Multi-wavelength study of light transmitted through a single marine centric diatom.

The characterization of partially coherent light transmission by micrometer sized valves of marine diatoms is an interesting optical challenge and, from the biological point of view, is of outmost relevance in order to understand evolution mechanisms of such organisms. In the present work, we have studied the transmission of light coming from a monochromator through single valves of Coscinodiscus wailesii diatoms. Incoming light is confined by the regular pore pattern of the diatom surface into a spot of few microns, its dimensions depending on wavelength. The effect is ascribed to the superposition of wavefronts diffracted by the pores' edges. Numerical simulations help to demonstrate how this effect is not present in the ultraviolet region of the light spectrum, showing one of the possible evolutionary advantages represented by the regular pores patterns of the valves.

Self-collimation of light over millimeter-scale distance in a quasi-zero-average-index metamaterial.

Inspired by the concept of complementary media, we experimentally demonstrate that an engineered metamaterial made of alternating, stripe layers of negatively refracting (photonic crystals) and positively refracting (air) materials strongly collimates a beam of near-infrared light. This quasi-zero-average-index metamaterial fully preserves the beam spot size throughout the sample for a light beam traveling through the metamaterial a distance of 2 mm-more than 1000 times the input wavelength lambda=1.55 microm. These results demonstrate the first explicit experimental verification of optical antimatter as proposed by Pendry and Ramakrishna [J. Pendry and S. Ramakrishna, J. Phys. Condens. Matter 15, 6345 (2003)10.1088/0953-8984/15/37/004], using two complementary media in which each n(eff)=-1 layer appears to annihilate an equal thickness layer of air.

An analytical approach to estimating aberrations in curved multilayer optics for hard x-rays: 2. Interpretation and application to focusing experiments.

Aberration effects are studied in parabolic and elliptic multilayer mirrors for hard x-rays, basing on a simple analytical approach. The interpretation of the underlying equations provides insight into fundamental limitations of the focusing properties of curved multilayers. Using realistic values for the multilayer parameters the potential impact on the broadening of the focal spot is evaluated. Within the limits of this model, systematic contributions to the spot size can be described. The work is complemented by a comparison with experimental results obtained with a W/B(4)C curved multilayer mirror.

An analytical approach to estimating aberrations in curved multilayer optics for hard x-rays: 1. Derivation of caustic shapes.

An analytical approach has been developed to derive aberration effects in parabolic and elliptic multilayer optics with weak interaction between photons and matter. The method is based on geometrical ray tracing including refraction effects up to the first order of the refractive index decrement delta. In the parabolic case, the derivation leads to simple parametric equations for the caustic shape. In the elliptic case, the analytical results more involved, but can be well approximated by the parabolic solution. Both geometries are compared with regard to the fundamental impact on their focusing properties.

Pendellösung effect in photonic crystals.

At the exit surface of a photonic crystal, the intensity of the diffracted wave can be periodically modulated, showing a maximum in the "positive" (forward diffracted) or in the "negative" (diffracted) direction, depending on the slab thickness. This thickness dependence is a direct result of the so-called Pendell osung phenomenon, consisting of the periodic exchange inside the crystal of the energy between direct and diffracted beams. We report the experimental observation of this effect in the microwave region at about 14GHz by irradiating 2D photonic crystal slabs of different thickness and detecting the intensity distribution of the electromagnetic field at the exit surface and inside the crystal itself.

A numerical wave-optical approach for the simulation of analyzer-based x-ray imaging.

An advanced wave-optical approach for simulating a monochromator-analyzer set-up in Bragg geometry with high accuracy is presented. The polychromaticity of the incident wave on the monochromator is accounted for by using a distribution of incoherent point sources along the surface of the crystal. The resulting diffracted amplitude is modified by the sample and can be well represented by a scalar representation of the optical field where the limitations of the usual 'weak object' approximation are removed. The subsequent diffraction mechanism on the analyzer is described by the convolution of the incoming wave with the Green-Riemann function of the analyzer. The free space propagation up to the detector position is well reproduced by a classical Fresnel-Kirchhoff integral. The preliminary results of this innovative approach show an excellent agreement with experimental data.

Diffractive-refractive optics in the Laue case: first experiment.

The possibility of sagittally focusing synchrotron radiation using an asymmetric Laue crystal with profiled surfaces has been experimentally demonstrated for the first time. The sample was a Si single crystal with two parallel cylindrical holes of diameter 8 mm. The axes of the holes formed an angle of 7.95 degrees with the (111) diffracting planes and were arranged vertically with respect to the diffracting planes. 15.35 keV synchrotron radiation was diffracted in the space between the holes. The minimum thickness of this Laue crystal was 0.5 mm. The diffracted beam formed an angle of 0.55 degrees with the exit surface. The experiment was performed at beamline BM05 at the ESRF. The length of the beamline was not sufficiently long to detect the focus, but the experiment clearly showed that the diffracted beam was sagittally convergent.

Large-distance refocusing of a submicrometre beam from an X-ray waveguide.

Among the several available X-ray optics for synchrotron radiation producing micrometre and submicrometre beams with high intensity, the X-ray waveguide (WG) can provide the smallest hard X-ray beam in one direction. A drawback of this optics is that, owing to the divergence at the exit, a nanometre-sized spot on the sample can only be obtained if this is within a few micrometres of the WG exit. Another limitation is that in planar WGs the beam is compressed in only one direction. Here, using a dynamically bent elliptical Si/Pt mirror, the guided X-ray beam has been refocused at approximately 1 m from the waveguide exit. The large working distance between the device and the submicrometre focus leaves some space for sample environment (vacuum chamber, furnace, cryostat, magnets, high-pressure device etc.) and allows cross-coupled geometries with two WGs for efficient compression in two directions.

Applications of dynamical diffraction under locally plane wave conditions: defects in nearly perfect crystals and X-ray refractometry.

In a previous paper, the concept of a locally plane wave was explained theoretically. In such a configuration, the fringe pattern recorded on the film can be considered as a phase analyser. Here the experimental analysis is presented, showing examples of interesting applications to X-ray refractometry and to the visualization of the strain field around isolated defects.

X-ray dynamical diffraction: the concept of a locally plane wave

The long distance between the source and the experiment and the small source size, now available at third-generation synchrotron sources, leads to new optical characteristics for X-ray diffraction. It is shown that, under certain conditions, the intensity received by a point located on the exit surface of a crystal is described by the diffraction of a locally plane wave. Each point along the surface is influenced by a plane wave with a varying departure from Bragg angle. In such a case, it is possible to visualize the rocking curve of the crystal as a function of the position along the exit surface. This represents a topographic method to obtain the reflectivity curve of the crystal instead of the usual goniometric method. Applications will be described in a forthcoming paper.