Abstract: We present a modular extreme ultraviolet (XUV) spectrometer system optimized for a broad spectral range of 12-41 nm (30-99 eV) with a high spectral resolution of λ/Δλ ≳ 784 ± 89. The spectrometer system has several operation modes for (1) XUV beam inspection, (2) angular spectral analysis, and (3) imaging spectroscopy. These options allow for a versatile use in high harmonic spectroscopy and XUV beam analysis. The high performance of the spectrometer is demonstrated using a novel cross-sectional imaging method called XUV coherence tomography.
Wuensche M, Fuchs S, Weber T, Nathanael J, Abel JJ, Reinhard J, Wiesner F, Huebner U, Skruszewicz SJ, Paulus GG, Roedel C

Abstract: The setup and first results from commissioning of a fast online photon energy spectrometer for the vacuum ultraviolet free electron laser at Hamburg (FLASH) at DESY are presented. With the use of the latest advances in detector development, the presented spectrometer reaches readout frequencies up to 1 MHz. In this paper, we demonstrate the ability to record online photon energy spectra on a shot-to-shot base in the multi-bunch mode of FLASH. Clearly resolved shifts in the mean wavelength over the pulse train as well as shot-to-shot wavelength fluctuations arising from the statistical nature of the photon generating self-amplified spontaneous emission process have been observed. In addition to an online tool for beam calibration and photon diagnostics, the spectrometer enables the determination and selection of spectral data taken with a transparent experiment up front over the photon energy of every shot. This leads to higher spectral resolutions without the loss of efficiency or photon flux by using single-bunch mode or monochromators.
S. Palutke, N. C. Gerken, K. Mertens, S. Klumpp, A. Mozzanica, B. Schmitt, C. Wunderer, H. Graafsma, K.-H. Meiwes-Broer, W. Wurth, and M. Martins

Abstract: We present HHG driven with a sub-cycle mJ-level parametric waveform synthesizer. The variation of the HHG spectral shape and yield as a function of the relative phase between the synthesizer channels is shown.
Yudong Yang, Giulio Maria Rossi, Roland E. Mainz, Fabian Scheiba, Giovanni Cirmi, and Franz X. Kaertner

Abstract: We show that attosecond pulse trains are a natural tool to control strong field processes such as high-order harmonic generation. Coherently combining an attosecond pulse train with an IR driving field, we predict and experimentally confirm enhancement and spectral narrowing of the harmonic yield at photon energies around 90 eV. The use of an attosecond pulse train to seed the harmonic generation process replaces tunneling ionization with a single-photon ionization step, therefore permitting the manipulation of the time-frequency properties of high-order harmonic generation already at the single-atom level.
J. Biegert; A. Heinrich; C. P. Hauri; W. Kornelis; P. Schlup; M. P. Anscombe; M. B. Gaarde; K. J. Schafer; U. Keller

Abstract: In this paper, results of spectral investigations of low temperature photoionized plasmas, created by irradiation of gases with intense pulses of extreme ultraviolet (EUV) radiation from a laser-produced plasma (LPP) source, are presented. The LPP source was based on a double-stream KrXe/He gas-puff target irradiated with 4 ns/0.8 J/10 Hz Nd:YAG laser pulses. The most intense emission from the source spanned a relatively narrow spectral region λ ≈ 10–12 nm; however, spectrally integrated intensity at longer wavelengths was also significant. The EUV beam was focused on a gas stream, injected into a vacuum chamber synchronously with the EUV pulses. Irradiation of gases resulted in formation of photoionized plasmas emitting radiation in the EUV range. Radiation spectra, measured for plasmas produced in various gases, are dominated by emission lines, originating from single charged ions. Significant differences in spectral intensities and distributions between plasmas created in neon and molecular gases were observed.
A. Bartnik, H. Fiedorowicz and P. Wachulak

Abstract: Coherent soft x-ray (SXR) sources enable fundamental studies in the important water window spectral region. Until now, such sources have been limited to repetition rates of 1 kHz or less, which limits count rates and signal-to-noise ratio for a variety of experiments. SXR generation at high repetition rate has remained challenging because of the missing high-power mid-infrared (mid-IR) laser sources to drive the high-harmonic generation (HHG) process. Here we present a mid-IR optical parametric chirped pulse amplifier (OPCPA) centered at a wavelength of 2.2 {\mu}m and generating 16.5-fs pulses (2.2 oscillation cycles of the carrier wave) with 25 W of average power and a peak power exceeding 14 GW at 100-kHz pulse repetition rate. This corresponds to the highest reported peak power for high-repetition-rate mid-IR laser systems. The output of this 2.2-{\mu}m OPCPA system was used to generate a SXR continuum extending beyond 0.6 keV through HHG in a high-pressure gas cell.
Justinas Pupeikis, Pierre-Alexis Chevreuil, Nicolas Bigler, Lukas Gallmann, Christopher R. Phillips, Ursula Keller

Abstract: We measured the dependence of the intensity of high harmonics on the gas pressure in slab waveguide. The phase matching of 61st high harmonic was obtained according to the quadratic dependence of the increase in efficiency with pressure. For higher order harmonics such as H91, periodic oscillations of the harmonic intensity with gas pressure was observed. We also studied the effect of the slit width of the slab waveguide on the high harmonics generation. The influence of the slit width on laser transverse mode and phase mismatching conditions can be used to explain this effect.
Peng Gu, Shi Bing, Liu Hai, Ying Song

Abstract: We present our attoline which is a versatile attosecond beamline at the Ultrafast Laser Physics Group at ETH Zurich for attosecond spectroscopy in a variety of targets. High-harmonic generation (HHG) in noble gases with an infrared (IR) driving field is employed to generate pulses in the extreme ultraviolet (XUV) spectral regime for XUV-IR cross-correlation measurements. The IR pulse driving the HHG and the pulse involved in the measurements are used in a non-collinear set-up that gives independent access to the different beams. Single attosecond pulses are generated with the polarization gating technique and temporally characterized with attosecond streaking. This attoline contains two target chambers that can be operated simultaneously. A toroidal mirror relay-images the focus from the first chamber into the second one. In the first interaction region a dedicated double-target allows for a simple change between photoelectron/photoion measurements with a time-of-flight spectrometer and transient absorption experiments. Any end station can occupy the second interaction chamber. A surface analysis chamber containing a hemispherical electron analyzer was employed to demonstrate successful operation. Simultaneous RABBITT measurements in two argon jets were recorded for this purpose.
R. Locher, M. Lucchini, J. Herrmann, M. Sabbar, M. Weger, A. Ludwig, L. Castiglioni, M. Greif, M. Hengsberger, L. Gallmann and U. Keller

Abstract: A femtosecond Ti:sapphire laser is focused into a modulated He-filled slab waveguide to generate high-order harmonics. The modulated slab waveguide is used to periodically vary the intensity of the laser pulse along the direction of propagation in order to implement quasi-phase matching of the high-order harmonics. Experimental results show that compared with the spectra emitted from an unmodulated slab waveguide, there is an obvious increase in the yield of high harmonics at wavelengths close to the cutoff region. This finding is in agreement with theory and calculations reported in previous research. Therefore, our experiment indicates that, as with the modulated hollow-core waveguide, a slab waveguide can also achieve quasi-phase matched high-order harmonics.
Peng Gu, Shi-Bing Liu and Hai-Ying Song

Abstract: High harmonics are generated in a newly designed gas-filled slab waveguide. A femtosecond Ti:sapphire laser is focused at an intensity of approximately 9 × 1014 W/cm2 into a 20-mm-long gas cell and a 20-mm-long gas-filled slab waveguide. An obvious extension of high harmonic orders is found in the guided configuration when the experimental result of high harmonic generation in the He atoms in the long gas cell is compared with that in the He-filled slab waveguide. Simple calculations and results from experiments performed with the gas cell and the gas-filled slab waveguide under different experimental conditions show that ionization-induced laser defocusing should be responsible for the lower cutoff position in the long gas cell. Our experimental results indicate that like the gas-filled capillary, the gas-filled slab waveguide restrains the effect of laser defocusing.
Peng Gu, Shi-Bing Liu and Hai-Ying Song

Abstract: We present measurements from laser-produced plasmas generated using cryogenic Xe targets and quantify the emission characteristics in the soft x-ray region (1 to 6 nm). The system is based on a LN2-cooled rotating drum, which allows for a high repetition rate, and Nd:YAG laser systems with energies up to 325 mJ on-target with pulse lengths of 130 ps, 600 ps, or 6.5 ns. High resolution spectra are measured using a grazing incidence spectrometer, and we present the first quantitative conversion efficiency (CE) measurements for Xe in this range. Data show CE values up to ∼1% at 6 nm and ∼0.08% at 1.5 nm (for 2% bandwidth and 2π solid angle), and there are lower limits on the required laser intensities and energies on target to achieve these efficiencies. In addition, the emission spot size is directly measured at 2 nm (620 eV) using a point-projection slit imaging method, with optimized emission spot sizes of ∼20 μm.
S. C. Bott-Suzuki, A. Bykanov, O. Khodykin, M. Tillack, S. Cordaro, and C. McGuffey

Abstract: We present HHG driven with a sub-cycle mJ-level parametric waveform synthesizer. The variation of the HHG spectral shape and yield as a function of the relative phase between the synthesizer channels is shown.
Yudong Yang, Giulio Maria Rossi, Roland E. Mainz, Fabian Scheiba, Giovanni Cirmi, and Franz X. Kärtner

Abstract: Coherent soft x-ray (SXR) sources enable fundamental studies in the important water window spectral region. Until now, such sources have been limited to repetition rates of 1 kHz or less, which limits count rates and signal-to-noise ratio for a variety of experiments. SXR generation at high repetition rate has remained challenging because of the missing high-power mid-infrared (mid-IR) laser sources to drive the high-harmonic generation (HHG) process. Here we present a mid-IR optical parametric chirped pulse amplifier (OPCPA) centered at a wavelength of 2.2 {\mu}m and generating 16.5-fs pulses (2.2 oscillation cycles of the carrier wave) with 25 W of average power and a peak power exceeding 14 GW at 100-kHz pulse repetition rate. This corresponds to the highest reported peak power for high-repetition-rate mid-IR laser systems. The output of this 2.2-{\mu}m OPCPA system was used to generate a SXR continuum extending beyond 0.6 keV through HHG in a high-pressure gas cell.
Justinas Pupeikis, Pierre-Alexis Chevreuil, Nicolas Bigler, Lukas Gallmann, Christopher R. Phillips, Ursula Keller