High-order Harmonic Source Research Articles

Phase-matching of high-order harmonic generation in the extreme ultraviolet region with orbital angular momentum.

High-order harmonics can generate vortex beams with orbital angular momentum (OAM) in the extreme ultraviolet region. However, experimental research on their phase-matching (PM) characteristics is limited. In this study, vortex high-order harmonic generation (HHG) in the extreme ultraviolet region was generated with Ar gas. Phase-matched HHG with OAM was obtained by optimizing the focus position, laser energy, and gas pressure. The dependence of the PM characteristics on these parameters was analyzed. In addition, we conducted an experimental analysis of the dimensional properties of vortex harmonics under PM conditions. This study is a contribution towards the intense vortex high-order harmonic light sources and their applications.

High-speed and wide-field nanoscale table-top ptychographic EUV imaging and beam characterization with a sCMOS detector.

We present high-speed and wide-field EUV ptychography at 13.5 nm wavelength using a table-top high-order harmonic source. Compared to previous measurements, the total measurement time is significantly reduced by up to a factor of five by employing a scientific complementary metal oxide semiconductor (sCMOS) detector that is combined with an optimized multilayer mirror configuration. The fast frame rate of the sCMOS detector enables wide-field imaging with a field of view of 100 µm × 100 µm with an imaging speed of 4.6 Mpix/h. Furthermore, fast EUV wavefront characterization is employed using a combination of the sCMOS detector with orthogonal probe relaxation.

Full-Brillouin-zone calculation of high-order harmonic generation from solid-state media

Solid-state high-order harmonic generation (HHG) continues to attract a lot of interest. From the theory and simulation standpoint, two issues are still open; The first is the so-called transition-dipole phase problem. It has been recognized that the dipoles must be treated as complex-valued quantities and that their corresponding Berry connections must be included to ensure phase-gauge invariance. However, while this has been successfully implemented for lower-dimensional systems, fully vectorial and three-dimensional simulations remain challenging. The second issue concerns the symmetry of the high-order harmonic response, when simulations sometimes fail to honor the symmetry of the crystalline material. This work addresses both of these problems with the help of a HHG-simulation approach which (a) is manifestly free of the transition-dipole phase problem, (b) does not require calculation of dipole moments, (c) can account for the contributions from the entire Brillouin zone, and (d) faithfully preserves the symmetry of the simulated crystalline material. We use the method to show that high-order harmonic sources are distributed throughout the Brillouin zone with various phase shifts giving rise to significant cancellations. As a consequence, for the simulated response to correctly capture the material symmetry, contributions from the entire Brillouin zone must be included. Our results have important implications for a number of HHG applications, including all-optical band and dipole reconstruction.

Strong-field coherent control of isolated attosecond pulse generation

Attosecond science promises to reveal the most fundamental electronic dynamics occurring in matter and it can develop further by meeting two linked technological goals related to high-order harmonic sources: improved spectral tunability (allowing selectivity in addressing electronic transitions) and higher photon flux (permitting to measure low cross-section processes). New developments come through parametric waveform synthesis, which provides control over the shape of field transients, enabling the creation of highly-tunable isolated attosecond pulses via high-harmonic generation. Here we demonstrate that the first goal is fulfilled since central energy, spectral bandwidth/shape and temporal duration of isolated attosecond pulses can be controlled by shaping the laser waveform via two key parameters: the relative-phase between two halves of the multi-octave spanning spectrum, and the overall carrier-envelope phase. These results not only promise to expand the experimental possibilities in attosecond science, but also demonstrate coherent strong-field control of free-electron trajectories using tailored optical waveforms.

Highly efficient soft x-ray spectrometer for transient absorption spectroscopy with broadband table-top high harmonic sources.

We present a novel soft x-ray spectrometer for ultrafast absorption spectroscopy utilizing table-top femtosecond high-order harmonic sources. Where most commercially available spectrometers rely on spherical variable line space gratings with a typical efficiency on the order of 3% in the first diffractive order, this spectrometer, based on a Hettrick–Underwood design, includes a reflective zone plate as a dispersive element. An improved efficiency of 12% at the N K-edge is achieved, accompanied by a resolving power of 890. The high performance of the soft x-ray spectrometer is further demonstrated by comparing nitrogen K-edge absorption spectra from calcium nitrate in aqueous solution obtained with our high-order harmonic source to previous measurements performed at the electron storage ring facility BESSY II.

Bright, polarization-tunable high repetition rate extreme ultraviolet beamline for coincidence electron–ion imaging

After decades of supremacy of the Titanium:Sapphire technology, Ytterbium-based high-order harmonic sources are emerging as a promising alternative for experiments requiring high flux of ultrashort extreme ultraviolet (XUV) radiation. In this article we describe a versatile experimental setup delivering XUV photons in the 10–50 eV range. The use of cascaded high-order harmonic generation enables us to reach 1.9 mW of average power at 18 eV. Several spectral selection schemes are presented, to isolate a single high-harmonic or a group of them. In the perspective of circular dichroism experiments, we produce highly elliptical XUV radiation using resonant elliptical high-harmonic generation, and circularly polarized XUV by bichromatic bicircular high-harmonic generation. As an illustration of the capacities of the beamline, we focus the XUV beam in a coincidence electron–ion imaging spectrometer, where we measure the photoelectron momentum angular distributions of xenon monomers and dimers.

Singleshot polychromatic coherent diffractive imaging with a high-order harmonic source.

Singleshot polychromatic coherent diffractive imaging is performed with a high-intensity high-order harmonic generation source. The coherence properties are analyzed and several reconstructions show the shot-to-shot fluctuations of the incident beam wavefront. The method is based on a multi-step approach. First, the spectrum is extracted from double-slit diffraction data. The spectrum is used as input to extract the monochromatic sample diffraction pattern, then phase retrieval is performed on the quasi-monochromatic data to obtain the sample's exit surface wave. Reconstructions based on guided error reduction (ER) and alternating direction method of multipliers (ADMM) are compared. ADMM allows additional penalty terms to be included in the cost functional to promote sparsity within the reconstruction.

Recent advances in ultrafast X-ray sources.

Over more than a century, X-rays have transformed our understanding of the fundamental structure of matter and have been an indispensable tool for chemistry, physics, biology, materials science and related fields. Recent advances in ultrafast X-ray sources operating in the femtosecond to attosecond regimes have opened an important new frontier in X-ray science. These advances now enable: (i) sensitive probing of structural dynamics in matter on the fundamental timescales of atomic motion, (ii) element-specific probing of electronic structure and charge dynamics on fundamental timescales of electronic motion, and (iii) powerful new approaches for unravelling the coupling between electronic and atomic structural dynamics that underpin the properties and function of matter. Most notable is the recent realization of X-ray free-electron lasers (XFELs) with numerous new XFEL facilities in operation or under development worldwide. Advances in XFELs are complemented by advances in synchrotron-based and table-top laser-plasma X-ray sources now operating in the femtosecond regime, and laser-based high-order harmonic XUV sources operating in the attosecond regime. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Wavelength-scale ptychographic coherent diffractive imaging using a high-order harmonic source

Ptychography enables coherent diffractive imaging (CDI) of extended samples by raster scanning across the illuminating XUV/X-ray beam, thereby generalizing the unique advantages of CDI techniques. Table-top realizations of this method are urgently needed for many applications in sciences and industry. Previously, it was only possible to image features much larger than the illuminating wavelength with table-top ptychography although knife-edge tests suggested sub-wavelength resolution. However, most real-world imaging applications require resolving of the smallest and closely-spaced features of a sample in an extended field of view. In this work, resolving features as small as 2.5 λ (45 nm) using a table-top ptychography setup is demonstrated by employing a high-order harmonic XUV source with record-high photon flux. For the first time, a Rayleigh-type criterion is used as a direct and unambiguous resolution metric for high-resolution table-top setup. This reliably qualifies this imaging system for real-world applications e.g. in biological sciences, material sciences, imaging integrated circuits and semiconductor mask inspection.

Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a Table-Top Femtosecond Soft X-ray Source.

We demonstrate the feasibility of soft X-ray absorption spectroscopy in the water window using a table-top laser-based approach with organic molecules and inorganic salts in aqueous solution. A high-order harmonic source delivers femtosecond pulses of short wavelength radiation in the photon energy range from 220 to 450 eV. We report static soft X-ray absorption measurements in transmission on the solvated compounds O=C(NH2)2, CaCl2, and NaNO3 using flatjet technology. We monitor the absorption of the molecular samples between the carbon (∼280 eV) and nitrogen (∼400 eV) K-edges and compare our results with previous measurements performed at the BESSYII facility. We discuss the roles of pulse stability and photon flux in the outcome of our experiments. Our work paves the way toward table-top femtosecond, solution-phase soft X-ray absorption spectroscopy in the water window.

Wavelength-scale Coherent Diffractive Imaging using a High-order Harmonic Source

Wavelength-scale Coherent Diffractive Imaging using a High-order Harmonic Source

Study of Higher Diffraction Order Contribution in a Flat Field Grating Spectrograph Using a High-Order Harmonic Source.

An experimental study has been carried out to characterize and investigate the performance of a flat-field grating spectrograph (FFGS) for higher diffraction order in the wavelength range of 80-170 Å. An aberration-corrected, mechanically ruled spherical grating with variable line spacing was used as a dispersive element in the spectrograph. The study was carried out using high-order harmonic radiation generated by an interaction of Ti:sapphire laser pulses with inert gas-filled cells. It was observed that the fraction of photons diffracted in the second diffraction order to that of the first order is as high as ∼ 65% at ∼ 80 Å wavelength, which reduces to ∼ 15% at ∼ 150 Å, whereas for the third diffraction order, the fraction was substantially lower, ∼ 15% at ∼ 100 Å. The observed results match well with reflectivity calculated using REFLEC software and also with the measurement carried out using INDUS-1 reflectivity beamline at RRCAT, Indore. The use of a high-order harmonic source for the study is advantageous, as the higher diffraction order contribution can be estimated from a single spectral image, which can be recorded in a single laser shot. The study will be useful for estimation of conversion efficiency-photon flux of a source using a variable line spaced (VLS) grating based spectrograph/monochromator.

High-order harmonic source spanning up to the oxygen K-edge based on filamentation pulse compression.

We present a 0.2 TW sub-two-cycle 1.8 µm carrier-envelope-phase stable source based on two-stage pulse compression by filamentation for driving high-order harmonic generation extending beyond the oxygen K absorption edge. The 1 kHz repetition rate, high temporal resolution enabled by the short 11.8 fs driving pulse duration, and bright high-order harmonics generated in helium make this an attractive source for solid-state and molecular-dynamics studies.

In situ ptychographic measurements of high-order harmonic sources from plasma mirrors: A theoretical and numerical study

Measuring the spatial properties of high-order harmonic beams produced by high-intensity laser-matter interactions directly in the target plane is very challenging due to the extreme physical conditions at stake in the interaction area. A measurement scheme has been recently developed to obtain this information experimentally, which consists in adapting a lensless imaging method known as ptychography. In this paper, we present a theoretical validation of this method in the case of harmonic generation from plasma mirrors, using a combination of simple modeling and 2D Particle-In-Cell simulations. This study investigates the concept of in situ ptychography and supports the analysis of experimental measurements presented in previous publications.

Band-Resolved Double Photoemission Spectroscopy on Correlated Valence Electron Pairs in Metals.

Correlated valence electrons in Ag and Cu are investigated using double photoemission spectroscopy driven by a high-order harmonic light source. Electron pairs consisting of two d electrons as well as pairs with one sp and one d electron are resolved in the two-dimensional energy spectrum. Surprisingly, the intensity ratio of sp-d to d-d pairs from Ag is 3 times higher than in the self-convoluted density of states. Our results directly show the band-resolved configurations of electron pairs in solids and emphasize a band-dependent picture for electron correlation even in these paradigmatic metals.

Performance of a ruthenium beam separator used to separate soft x rays from light generated by a high-order harmonic light source.

We describe the design and fabrication of a ruthenium beam separator used to simultaneously attenuate infrared light and reflect soft x rays. Measurements in the infrared and soft x-ray regions showed the beam separator to have a reflectivity of 50%-85% in the wavelength region from 6 to 10 nm at a grazing incidence angle of 7.5 deg and 4.3% at 800 nm and the same angle of grazing incidence, indicating that the amount of attenuation is 0.05-0.09. These results show that this beam separator could provide an effective means for separating IR light from soft x rays in light generated by high-order harmonic generation sources.

Attosecond beamline with actively stabilized and spatially separated beam paths.

We describe a versatile and compact beamline for attosecond spectroscopy. The setup consists of a high-order harmonic source followed by a delay line that spatially separates and then recombines the extreme-ultraviolet (XUV) and residual infrared (IR) pulses. The beamline introduces a controlled and actively stabilized delay between the XUV and IR pulses on the attosecond time scale. A new active-stabilization scheme combining a helium-neon-laser and a white-light interferometer minimizes fluctuations and allows to control delays accurately (26 as rms during 1.5 h) over long time scales. The high-order-harmonic-generation region is imaged via optical systems, independently for XUV and IR, into an interaction volume to perform pump-probe experiments. As a consequence of the spatial separation, the pulses can be independently manipulated in intensity, polarization, and frequency content. The beamline can be combined with a variety of detectors for measuring attosecond dynamics in gases, liquids, and solids.

Coherent diffractive imaging microscope with a high-order harmonic source.

We report the generation of highly coherent extreme ultraviolet sources with wavelengths around 30 and 10 nm by phase-matched high-order harmonic generation (HHG) in a gas cell filled with argon and helium, respectively. We then perform coherent diffractive imaging (CDI) by using a focused narrow-bandwidth HHG source with wavelength around 30 nm as an illumination beam for two kinds of samples. The first is a transmission sample and the second is a absorption sample. In addition, we report the successful reconstruction of a complex absorption sample using a tabletop high-harmonic source. This will open the path to the realization of a compact soft x-ray microscope to investigate biological samples such as membrane proteins.

Efficient and tunable high-order harmonic light sources for photoelectron spectroscopy at surfaces

With the recent progress in high-order harmonic generation (HHG) using femtosecond lasers, laboratory photoelectron spectroscopy with an ultrafast, widely tunable vacuum-ultraviolet light source has become available. Despite the well-established technique of HHG-based photoemission experiments at kilohertz repetition rates, the efficiency of these setups can be intrinsically limited by the space-charge effects. Here we present recent developments of compact HHG light sources for photoelectron spectroscopy at high repetition rates up to megahertz, and examples for angle-resolved photoemission experiments are demonstrated.

Rare-Gas Clusters in Intense Extreme-Ultraviolet Pulses from a High-Order Harmonic Source

We report evidence for two previously unidentified effects in the ionization of rare-gas clusters by intense extreme-ultraviolet pulses. First, electron spectra indicate multistep photoemission with increasing isotropy for larger clusters due to electron-atom collisions. Second, very slow (meV) electrons are interpreted as the first experimental evidence for Rydberg-like atomic state formation in the nanoplasma expansion. Only small fractions of Xe2+ ions were found, in sharp contrast to previous results recorded under comparable conditions [Murphy etal., Phys. Rev. Lett. 101, 203401 (2008).