Soft X-ray free-electron lasers (FELs) have gained significant attention as a research tool in X-ray ultrafast spectroscopy due to their ultra-high pulse brightness and ultra-short duration. Combined with an independent optical laser to perform pump-probe experiments with time resolution has wide-ranging application value and can have great impact on ultrafast dynamics research in fields such as energy catalysis, solid state physics, materials science, and biology. However, the inherent temporal and spatial jitter of soft X-ray FEL pulses significantly limits the time resolution in these experiments due to the low level of synchronization between the two independent light sources. Here, we present a spatiotemporal coupling device suitable for soft X-ray FELs. The device uses a self-designed four-blade slit device which is suitable for ultra-high vacuum environments to complete the spatial coupling between the two foci of both the soft X-ray FEL and optical laser, reducing the negative effects caused by spatial jitter of soft X-ray FEL beam spots. Based on this, a wavefront-splitting scheme is used to reflect and separate approximately 30% of the soft X-ray FEL beam for arrival time diagnosis. Based on the principle of transient decrease in the reflectivity of semiconductor material surfaces induced by X-rays, precise time measurement is achieved on a shot-by-shot basis through spectral encoding. After experiments, the data is rearranged according to the arrival time delay between the two pulses, effectively increasing the time resolution of the pump-probe experiment to the femtosecond scale.
Spherically Bent Crystal Analyzers (SBCAs) are the core optical components of x-ray spectrometers. They have been widely used in many x-ray spectroscopy end-stations at synchrotron radiation and x-ray free electron laser facilities around the world. Owing to the monochromatic and focal properties of SBCAs, x-ray spectrometers with high efficiency and high energy resolution can be well applied to the study of x-ray absorption spectroscopy (XAS) and emission spectroscopy (XES). Hence, the quality of SBCAs is the key factor in determining the performance of x-ray spectrometers. Previously, we have investigated the focal properties of Si(444) SBCAs by using a laboratory’s Rowland circle device. However, the original device is limited by movement distance of motors and the space between detector and x-ray source. It is only applicable to SBCAs with a radius of curvature of 500 mm, and the maximum Bragg angle is 86°. Here, we present a new simple near backscattering detection device, which is based on a long linear guideway, to inspect the surface morphology, crystal face morphology, and focal performance of SBCAs at Bragg angle of 88°. By simply adjusting the distance between the source, SBCAs, and detector, focal performance at the focal point can be detected, while crystal face can be imaged off the focal point. By switching the x-ray source to the LED light source, surface morphology is able to be imaged as well. Furthermore, SBCAs with different radius of curvatures within 1000 mm are all measurable by this new device.
Transient structural information of matter can be obtained by time-resolved X-ray measurement, such as ultrafast X-ray diffraction (UXRD) and ultrafast X-ray absorption Spectroscopy (UXAS). A time-resolved ultrafast X-ray source is necessary for ultrafast X-ray spectroscopy measurements, such as XFEL or synchrotron radiation source. Because of the high cost of X-ray free electron laser (XFEL) and synchrotron radiation source, we designed a laboratory ultrafast plasma X-ray source driven by 800Hz high-energy laser. The X-ray pulse duration is shorter than 100fs. Copper was chosen as the target material of the source, and the expected photon flux can reach 107 photons/s. The target material can also be replaced by other common target materials.
Brilliant synchrotron radiation and x-ray free-electron laser facilities have promoted the development of x-ray spectroscopy, especially the resonant inelastic x-ray scattering and femtosecond time-resolved x-ray spectroscopy. Comparing with the flat crystals, the spherically bent crystals (SBCs) are widely used in the relevant x-ray spectrometers for focusing, which will dramatically improve the detection efficiency. Therefore, focal properties of the SBCs should be experimentally well-investigated. Here, we build a device based on the Rowland circle geometry to study the focusing performance of Si(444) SBCs at different Bragg angles from 60° to 86° with the area detector on(off) the Rowland circle. As a result, the SBCs shows different focal properties in the Rowland plane (sagittal direction) and out of the Rowland plane (meridional direction) due to the unequal focal length. These two foci are separated, while the sagittal focus is on the Rowland circle and the meridional focus is off the Rowland circle. The focused spot on the detector is arcuate, exhibiting the best meridional focal quality near backscatter, and spreading rapidly when the Bragg angle downs to 60°. The sagittal focal quality stays as good as 1.5 pixels on the detector regardless of the Bragg angles.
In many single-pulse experiments of X-ray free electron lasers, the spectrum and intensity distribution of the self-amplifying spontaneous radiation beams fluctuate significantly. It is necessary to perform accurate spectral characterization of each pulse. In this paper, we present an in-line spectrometer that can observe the distribution of energy and incident intensity of single pulse X-ray photons in real time. The X-ray diffraction is achieved by using a high precision transmission crystal bending to fixed pressure bending, and the spectrum is recorded by a spatial resolution detector. At the same time, most of the incident flux is transmitted to the downstream experiment. In this paper, based on the X-ray crystal diffraction theory, geometric optical path designs were carried out, high-precision transmission curved-crystal with fixed bending was developed, the optical path of in-line transmission spectrometer was built, and the copper Kα1 and Kα2 obtained by the test were used for spectral calibration. The experimental results show that each pixel on the detector corresponds to 0.43eV, and the half-width of Kα1 is 3.44 eV. The Single-pulse spectrometer can be used for hard x-ray free electron lasers spectroscopy experiments.
Benefitting from the ultra-broad transparency window, high refractive index and exceptional thermal conductivity, diamond has attracted numerous spotlights on the optical applications and the academic researches. However, the practical deployments of high-quality diamond element were obstructed by the immature manufacture techniques. Here we designed a new mask system for diamond etching that utilizes SiO2 as the main hard mask layer and Cr as the adhesive layer. The cross-section morphology revels three distinct characteristics including an upper slope, a middle vertical sidewall and a trench at the bottom because of the retreat of thick SiO2 mask during the etching process. The completely different etching profile from the previous metal masks has the potential to realize higher aspect ratio etching than traditional metal masks for diamond elements.
X-ray spectroscopy is an important technique for studying the material electronic structure, oxidation state and coordination, which have wide applications in energy catalysis, environmental science fields. The crystals diffract X-rays because their internal atoms are spatially ordered and the lattice spacing is on the nanometer scale, which is similar to the X-ray wavelength. In this paper, a technique based on bending and epoxy adhesive is proposed to fabricate a bent crystal analyzer. The radius of convex surface is 1‰ smaller than the concave one. The wafers and spherical substrates were cleaned with acetone and ethanol in an ultra-clean room. To remove residual organic compounds, UV ozone cleaning procedure should be used. The results show that the measured curvature radius of the bent crystal analyzer is 1000.550 mm; the surface RMS of the surface is 1.34λ and the energy resolution is better than 5 eV, which can distinguish Cu Kα1 and Kα2 fluorescence lines.
A time-resolved soft X-ray emission spectrometer covering 250-620 eV is presented for the study of chemical reaction processes. Contrary to conventional time-resolved spectrometer, our spectrometer can obtain a two-dimensional timeenergy map in single shot by adding an imaging mirror to the flat-field spectrometer. The temporal changes are spatially encoded in the footprint of the probe X-ray beam on the sample via grazing incidence geometry. The flat-field spectrometer design is chosen to alleviate the aberration of the imaging mirror. The spectrometer is optimized at 400 eV, targeting at over 2000 resolving power and sub-picosecond time resolution.
As the spectrum of each pulse from XFEL (X-ray free electron lasers, XFEL) undulator (often referred as pink X-ray) varied between each other, it is necessary to measure the intensity and spectrum of each pulse. The major parameters of the spectrometer are the facet of bent crystal, the Bragg angle, the transmittance (related to the material, its thickness and transmission angle), energy range and energy resolution. An ultrathin Si wafer was put above a concave lens and under a convex lens. Bending was achieved by applying the pressure, then the Si wafer with 25μm thickness would be regularly bent onto the concave profile until the convex substrate, Si wafer and the concave lens were overlap. The cylindrical substrates were provided with holes to let the FEL beam through the crystal, and the radius of curvature could be varied from 0.2m to 1m. The crystal orientation and miscut angle were measured by a high resolution X-ray diffractometer. Finally, the energy dispersion spectra were measured by a spectrometer built in laboratory.
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