High Harmonic Generation Source Research Articles

Monolithic focused reference beam X-ray holography

Fourier transform holography is a highly efficient and robust imaging method, suitable for single-shot imaging at coherent X-ray sources. In its common implementation, the image contrast is limited by the reference signal generated by a small pinhole aperture. Increased pinhole diameters improve the signal, whereas the resolution is diminished. Here we report a new concept to decouple the spatial resolution from the image contrast by employing a Fresnel zone plate to provide the reference beam. Superimposed on-axis images of distinct foci are separated with a novel algorithm. Our method is insensitive to mechanical drift or vibrations and allows for long integration times common at low-flux facilities like high harmonic generation sources. The application of monolithic focused reference beams improves the efficiency of high-resolution X-ray Fourier transform holography beyond all present approaches and paves the path towards sub-10 nm single-shot X-ray imaging.

Towards Zeptosecond-scale Pulses from X-ray Free-electron Lasers

The aim of the workshop was to consider what developments might be achievable in the next generation of light sources. One area of particular interest is the generation of ultra-short pulses of light. High harmonic generation (HHG) sources are now capable of reaching pulse durations in the range of tens to hundreds of attoseconds, thereby facilitating the study of electron dynamics. Here we consider the potential of free-electron lasers (FELs) to contribute to this field. First we make the case for FELs in general as a particularly promising source of ultra-short pulses, and summarise some of the relevant proposals. Different classes of ultra-short pulse techniques are identified—each with respective merits and potentially better suited to different types of application. Particular focus is given to highlighting a recent proposal by the authors to generate trains of few-cycle pulses from amplifier FELs, which if applied at hard x-ray wavelengths could generate pulses at the single attosecond scale, or even shorter—into the zeptosecond scale.

Generation of highly coherent extreme ultraviolet source and its application in diffraction imaging

We report the generation of small bandwidth highly coherent extreme ultraviolet radiation and the application of this source in coherent diffractive transmission microscope. Using a focussed narrow-bandwidth high harmonic generation (HHG) source with wavelength around 30nm we achieve a resolution of ∼45nm with a sample size down to 3μm×3μm in a very short exposure time of <8s. The experimental scheme is able for imaging sub-10nm scale objects with an inexpensive table-top, commercially available femtosecond laser system.

A high-harmonic generation source for seeding a free-electron laser at 38 nm

Direct seeding with a high-harmonic generation (HHG) source can improve the spectral, temporal, and coherence properties of a free-electron laser (FEL) and shall reduce intensity and arrival-time fluctuations. In the seeding experiment sFLASH at the extreme ultraviolet FEL in Hamburg FLASH, which operates in the self-amplified spontaneous emission mode (SASE), the 21st harmonic of an 800 nm laser is refocused into a dedicated seeding undulator. For seeding, the external light field has to overcome the noise level of SASE; therefore, an efficient coupling between seed pulse and electron bunch is mandatory. Thus, an HHG beam with a proper divergence, width, beam quality, Rayleigh length, pointing stability, single-shot pulse energy, and stability in the 21st harmonic is needed. Here, we present the setup of the HHG source that seeds sFLASH at 38.1 nm, the optimization procedures, and the necessary diagnostics.

Soft X-Ray Magneto-Optics: Probing Magnetism by Resonant Scattering Experiments

The most advanced X-ray sources (third generation synchrotrons, linear free-electron lasers and high-harmonic generation sources) widen the range of application of X-ray scattering techniques considerably. Beyond flux and brilliance, improvements in polarization tuneability, degree of coherence and selectable time-structure promoted new methods for investigating the electronic and magnetic properties of solids. The soft X-ray range (50-2000 eV) is well suited for studying magneto-optical effects in laterally confined submicron sized objects, either artificially built or self-assembled. First, by tuning the photon energy at a core resonance, one provides the X-ray scattering technique with element selectivity. Second, resonant excitations make the optical constants sensitive to the local magnetization by introducing large off-diagonal elements in the dielectric tensor; since magnetic effects are stronger when the core excitation produces a dipolar transition to final states involving the magnetic orbitals ( 3d for the first row TM; 4 f for RE), the most interesting resonances for X-ray magneto-optics [(2, 3) p → 3 d and (3, 4) d → 4 f] are all located in the soft X-ray region. Finally, the wavelengths corresponding to soft X-rays are very well suited for scattering studies of nanometer- to micrometer-sized magnetic structures. We will present the results of recent soft X-ray resonant scattering experiments, showing that the combination of element selectivity, magnetic sensitivity and structural analysis can help disentangling and understanding the magnetic properties of complex self-assembled periodic systems. Lastly, recent applications of coherent scattering to the X-ray holographic imaging of magnetic domains will be presented.

Photoelectron kinetic and angular distributions for the ionization of aligned molecules using a HHG source

We present an experimental and theoretical investigation of the angular distributions of electrons ejected in aligned molecules by extreme ultra-violet ionization using a high harmonic generation (HHG) source. Impulsive alignment in O2, N2 and CO molecules was achieved using a near-IR laser pulse and the photoelectron angular distribution after ionization by a fs harmonic comb composed of harmonic H11 to H29 (17.5–46 eV) was recorded at the maximum of both alignment and anti-alignment. The experiment reveals signatures that are specific for the electronic orbitals that are ionized as well as the onset of the influence of the molecular structure and is well reproduced by theoretical calculations based on the multichannel Schwinger configuration interaction method.

Time-of-flight-photoelectron emission microscopy on plasmonic structures using attosecond extreme ultraviolet pulses

We report on the imaging of plasmonic structures by time-of-flight-photoemission electron microscopy (ToF-PEEM) in combination with extreme ultraviolet (XUV) attosecond pulses from a high harmonic generation source. Characterization of lithographically fabricated Au structures using these ultrashort XUV pulses by ToF-PEEM shows a spatial resolution of ∼200 nm. Energy-filtered imaging of the secondary electrons resulting in reduced chromatic aberrations as well as microspectroscopic identification of core and valence band electronic states have been successfully proven. We also find that the fast valence band electrons are not influenced by space charge effects, which is essentially important for attosecond nanoplasmonic-field microscopy realization.

Coherent Control of High Harmonic Generation via Dual-Gas Multijet Arrays

High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources (>2) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.

High harmonic generation as a seed source for free electron lasers

The design of a high harmonic generation (HHG) source for seeding the proposed New Light Source free electron laser (FEL) is presented. The seed source is designed to cover the energy range from 50 to 100 eV with a peak power of 400 kW and a 20 fs FWHM pulse length and is tuneable to provide continuous coverage over the full 50–100 eV range. The seed pulses are produced by HHG in rare gases in the loose-focusing regime, driven by a high-power kHz laser system. Continuous tunability is achieved by varying the wavelength of the drive laser over ±25 nm from the 800 nm centre wavelength. Future prospects for increasing the HHG yield and short wavelength cut-off by using multi-wavelength generation schemes and longer drive wavelengths are considered, as is the potential for FEL seeding at higher repetition rates.

Analysing a two-electron wavepacket by semiclassically propagating its Fourier components in space

In the last few years, the development of high order harmonic generation sources and free electron lasers delivering ultra-intense and ultra-short VUV-XUV pulses has made it possible to study nonlinear processes in atoms and molecules on the electronic time scale. The theoretical support required by the ongoing experiments comes notably in the form of numerical tools intended to solve the time-dependent Schrödinger equation. The wavepacket produced in these approaches has a multichannel character and its analysis in terms of the observed physical channels is a problem in itself. Various solutions have been proposed so far, which all suffer from one or another inconvenience, ranging from very heavy computational costs to the inability to characterize differential cross sections. The purpose of this paper is to propose a new, low-cost and complete method of analysis. It consists in propagating the Fourier components of the wavepacket with respect to the hyper-radius all the way to the genuine asymptotic region where the various channels disentangle from each other based on their kinematics. We demonstrate the feasibility and versatility of this proposal by applying it to two different time-propagation codes in the case of one-photon double ionization of helium using short pulses.

Extreme ultraviolet radiation for coherent diffractive imaging with high spatial resolution

Using different noble gases, argon, neon and helium, we are able to generate by high-harmonic generation (HHG) just a few harmonic orders in the spectral range 10–35 nm with a photon flux of ∼ 2.1012 photons/(harmonic cm2 s) for argon and ∼ 1010 photons/(harmonic cm2 s) for helium. The few-harmonic-order radiation is used for coherent diffractive imaging directly without any spectral filter. A spatial resolution of ∼ 100 nm is achieved using a ∼ 30 nm HHG source.

Tri-material multilayer coatings with high reflectivity and wide bandwidth for 25 to 50 nm extreme ultraviolet light

Magnesium/silicon carbide (Mg/SiC) multilayers have been fabricated with normal incidence reflectivity in the vicinity of 40% to 50% for wavelengths in the 25 to 50 nm wavelength range. However many applications, for example solar telescopes and ultrafast studies using high harmonic generation sources, desire larger bandwidths than provided by high reflectivity Mg/SiC multilayers. We investigate introducing a third material, Scandium, to create a tri-material Mg/Sc/SiC multilayer allowing an increase the bandwidth while maintaining high reflectivity.

Near diffraction limited coherent diffractive imaging with tabletop soft x-ray sources

Tabletop coherent x-ray sources hold great promise for practical nanoscale imaging, in particular when coupled with diffractive imaging techniques. In initial work, we demonstrated lensless diffraction imaging using a tabletop high harmonic generation (HHG) source at 29 nm, achieving resolutions ∼ 200 nm. In recent work, we significantly enhanced our diffractive imaging resolution by implementing a new high numerical aperture (up to NA=0.6) scheme and field curvature correction where we achieved sub-100 nm resolution. Here we report the first demonstration of Fourier transform holography (FTH) with a tabletop SXR source, to acquire images with a resolution ≈ 90 nm. The resolution can be refined by applying phase retrieval. Additionally, we show initial results from FTH with 13.5 nm HHG radiation and demonstrate ∼ 180 nm resolution.

Diffractive imaging using a polychromatic high-harmonic generation soft-x-ray source

A new approach to diffractive imaging using polychromatic diffraction data is described. The method is tested using simulated and experimental data and is shown to yield high-quality reconstructions. Diffraction data produced using a high-harmonic generation source are considered explicitly here. The formalism can be readily adapted, however, to any short-wavelength source producing a discrete spectrum and possessing sufficient spatial coherence.

Multiple wavelength diffractive imaging

We demonstrate coherent diffraction imaging using multiple harmonics from a high-harmonic generation source. An algorithm is presented that builds the known incident spectrum into the reconstruction procedure with the result that the useable flux is increased by more than an order of magnitude. Excellent images are obtained with a resolution of $(165\ifmmode\pm\else\textpm\fi{}5)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and compare very well with images from a scanning electron microscope.

Vacuum space-charge effects in solid-state photoemission

Solid-state photoemission spectroscopy relies to a large part on pulsed photon sources: third-generation synchrotron-radiation sources and ultrafast laser systems in particular. Especially when the photon pulses are intense, Coulombic repulsion between the emitted electrons will be a limiting factor for photoemission experiments aiming at highest energy and angle resolutions. In the present work, the propagation of the photoelectron cloud to the detector is studied with a full $N$-body numerical simulation. The influence of various parameters, in particular number of electrons per pulse, source size, pulse duration, kinetic-energy and emission-angle distributions as well as presence of mirror charges in the sample, is investigated in detail. Previous experimental results obtained with various picosecond and femtosecond light sources are successfully reproduced and the general resolution limits of solid-state photoemission using pulsed photon sources are explored. The results are potentially important for the design and interpretation of photoemission experiments with next-generation light sources, such as free-electron lasers and high-harmonic generation sources.

Helium double ionization signals under soft-x-ray coherent radiation

In this work we present theoretical ionization yields of helium for photon energies where direct two-photon double electron ejection is allowed. We explore the photon energies and the laser peak intensities of the xuv pulse where the direct double ionization of helium is expected to dominate over the sequential channel. This is achieved with the separate calculation of the partial double ionization yield (sequential and direct). We generally conclude that a careful choice of the photon energy in combination with the peak xuv intensity can result in the clear dominance of one signal (direct) over the other (sequential). Avoiding photons where resonant He+ enhancement may appear, relative double ionization signals are less sensitive to pulse duration and the ideal candidate for the observation of the direct double ionization signal appears to be photons around 45 eV (29th harmonic of a Ti:Sapphire 800 nm laser), assuming the radiation is restricted to high order harmonic generation sources. For continuously tunable sources, such as the free electron lasers, any frequency around the minima between intermediate resonances will be as convenient. In addition, we have demonstrate that beyond a certain intensity, depending on wavelength and pulse duration, the signal from the direct will be overwhelmed by the sequential.

Tunable light sources based on high harmonics generation for time-resolved VUV spectroscopy

The project of set-up for time resolved femtosecond VUV spectroscopy, which is based on a high harmonic generation source and a VUV monochromator is presented. We discuss the generation conditions of quasi-continuous VUV spectrum via HHG in Ar using the intense chirped IR fundamental pulse and the relatively weak second harmonic of IR fundamental. The desired narrow VUV spectral range is selected by time compensated double monochromator with concave spherical diffraction gratings.

Coherent x-ray Raman spectroscopy: a nonlinear local probe for electronic excitations.

Nonlinear x-ray four-wave mixing experiments are becoming feasible due to rapid advances in high harmonic generation and synchrotron radiation coherent x-ray sources. By tuning the difference of two x-ray frequencies across the valence excitations, it is possible to probe the entire manifold of molecular electronic excitations. We show that the wave vector and frequency profiles of this x-ray analogue of coherent Raman spectroscopy provide an excellent real-space probe that carries most valuable structural and dynamical information, not available from spontaneous Raman techniques.