer focal length the grating would be damaged by the driving laser. For the set ups described here we decided to accept these disadvantages as they turned out to be acceptable (see focus and flux measurements in section III).…”

“…In set up I we used a gas jet as the HHG medium. The focal length of the lens, the laser pulse energy, the numerical aperture of the laser at the lens position, the length of the rare gas generation medium along the laser propagation axis and the gas medium pressure need to be optimized for a given generation gas to reach maximum VUV flux (1,2,4,(8)(9)(10)(11)(12)(13)(14).…”

“…Often, the technological and methodological challenges for the generation and application of attosecond pulses are very different from those for femtosecond pulses. As we are describing our approach with femtosecond resolution we shall concentrate in the following on femtosecond pulses from High Harmonic Generation (HHG) (1,2,(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). The applications of such pulses range from spectroscopy (1,2,(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27) to microscopy (28,29) and coherent diffraction or holography (30)(31)(32)(33).…”

Resonant third harmonic generation of KrF laser in Ar gas

“…Interestingly, further 3rd harmonic intensity enhancement was observed for laser parameters causing a visible plasma column creation in the gas jet which can be attributed to nonlinear propagation effects (also found in high-order harmonic generation (HHG) with Ti:Sa lasers). 22 This led to extending of favorable condition for efficient HHG beyond the coherence and absorption lengths. To our knowledge the effect relays on spatio-temporal reshaping of the fundamental laser pulse propagating under filament-like conditions resulting in a spatial flattening of Gaussian pulse distribution (thus a greater part of the beam profile matches to optimum HHG), changing of the geometrical phase-matching contribution to Eq.…”

Spatio–spectral structures in high-order harmonic beams generated with Terawatt 10-fs pulses

“…It is due to the half cycle periodicity of the HHG process that the attosecond pulses of the train interfere with each other and spectral structures related to the number of pulses in the train can appear. In addition, spectral splitting in the harmonic spectra due to the microscopic response 30 or due to propagation in the medium 31,32 has also been reported.…”