X-ray Free Electron Laser (XFEL) radiation may transform diamond into graphite. Two X-ray pulses were used; the first as pump to trigger the phase transition and the second as probe performing X-ray diffraction. The experiment was performed at the SACLA XFEL facility at the beamline 3 experimental hutch 5. The samples were polycrystalline diamond. The pump and probe photon energies were 7 and 10.5 keV, respectively, and the delay between the X-ray pulses was varied from 0 to 286 fs. To provide a range of energy densities, the X-ray focus was adjusted between 150 nm and 1 um. The (111), (220) and (311) diffraction peaks were observed. The intensity of each diffraction peak decreased with time indicating a disordering of the crystal lattice. From a Debye-Waller analysis, the root-mean-square (rms) atomic displacement perpendicular to particular lattice planes are calculated. At higher fluences, the rms atomic displacement perpendicular to the (111) planes is significantly larger than that perpendicular to the (220) or (311) planes. By accepting two successive XFEL pulses at a time delay of 33 ms, graphite (002) diffraction was observed beginning at a threshold dose of 1.7 eV/atom. The experimental results will be compared with calculations using a hybrid model based on tight-binding molecular dynamics.
Irradiation effects of poly(methyl methacrylate) (PMMA) induced by femtosecond-pulsed extreme ultraviolet (EUV) were investigated using Soft X-ray free electron laser (SXFEL) for realization of next generation extreme ultraviolet free electron laser (EUV-FEL) lithography. The sensitivity of PMMA upon exposure to femtosecond-pulsed SXFEL was much higher than that measured for conventional nanosecond-pulsed EUV source. The sensitivity enhancement upon exposure to femtosecond-pulsed SXFEL is similar to the result obtained using laser-induced-plasma based Soft X-ray laser (SXRL) (picosecond-pulsed EUV). This result speculates the reactions induced by femtosecond-pulsed SXFEL and picosecond-pulsed XRL were almost same, but it was different from those induced by nanosecond-pulsed EUV.
We evaluated laser-induced damage thresholds (LIDTs) on silica glasses by two kinds of ultra-short soft X-ray laser pulses (13.5 nm, 70 fs, and 13.9 nm, 7 ps). The comparison of our experimental results and the reported values observed by nanosecond soft X-ray pulse revealed a pulse width dependence of LIDTs on silica glasses in the soft X-ray region. The relationship between the pulse width and LIDTs provided valuable discussions of the laser-induced damage mechanism.
Tight XFEL focusing is very important for significantly enhancing photon flux density, which is highly demanded by users exploring nonlinear X-ray optics. However, focusing XFEL down to 10 nm or less is so difficult from the viewpoints of both optical fabrication and optical alignment. The former can be overcome using techniques of wavefront sensing and fine shape correction. For the latter, techniques for directly measuring beam size on the focus without an influence of vibration of nanobeam are required. We have developed a technique for determining the size of nanobeam on the focus using an intensity interferometer, based on the Hanbury Brown and Twiss effect, of X-ray fluorescence emitted from a thin film inserted into the focus. The spatial coherence of X-ray fluorescence observed far from the focus depends on the distance from the focus and emission region of X-ray fluorescence. Therefore, the measured coherence can determine the size of X-ray nanobeam. This method has advantages that vibration of nanobeam does not affect the result and the setup is so simple.
A demonstration experiment was performed using a 100 nm focusing system based on total reflection KB mirrors at SACLA. X-ray fluorescence (8 keV) emitted from a thin Cu film by irradiation of focused XFEL pulses (12 keV) was detected shot-by-shot with a dual MPCCD. Analyses of approximately 1000 images based on the autocorrelation revealed that the beam size obtained with this method is in good agreement with one obtained with the wire scan method.
Focusing X-ray free-electron lasers (XFELs) is very important for producing ultra-intense X-ray nanoprobes. We have developed a system based on multilayer Kirkpatrick–Baez (KB) mirrors to focus XFELs to 10 nm or less at the SPring-8 Angstrom Compact free-electron LAser (SACLA) facility. The mirror optics in the system are designed with a large NA of greater than 0.01 to produce a diffraction-limited size of 6 nm at 9 keV. We constructed a precise X-ray grating interferometer based on the Talbot effect, and succeeded in fabricating near-perfect focusing mirrors with wavefront aberrations of λ/4.
However, strict error tolerances for mirror alignment can prevent sub-10 nm focusing. Errors of perpendicularity, incident angle, and astigmatism cause aberration on the focusing wavefront and characteristically change the beam shape. In particular, the required accuracy of the incident angle is 500 nrad. Due to shot-by-shot variations in the XFEL beam position and vibration of the optics, a single-shot diagnosis of beam shape is essential to align the mirrors quickly and accurately. By improving the method proposed by Sikorski et al. at the Stanford Linear Accelerator Center (SLAC), National Accelerator Laboratory, we propose a nanobeam diagnosis method based on the speckle pattern observed under coherent scattering. Computer simulation revealed that speckle size and beam size are inversely proportional. Platinum particles with a diameter of 2 nm were prepared and irradiated with X-rays to obtain a speckle pattern. Our experimental results demonstrate the successful estimation of beam shape and the alignment of all mirrors with the required accuracies.
We present the study of optical and spectral properties of radiation-induced stable point defects, known as color centers (CCs), in lithium fluoride (LiF) for the detection of 10 keV XFEL beam at Spring-8 Angstrom Compact free electron LAser (SACLA) in Japan. A thick LiF crystal was irradiated in four spots with 10 keV XFEL beam (pulse duration = 10 fs) with different number of accumulated shots. After irradiation the colored-LiF spots were characterized with an optical microscope in fluorescence mode and their photoluminescence intensity and spectra were analyzed.
This article reports the progress in the beamlines at the SPring-8 Angstrom Compact free electron LAser (SACLA). The beamline optical and diagnostics systems have been upgraded to further accelerate the scientific applications of X-ray free-electron lasers (XFELs). End-station instruments have also been developed to provide user-friendly experimental platforms which allow efficient data collection. Along with the upgrades of beamlines and experimental stations, we have established reliable and efficient procedures of the beamline operation.
X-ray free-electron lasers (XFELs) that utilize intense and ultra-short pulse X-rays may damage optical elements. We investigated the damage fluence thresholds of optical materials by using an XFEL focusing beam that had a power density sufficient to induce ablation phenomena. The 1 μm focusing beams with 5.5 keV and/or 10 keV photon energies were produced at the XFEL facility SACLA (SPring-8 Angstrom Compact free electron LAser). Test samples were irradiated with the focusing beams under normal and/or grazing incidence conditions. The samples were uncoated Si, synthetic silica glass (SiO2), and metal (Rh, Pt)-coated substrates, which are often used as X-ray mirror materials.
We proposed a split and delay optics setup with Si(220) crystals combined with Kirkpatric-Baez mirror optics for x-ray
pump-x-ray probe experiments at x-ray free-electron laser facilities. A prototype of the split-delay optics and its
alignment procedure were tested at BL29XUL of SPring-8. The horizontal focal profile, measured via double-beam
operation, showed good spatial overlap between the split beams with an FWHM of 100 nm, near the diffraction limit at
10 keV. High throughputs of the split-delay optics of 12% (upper) and 7.4% (lower) were obtained. The throughputs can
be improved to 30% and 20% by optimizing the upper and lower central energy, respectively.
X-ray transmission properties of a thin HPHT IIa diamond crystal were characterized around Bragg diffraction, using a
pseudo plane-wave setup at the 1-km beamline of SPring-8. Monochromatic x-rays of 19.75 keV were used for diamond
400 reflection from 120-μm-thick (001) diamond crystals, and 9.44-keV x-rays were used for diamond 111 reflection
from 180-μm-thick (111) crystals. These thin crystals were mounted on the aluminum plate using an ultraviolet-cured
resin. Several thin crystals showed rocking curve broadening due to bend. However, by limiting a small area of the
crystal, transmittance curves agreed well with those of calculation. We can select a practically usable region for various
applications: phase retarder, beam splitter, and also self-seeding of x-ray free electron laser.
We have developed a new method to fabricate ultrathin silicon single crystals, which can be used as spectral beam
splitters for the hard x-ray regime, based on a reactive dry etching process using plasma at atmospheric pressure. The
high crystalline perfection of the crystals was verified by both topographic and high-resolution rocking curve
measurements using coherent x-rays at the 1-km-long beamline, BL29XUL of SPring-8. The development of thin
crystals enables the construction of a split-delay unit and the provision of a dedicated branch for photon diagnostics. By
using a 20-μm-thick Si(111) crystal in the symmetric Bragg geometry as a component of a Si(111) double-crystal
monochromator, an arrival-time monitor using a destructive manner has been upgraded to a non-destructive method at
SPring-8 Angstrom Compact free-electron LAser. Using the splitting crystals in a helium atmosphere can prevent
oxidation, which can introduce a lattice distortion.
X-ray free electron lasers (XFELs) with intense and ultra-short pulse X-rays possibly induce damage to optical elements.
We investigated the damage thresholds of optical materials by using focusing XFEL beams with sufficient power density
for studying ablation phenomena. 1-μm focusing beams with 10 keV photon energy were produced at the XFEL facility
SACLA (SPring-8 Angstrom Compact free electron LAser). The focusing beams irradiated samples of rhodium-coated
substrate, which is used in X-ray mirror optics, under grazing incident condition.
With the development of hard X-ray free electron lasers, there is a pressing need to experimentally determine the single shot damage limits of presently used and potential future optical coating materials. To this end we present damage results, and analysis of fluence threshold limits, from grazing incidence geometry experiments conducted at the Spring-8 Angstrom Compact free electron LAser (SACLA) on Carbon coatings at 7 and 12 keV photon energies.
New physical-vapor-deposited (PVD) beryllium foils were characterized using coherent x-rays at the 1-km-long
beamline in the SPring-8. Non-uniformity in the 150 μmx150 μm area is 3% (rms) for 0.1-nm x-rays and 5% for 0.15-nm x-rays which are almost similar value to that of previous PVD foils. The PVD beryllium foil has a capability for
synchrotron radiation and x-ray free electron laser applications with spatially coherent x-rays.
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