Diffraction gratings are key components in experiments with UV, VUV and soft x-ray radiation at synchrotron radiation and free-electron laser (FEL) facilities. They areused in monochromators to produce beams with narrow energy distribution, and in analyzers to provide high-resolution spectral analysis of scattered radiation. Today, such diffraction gratings consist of a large number of periodic grooves (typically tens to thousands per millimeter) arranged on the reflecting surface on a plane or concave substrate. The performance of the grating is also ultimately limited by the surface quality of the substrate. For best spectral resolution, the substrates need to be ultra-precise and superpolished, with typical lengths of 100 – 300 mm and shape errors of 1-2 nanometers, slope errors in the order of 100 nrad, and a roughness in the Ångstrom scale.
Today, the line patterns of x-ray gratings are mostly produced either by holographic exposures of interfering laser beams into a photo resist, or by ruling machines that imprint the grating groves in a line-by-line manner into a soft metal layer.
As an alternative to these established methods, we present a strategy of grating fabrication by electron-beam lithography (EBL). This technique can expose arbitrary patterns with high resolution and placement accuracy at a speed sufficient for x-ray grating fabrication.
We have performed optical metrology of a low-cost grating substrate, which served as the basis for an aligned exposure of a corrected grating pattern to yield a laminar grating with a constant deflection angle along the substrate surface. We have confirmed that the effects of slope errors were strongly compensated over an extended wavelength range by using synchrotron radiation in the soft x-ray range.
This work shows a path for circumventing the limited availability of high quality reflective substrates to obtain gratings with performance beyond what is possible today.
The design for a new XUV-Optics Beamline is presented. The collimated plane grating monochromator (PGM-)
beamline at a bending magnet is setup at the BESSY-II synchrotron radiation facility within the framework of the
blazed-grating production facility. Coupled to a versatile four-circle (ten axes) UHV- reflectometer as a permanent end
station the whole setup is dedicated to at-wavelength characterization and calibration of the in-house produced precision
gratings and novel nano-optical devices as well as mirrors, multilayered systems etc. It is also open to external projects
employing reflectometry, spectroscopy or scattering techniques. According to its purpose, this beamline has specific
features, such as: very high spectral purity, provided by two independent high order suppression systems, an advanced
aperture system for suppression of stray light and scattered radiation, a broad energy range between 10 eV and 2000 eV,
small beam divergence and spot size on the sample. Thus this Optics Beamline will become a powerful metrology tool
for reflectivity measurements in s- or p-polarisation geometry with linearly or elliptically polarized light on real optics up
to 360 mm length and 4 kg weight.
The European XFEL is a large facility under construction in Hamburg, Germany. It will provide a transversally fully
coherent X-ray radiation with outstanding characteristics: high repetition rate (up to 2700 pulses with a 0.6 milliseconds
long pulse train at 10Hz), short wavelength (down to 0.05 nm), short pulse (in the femtoseconds scale) and high average
brilliance (1.6•1025 photons / s / mm2 / mrad2/ 0.1% bandwidth). Due to the very short wavelength and very high pulse
energy, mirrors have to present high quality surface, to be very long, and at the same time to implement an effective
cooling system. Matching these tight specifications and assessing them with high precision optical measurements is very
challenging. One of the three foreseen beamlines operates in the soft X-ray range and it is equipped with a diffractive
monochromator. The monochromator is using a variable line spacing grating that covers the wavelength range from 4.6nm to 0.41 nm (energies from 270eV to 3000eV). The grating profile is blazed, and due to the small angle and relatively
few lines/mm, it is also very challenging to realize and to be characterized. In this contribution we discuss about the
requirements of the optics involved in the soft X-ray monochromator. We describe mirror and grating specifications, and
the tests that could be carried out during and after the manufacturing in order to ensure the specifications match.
Material science research in the soft-X ray regime at the Swiss Light Source accommodates five beamlines where the
monochromators rely on in-vacuum angular encoders for positioning mirror and gratings. Despite the factory-calibration
of the quadrature signals from these rotary encoders, the energy linearization for spectroscopic data requires accurate
calibration of the encoder quadrature signals. We characterize the interpolation errors and describe the Heydemann
correction algorithm for the quadrature signals for improving the energy linearization on a scale comparable with the
incremental encoder interpolation interval. Experimental data are shown where such errors produce sizeable effects in
soft-X ray spectroscopy and for which the correction algorithm efficiently improves the short-range non-linearity.
Free electron lasers (FEL) for the hard- and soft X-ray regime are promoted in many projects around the world and the first VUV-FEL with wavelengths down to 20 nm is already in operation at DESY in Hamburg. They open up new domains in coherent radiation, time resolution and intensity and extend current X-ray science.
The BESSY project of a 2nd generation FEL facility proposes the implementation of an externally seeded FEL with significantly improved beam quality compared to the conventional self seeded SASE-FELs.
The BESSY design as described in the technical design report1
provides full tunability of photon energies from
24 eV to 1 keV, complete polarisation control and reproducible
pulse structures with pulse lengths in the femtosecond range.
The external seeding ensures the synchronisation to other laser
sources necessary for pump-probe experiments and provides shot-to-shot reproducible pulse properties.
We present new experimental data on diffraction efficiency measurements on gratings for the first undulator beamlines
at BESSY II. The measured data will be compared with results from electromagnetic theory. A good suppression of higher orders, i.e. the sum of higher orders from higher energies which are diffracted into the same angle as the first order, has been an important point during the design process of the beamlines at BESSY II. For this purpose lamellar grating structures have been optimized and specified. The measurements were carried out with a triple axis vacuum diffractometer at the BESSY I PM 4 beamline in an energy range from 70 to 1200 eV. We measured the diffraction efficiencies of the first order and the corresponding second and third order at discrete energies E, i.e. second and third order efficiency at 2E and 3E respectively. To improve the accuracy of the measurements, the higher orders from the PM 4 beamline had to be taken into account. We used a calculation scheme starting at the highest energy, in which the diffraction efficiency of the grating under test and the higher orders from the monochromator have been evaluated simultaneously. The calculated higher orders of the monochromator were then recursively used as input for the calculation at lower energies. The gratings were measured at different angular settings, hence enabling different
degrees of high order suppression. It was determined, that with appropriate angular settings, the higher orders of monochromator gratings can be significantly reduced.