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.

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.

We study high-order harmonic generation in argon driven by an intense 800 nm laser field and a small fraction of its second harmonic. The intensity and divergence of the emitted even and odd harmonics are strongly modulated as a function of the relative delay between the two fields. We provide a detailed analysis of the underlying interference effects. The interference changes drastically when approaching the cutoff region due to a switch of the dominant trajectory responsible for harmonic generation.High-order harmonic generation (HHG) from the interaction of an intense infrared (IR) laser field and a gas target provides a coherent table-top radiation source in the extreme ultraviolet (XUV) range, of interest for a number of applications, in particular the production of attosecond light pulses [1,2]. The underlying physics of HHG is well described by the so-called three-step model [3][4][5]: an electron wave packet is created by tunneling through the Coulomb barrier deformed by the laser field; it is subsequently accelerated by the laser field; and returns to the atom where it recombines to the ground state, leading to the production of an XUV light burst. This process is repeated every half-cycle of the IR laser field, resulting in an attosecond pulse train (APT) with a pulse separation of one-half IR period and to a spectrum of odd harmonics.There is a growing interest to achieve even better control of the generation process [6], e.g., to obtain higher conversion efficiency or to tailor attosecond pulses or pulse trains for specific applications. Two-color HHG driven by an IR laser and its second harmonic (blue) provides subcycle control of the generating electric field, with the interesting property that two consecutive half-cycles become different, and not simply opposite in sign. This breakdown of the electric field inversion symmetry has been used for several applications, e.g., the generation of even and odd high-harmonics with increased conversion efficiency [7,8] and the production of attosecond pulse trains with one pulse per IR cycle [9,10]. In some conditions, when the intensity of the second harmonic is much weaker than that of the fundamental laser field, even harmonics can be used to provide information about the generation process [11][12][13].In this article, we investigate both experimentally and theoretically high-order harmonic generation driven by a two-color laser field consisting of a 800 nm fundamental and a fraction of its second harmonic. The even and odd harmonic intensities are found to be modulated as a function of IR-blue delay, forming in some cases a rich interference pattern (Fig. 1). We investigate how these oscillations depend on harmonic energy and intensity of the blue field and how the spatial profiles of the emitted harmonics are affected. We provide an interpretation based on quasiclassical calculations.Experiments were performed using an amplified 10 Hz titanium sapphire laser system delivering 40 fs pulses at 800 nm with energy up to 1 J. The results presented in this ar...

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The recently discovered, chlorophyll-f-containing, far-red photosystem II (FR-PSII) supports far-red light photosynthesis. Participation and kinetics of spectrally shifted far-red pigments are directly observable and separated from that of bulk chlorophyll-a. We present an ultrafast transient absorption study of FR-PSII, investigating energy transfer and charge separation processes. Results show a rapid subpicosecond energy transfer from chlorophyll-a to the long-wavelength chlorophylls-f/d. The data demonstrate the decay of an ∼720-nm negative feature on the picosecond-to-nanosecond timescales, coinciding with charge separation, secondary electron transfer, and stimulated emission decay. An ∼675-nm bleach attributed to the loss of chl-a absorption due to the formation of a cation radical, PD1+•, is only fully developed in the nanosecond spectra, indicating an unusually delayed formation. A major spectral feature on the nanosecond timescale at 725 nm is attributed to an electrochromic blue shift of a FR-chlorophyll among the reaction center pigments. These time-resolved observations provide direct experimental support for the model of Nürnberg et al. [D. J. Nürnberg et al., Science 360, 1210–1213 (2018)], in which the primary electron donor is a FR-chlorophyll and the secondary donor is chlorophyll-a (PD1 of the central chlorophyll pair). Efficient charge separation also occurs using selective excitation of long-wavelength chlorophylls-f/d, and the localization of the excited state on P720* points to a smaller (entropic) energy loss compared to conventional PSII, where the excited state is shared over all of the chlorin pigments. This has important repercussions on understanding the overall energetics of excitation energy transfer and charge separation reactions in FR-PSII.