Report of the Basic Energy Sciences Workshop on Compact Light Sources

The Compact Light Source (CLS) is a new type of x-ray source, a miniature synchrotron light source, which has been developed at Lyncean Technologies, Inc. The CLS is a near-monochromatic, tunable, homelab-size x-ray source with up to three beamlines that can be used like the x-ray beamlines at the synchrotrons--but it is about 200 times smaller than a synchrotron light source. The compact size is achieved using a laser undulator and a miniature electron-beam storage ring. The photon flux on a sample will be comparable to the flux of highly productive synchrotron beamlines. This presentation will first introduce the basic principles of the Compact Light Source and show how it can bring the quality, tunability and flux of a synchrotron beam line into an x-ray scientist’s local laboratory. At Lyncean Technologies, Inc. we have recently completed the construction of a production prototype source with funding from theNIGMS Protein Structure Initiative. The presentation will show details of the electron storage ring, laser system and x-ray optics and conclude with results of our initial testing of the prototype CLS and x-ray optics. m10.o02

In this paper, we present our latest results on development of a high harmonic generation (HHG) based EUV source at KMLabs. We have successfully demonstrated the first commercially available HHG source that meets critical specifications for metrology and imaging applications. We attribute this performance to our patented hollow core fiber technology and the modularized beamline developed to reliably characterize this EUV source.

In the last decade, there has been an increasing demand for a compact Inverse Compton Light Source (ICLS) which is capable of producing high-quality X-rays by colliding an electron beam and a high-quality laser. It is only in recent years when both SRF and laser technology have advanced enough that compact sources can approach the quality found at large installations such as the Advanced Photon Source at Argonne National Laboratory. Previously, X-ray sources were either high flux and brilliance at a large facility or many orders of magnitude lesser when produced by a bremsstrahlung source. A recent compact source was constructed by Lyncean Technologies using a storage ring to produce the electron beam used to scatter the incident laser beam. By instead using a linear accelerator system for the electron beam, a significant increase in X-ray beam quality is possible, though even subsequent designs also featuring a storage ring offer improvement. Preceding the linear accelerator with an SRF reentrant gun allows for an extremely small transverse emittance, increasing the brilliance of the resulting X-ray source. In order to achieve sufficiently small emittances, optimization was done regarding both the geometry of the gun and the initial electron bunch distribution produced off the cathode. Using double-spoke SRF cavities to comprise the linear accelerator allows for an electron beam of reasonable size to be focused at the interaction point, while preserving the low emittance that was generated by the gun. An aggressive final focusing section following the electron beam's exit from the accelerator produces the small spot size at the interaction point which results in an X-ray beam of high flux and brilliance. Taking all of these advancements together, a world class compact X-ray source has been designed. It is anticipated that this source would far outperform the conventional bremsstrahlung and many other compact ICLSs, while coming closer to performing at the levels found at large facilities than ever before. The design process, including the development between subsequent iterations, is presented here in detail, with the simulation results for this groundbreaking X-ray source.

The rapid growth of nanotechnology has increased the need for fast nanoscale imaging. X-ray free electron laser (XFEL) facilities currently provide such coherent sources of directional and high-brilliance X-ray radiation. These facilities require large financial investments for development, maintenance, and manpower, and thus, only a few exist worldwide. In this article, we present an automated table-top system for XUV coherent diffraction imaging supporting the capabilities for multispectral microscopy at high repetition rates, based on laser high harmonic generation from gases. This prototype system aims towards the development of an industrial table-top system of ultrafast soft X-ray multi-spectral microscopy imaging for nanostructured materials with enormous potential and a broad range of applications in current nanotechnologies. The coherent XUV radiation is generated in a semi-infinite gas cell via the high harmonic generation of the near-infrared femtosecond laser pulses. The XUV spectral selection is performed by specially designed multilayer XUV mirrors that do not affect the XUV phase front and pulse duration.

Coherent light pulses of few to hundreds of femtoseconds (fs) duration haveprolically served the eld of ultrafast phenomena. While fs pulses addressmainly dynamics of nuclear motion in molecules or lattice in the gas, liquidor condensed matter phase, the advent of attosecond (asec) pulses has inrecent years provided direct experimental access to ultrafast electron dynamics.However, there are processes involving nuclear motion in molecules andin particular coupled electronic and nuclear motion that occur in the few fsor even sub-fs time scale. Electronic excitations in molecules are commonlyin the VUV/XUV spectral region. Until recently most of the XUV sourceswere lacking either sucient pulse energy ( High Order Harmonic Generation(HOHG) sources) or ultrashort pulse duration (free electron lasers (FEL) [1]),thus preventing access to XUV-pump-XUV-probe measurements in the 1fsor asec temporal scale. In this work, by loose focusing a multi-cycle, highpower, fs Infra-Red (IR) laser (35fs, 160mJ=pulse, 805nm) into a Xenongas jet, the non-linear interaction results in frequency up-conversion and,under proper experimental conditions for Phase Matching (PM), in the formationof Attosecond Pulse Trains (APT). The application of InterferometricPolarization Gating (IPG) [88, 89] to this scheme allows the generation ofenergetic, broadband, coherent extreme-ultraviolet (XUV) continuum radiation.Although the temporal properties of the XUV pulse are aected by thelack of Carrier-Envelope Phase (CEP) stabilization [86], the XUV intensityis high enough to induce XUV multi-photon absorption in atomic [75] ormolecular system.So far, experimental eorts on this time scale have been restricted to XUVIRpump-probe schemes [34, 37, 41, 44, 73, 94], or in-situ electron-ion collisionmethods [59]. As the IR eld induced potential may distort the molecular potential[33], the use of IR-free technique to investigate the intrinsic moleculardynamics is of central importance.The present work focuses on the developmentof techniques for the investigation of such dynamics utilizing solelyXUV radiation and demonstrates proof of principle time resolved and XUVpump - XUV probe experiments, tracking ultrafast molecular dynamics.In order to induce the aforementioned XUV multi-photon processes, highXUV photon ux is required. In order to increase the XUV intensity, thesetup was upgraded to allow the use of 3m or 6m lenses for loose focus con-guration [36], and of a Barium Borate (BBO) crystal, for 2-color drivenHOHG [9, 14, 19, 25, 40, 95]. Although in our case the XUV intensity doesnot benet from the 2-color generation scheme, there is the possibility in theHOHG process to control the electron re-collision trajectories by acting on the parameters of the two laser elds [10]. With this option, the use of longtrajectories is convenient due to their higher energy content. The temporalcharacterization of the HOH obtained by the long trajectories in a 2-colordriving eld is an unexplored eld of investigation. We succeeded in thistask, recording 2nd order Interferometric Volume Auto Correlation [84, 85]traces by using, for the rst time, a multiply ionized atom [8, 87] as a nonlineardetector. The results show clearly the existence of a structure on the1fs scale.Exploiting the short duration of the above mentioned high energy pulses, allthe optically allowed excited states of H2 are coherently populated. Monitoringthe pump-probe delay dependent yield of protons, nuclear and electronic1fs scale dynamics are subsequently investigated and compared to the resultsof ab initio calculations. The revealed dynamics reects the intrinsicmolecular behavior as the XUV probe pulse, despite its still high intensity,hardly distorts the molecular potential. It was further feasible to follow theopening of the dissociative ionization channel through the 2+g (2pu) state,due to the stretching of the molecule. This is visible as a build-up of non-zerokinetic energies proton signal during the rst fs of the delay time.Utilizing individual harmonics, single (5th+5th; 7th+7th harmonic) and twocolor(5th + 9th harmonic) time resolved spectroscopy has been performedin molecular Oxygen, aiming at settling existing discrepancies of previousexperiments [23, 64]. Finally an attempt in observing energy resolved directtwo photon atomic double ionization [43, 48] through energy resolved electronspectroscopy has identied necessary actions and improvements for asuccessful implementation of such an experiment.

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.

Through generation of coherent extreme ultraviolet (XUV) radiation, high-harmonic generation (HHG) has opened up a wealth of possibilities for table-top research, ranging from attosecond dynamics to high resolution coherent imaging. The ability to perform interferometry with HHG sources would enable powerful coherent optical techniques such as Fourier transform spectroscopy in the XUV spectral range. However, interferometry and Fourier transform spectroscopy (FTS) remain challenging, due to the need for a pulse pair with a controllable, yet stable, attosecond delay. To overcome this challenge we have developed a method to produce such ultrastable XUV pulse pairs, using a birefringence-based common path interferometer. This interferometer is suitable for high-intensity laser pulses, which enables the generation of fully coherent XUV pulse pairs through HHG. The resulting pulse pairs have sub-attosecond timing variation, a tunable time delay and a clean Gaussian spatial mode profile, as shown in Figure 1 [1].

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.

The emission of high-order harmonics from solids under intense laser-pulse irradiation is revolutionizing our understanding of strong-field solid-light interactions, while simultaneously opening avenues towards novel, all-solid, coherent, short-wavelength table-top sources with tailored emission profiles and nanoscale light-field control. To date, broadband spectra in solids have been generated well into the extreme-ultraviolet (XUV), but the comparatively low conversion efficiency in the XUV range achieved under optimal conditions still lags behind gas-based high-harmonic generation (HHG) sources. Here, we demonstrate that two-color high-order harmonic wave mixing in a fused silica solid is more efficient than solid HHG driven by a single color. This finding has significant implications for compact XUV sources where gas-based HHG is not feasible, as solid XUV wave mixing surpasses solid-HHG in performance. Moreover, our results enable utilizing solid high-order harmonic wave mixing as a probe of structure or material dynamics of the generating solid, which will enable reducing measurement times compared to the less efficient regular solid HHG. The emission intensity scaling that follows perturbative optical wave mixing, combined with the angular separation of the emitted frequencies, makes our approach a decisive step for all-solid coherent XUV sources and for studying light-engineered materials.

While large-scale synchrotron sources provide a highly brilliant monochromatic X-ray beam, these X-ray sources are expensive in terms of installation and maintenance, and require large amounts of space due to the size of storage rings for GeV electrons. On the other hand, laboratory X-ray tube sources can easily be implemented in laboratories or hospitals with comparatively little cost, but their performance features a lower brilliance and a polychromatic spectrum creates problems with beam hardening artifacts for imaging experiments. Over the last decade, compact synchrotron sources based on inverse Compton scattering have evolved as one of the most promising types of laboratory-scale X-ray sources: they provide a performance and brilliance that lie in between those of large-scale synchrotron sources and X-ray tube sources, with significantly reduced financial and spatial requirements. These sources produce X-rays through the collision of relativistic electrons with infrared laser photons. In this study, an analysis of the performance, such as X-ray flux, source size and spectra, of the first commercially sold compact light source, the Munich Compact Light Source, is presented.

High harmonic generation (HHG) from solids shows great application prospects in compact short-wavelength light sources and as a tool for imaging the dynamics in crystals with subnanometer spatial and attosecond temporal resolution. However, the underlying collision dynamics behind solid HHG is still intensively debated and no direct mapping relationship between the collision dynamics with band structure has been built. Here, we show that the electron and its associated hole can be elastically scattered by neighboring atoms when their wavelength approaches the atomic size. We reveal that the elastic scattering of electron/hole from neighboring atoms can dramatically influence the electron recombination with its left-behind hole, which turns out to be the fundamental reason for the anisotropic interband HHG observed recently in bulk crystals. Our findings link the electron/hole backward scattering with Van Hove singularities and forward scattering with critical lines in the band structure and thus build a clear mapping between the band structure and the harmonic spectrum. Our work provides a unifying picture for several seemingly unrelated experimental observations and theoretical predictions, including the anisotropic harmonic emission in MgO, the atomic-like recollision mechanism of solid HHG, and the delocalization of HHG in ZnO. This strongly improved understanding will pave the way for controlling the solid-state HHG and visualizing the structure-dependent electron dynamics in solids.

While conventional x-ray tube sources reliably provide high-power x-ray beams for everyday clinical practice, the broad spectra that are inherent to these sources compromise the diagnostic image quality. For a monochromatic x-ray source on the other hand, the x-ray energy can be adjusted to optimal conditions with respect to contrast and dose. However, large-scale synchrotron sources impose high spatial and financial demands, making them unsuitable for clinical practice. During the last decades, research has brought up compact synchrotron sources based on inverse Compton scattering, which deliver a highly brilliant, quasi-monochromatic, tunable x-ray beam, yet fitting into a standard laboratory. One application that could benefit from the invention of these sources in clinical practice is coronary angiography. Being an important and frequently applied diagnostic tool, a high number of complications in angiography, such as renal failure, allergic reaction, or hyperthyroidism, are caused by the large amount of iodine-based contrast agent that is required for achieving sufficient image contrast. Here we demonstrate monochromatic angiography of a porcine heart acquired at the MuCLS, the first compact synchrotron source. By means of a simulation, the CNR in a coronary angiography image achieved with the quasi-mono-energetic MuCLS spectrum is analyzed and compared to a conventional x-ray-tube spectrum. The results imply that the improved CNR achieved with a quasi-monochromatic spectrum can allow for a significant reduction of iodine contrast material.

I was surprised to discover from Alexander Helleman's article (News & Comment, [16 Feb., p. 902][1]) that we had reached “consensus” on future x-ray generation at the International Committee on Future Accelerators Workshop on 4th Generation Light Sources, hosted by the European Synchrotron Radiation Facility in Grenoble, France. In fact, the group of more than 100 international scientists found it difficult even to agree on the definition of “4th Generation,” let alone have a precise view of the future. While linear accelerator (linac)-based free electron lasers (FELs) did have their strong advocates, as Helleman describes, there was more to the meeting than that. FELs have already demonstrated success as powerful infrared facilities in Europe, the United States, and elsewhere, mostly on the basis of relatively low-energy electron linacs. However, electron storage rings are likely to remain better value for the money in the higher energy range from 100 megaelectron volts to at least several gigaelectron volts; such energies are needed for the next step of user facilities using very ultraviolet, extreme ultraviolet, and even soft x-ray output. Of course, if an accelerator has already been funded for other purposes [such as the TESLA linac at DESY (Germany's particle physics laboratory near Hamburg)], then it makes sense to exploit it for FEL development. In contrast to such unusual (and technologically demanding) linacs, there are already large numbers of storage rings in use or planned around the world, and it seems logical to explore the use of FELs on these as a favored option. Successful demonstrations have been conducted for more than 10 years, and one UV user facility now exists at the LURE center near Paris. It seems likely that the present Russian world record for FEL output wavelength [240 nanometers (nm)] will be broken later this year as the Duke University storage ring in the United States comes on stream. The workshop concluded that storage ring FEL technology should reach 50 nm within 2 years and 20 nm soon afterwards (the possibility of 4 nm was also discussed). It is far too early to write an obituary for both the 3rd Generation Light Sources and the FELs they are likely to contain in the future. As usual, a number of complementary sources are going to emerge, and each will have its application. [1]: /lookup/volpage/272/902

Extreme ultraviolet (EUV) lithography is expected to be used for 3 nm technology nodes and beyond, yet the need for actinic mask metrology and inspection remains a critical challenge. In this study, we demonstrate an EUV ptychography microscope as a high-harmonic generation-based actinic mask imaging tool. A series of diffraction patterns on an EUV mask is used to reconstruct both the amplitude and phase information of the periodic patterns using ptychographic algorithms. The results show that the EUV ptychography microscope has the potential for determining the actinic metrology of EUV masks and providing phase information for EUV mask development.

In the original version of this Article, the affiliation for Luca Poletto was incorrectly given as ‘European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Hamburg, Germany’, instead of the correct ‘CNR, Istituto di Fotonica e Nanotecnologie Padova, Via Trasea 7, 35131 Padova, Italy’. This has now been corrected in both the PDF and HTML versions of the Article.