High-energy x-rays from a synchrotron source are well suited for numerous applications, such as studies of
materials structure and stress in bulk or extreme environments. Some of these methods require high spatial
resolution. Planar kinoforms are shown to focus monochromatized undulator radiation in the 50–100 keV
range down to 0.2–1.5 μm beam sizes at 0.25–2 m focal distances. These lenses were fabricated by reactive ion
etching of silicon. At such high x-ray energies, these optics can offer substantial transmission and lens aperture.
Ultra-low emittance third-generation synchrotron radiation sources such as the NSLS-II offer excellent opportunities for
the development of experimental techniques exploiting x-ray coherence. Coherent light scattered by a heterogeneous
sample produces a speckle pattern characteristic for the specific arrangement of the scatterers. This may vary over time,
and the resultant intensity fluctuations can be measured and analyzed to provide information about the sample dynamics.
X-ray photon correlation spectroscopy (XPCS) extends the capability of dynamic light scattering to opaque and turbid
samples and extends the measurements of time evolution to nanometer length scales. As a consequence XPCS became
crucial in the study of dynamics in systems including, but not being limited to, colloids, polymers, complex fluids,
surfaces and interfaces, phase ordering alloys, etc. In this paper we present the conceptual optical design and the
theoretical performance of the Coherent Hard X-ray (CHX) beamline at NSLS-II, dedicated to XPCS and other coherent
scattering techniques. For the optical design of this beamline, there is a tradeoff between the coherence needed to
distinguish individual speckles and the phase acceptance (high intensity) required to measure fast dynamics with an
adequate signal-to-noise level. As XPCS is a "photon hungry" technique, the beamline optimization requires maximizing
the signal-to-noise ratio of the measured intensity-intensity autocorrelation function. The degree of coherence, as
measured by a two-slit (Young) experiment, is used to characterize the speckle pattern visibilities. The beamline
optimization strategy consists of maximization of the on-sample intensity while keeping the degree of coherence within
the 0.1-0.5 range. The resulted design deviates substantially from an ad-hoc modification of a hard x-ray beamline for
XPCS measurements. The CHX beamline will permit studies of complex systems and measurements of bulk dynamics
down to the microsecond time scales. In general, the 10-fold increase in brightness of the NSLS-II, compared to other
sources, will allow for measurements of dynamics on time-scales that are two orders of magnitude faster than what is
currently possible. We also conclude that the common approximations used in evaluating the transverse coherence
length would not be sufficiently accurate for the calculation of the coherent properties of an undulator-based beamline,
and a thorough beamline optimization at a low-emittance source such as the NSLS-II requires a realistic wave-front
We have evaluated the applicability of vertically-focusing kinoform lenses for tailoring the vertical coherence
length of storage-ring undulator x-ray beams so that the entirety of the coherent flux can be used for small
angle multi-speckle x-ray photon correlation spectroscopy (XPCS) experiments. We find that the focused beam
produced by a kinoform lens preserves the coherence of the incident unfocused beam and that at an appropriate
distance downstream of the focus, the diverging beam produces speckles nearly identical to those produced by
an equivalently-sized unfocused beam. We have also investigated the effect of imperfect beamline optics on the
observed coherence properties of the beam. Via phase contrast imaging and beam-divergence measurements,
we find that a horizontally-deflecting mirror in our beamline precludes us from seeing the true radiation source
point but instead acts as an apparent source of fixed size at the center of our insertion device straight section.
Finally, we discuss how expected near-future optimization of these optics will greatly benefit XPCS measurements
performed at beamline 8-ID-I at the Advanced Photon Source.
While hard x-rays have wavelengths in the nanometer and sub-nanometer range, the ability to focus them is limited by the quality of sources and optics, and not by the wavelength. A few options, including reflective (mirrors), diffractive (zone plates) and refractive (CRL's) are available, each with their own limitations. Here we present our work with kinoform lenses which are refractive lenses with all material causing redundant 2π phase shifts removed to reduce the absorption problems inherently limiting the resolution of refractive lenses. By stacking kinoform lenses together, the effective numerical aperture, and thus the focusing resolution, can be increased. The present status of kinoform lens fabrication and testing at Brookhaven is presented as well as future plans toward achieving nanometer resolution.
One application of Kinoform Fresnel Lenses is to generate small focal spots of hard X-ray photons with high gain for micro-diffraction experiments. A Kinoform lens can be obtained from a refractive lens by deleting material such that at the design photon energy, the deleted regions correspond to with modulo 2π phase-shifts in the phase front. At photon energies different from the design photon energy, the phase jumps are no longer 2π, and the diffractive properties of the kinoform become more significant. We present measurements and calculations of spot size versus photon energy.
Kinoform lenses avoid the absorption losses from a comparable refractive lens by removing all material causing redundant 2π phase shifts. Such optics allow high resolution imaging with a theoretical 94% focusing efficiency. While fabrication of kinoform lenses for two-dimensional focusing is difficult, standard lithographic processes can be utilized to fabricate optics in silicon which produce a line focus. By putting two single-dimension kinoform lenses in a crossed-pair arrangement, a two-dimensional spot is achieved. First attempts at imaging with a crossed pair of kinoform lenses are presented.
To image weakly absorbing materials (e.g. biological specimens, thin films, etc.) with hard x-ray photons, phase-contrast methods have to be applied to enhance the image contrast. Micro-fabricated Fresnel prisms in silicon have been manufactured to enable wavefront division of the incoming x-ray beam for phase-contrast applications. To maximize the efficiency and aperture of these optics, multiples of 2π phase-shifting regions in a conventional prism structure have been deleted, leading to structures that are arrays of micro-prisms. We show preliminary results of x-ray beam deflection using a variety of micro-prism arrays at the NSLS X13B undulator beamline at 11.3 keV.