X-ray Waveguide Research Articles

Numerical Simulation of C/C/C Planar X-ray Waveguides

A numerical simulation was done to check the possibility of using planar C/C/C multilayers with density contrast 0.2 and 0.7 g/cm3 as an X-ray waveguide. After an optimization procedure, suitable waveguide layer thicknesses were found which provide a high degree of resonant standing wave field intensity enhancement in the core layer at incident beam energy of 13 keV. The obtained results were compared with those of the Mo/Be/Mo waveguide, whose high waveguiding capability at the same energy value was reported in the 1990s. The comparison shows that standing wave field intensity resonant enhancement provided by C/C/C planar multilayers is very high and, consequently, a guided beam can be well detected.

X-ray waveguides for laboratory sources

A planar x-ray waveguide (WG) consisting of a diamond-like carbon layer sandwiched between two Ni layers is used to demonstrate for the first time the possibility of using WGs as optical elements with standard laboratory sources. The exit beam profile for the first two guided modes has been measured and analysed to demonstrate the high degree of beam coherence. The exit divergence and the beam size are reported and compared with theoretical predictions. Other relevant parameters, such as efficiency and gain, have been measured. This opens up interesting perspectives for the production of nanometre-sized x-ray beams with table-top laboratory sources.

Real-Time Evolution of the Distribution of Nanoparticles in an Ultrathin-Polymer-Film-Based Waveguide

We report a novel application of an ultrathin-polymer-film-based, resonance-enhanced x-ray waveguide as a real-time nanoprobe for elucidating dilute, yet disordered, gold nanoparticles embedded in the polymer matrix. This nanoprobe promises a sensitivity enhancement of several orders of magnitude, hence revealing in real time the lateral nanoparticle distribution with subnanometer spatial resolution. We observed that the motion of the nanoparticles is strongly anisotropic, with in-plane coalescence taking place more rapidly than out-of-plane diffusion, which can ultimately facilitate the formation of two-dimensional structures.

X-ray waveguide phenomenon in copper phthalocyanine thin film

A white X-ray beam was irradiated on a copper phthalocyanine (CuPc) thin film under grazing incidence condition, and X-rays propagated in the film due to the waveguide phenomenon were observed by an energy-dispersive detector. Energy profiles of guided X-rays showed four specific resonant modes. By comparing them with the calculated ones, a three-layer model of CuPc film was proposed.

Finite-difference field calculations for one-dimensionally confined X-ray waveguides

The propagation of X-rays in one-dimensionally confined waveguides is analyzed by means of finite-difference calculations based on the parabolic wave equation. We compare the numerical results to an analytical approach and investigate the transmission and coherence properties of a waveguide depending on its length and the number of guided modes corresponding to the guiding layer thickness.

Two-dimensional X-ray waveguides on a grating

Two-dimensional X-ray waveguides are promising optical elements providing a highly coherent and divergent beam for possible applications in coherent X-ray imaging and scattering. We have fabricated arrays of 2D waveguide nanostructures by e-beam lithography. To characterize the device a parallel synchrotron beam has been coupled into the front side and the farfield diffraction pattern has been mapped as a function of the angle of incidence.

An automatic analysis of strain-depth profile in X-ray microdiffraction

The increased demanding of high spatial resolution analysis for local strain/stress measurements gave an impulse for developing new X-ray microdiffraction technique. In particular spatial resolution of the order of 100–300 nm can be recently obtained using an X-ray waveguides as optical element. However, the great number of datasets which must be acquired and analyzed to probe the full field of strain variations renders the high-resolution technique not suitable for systematic analysis. In this communication, we present a data treatment procedure for an automatic analysis of microdiffraction profiles measured with X-ray waveguide to obtain quantitative information about local strain variations. The presented procedure allows to extract a depth-dependent strain profile directly from the measured data to be used as initial guess for calculating the diffraction profile by means of the dynamical theory in the Takagi–Taupin recursive formalism. Then a Monte Carlo fitting data refinement is applied to optimize the strain profile.

Nanometer focusing of X-rays with modified reflection zone plates

Transmission Fresnel zone plates and x-ray waveguides are presently the best high-resolution optical elements used in x-ray microscopy and micro-probing. The spatial resolution of zone plates is defined not only by the technological limit of about 10 nm but also by the effects of volume diffraction, even for high-order zone plates. The physical nature of a waveguide limits the theoretical spatial resolution to 10 nm, corresponding to the minimum thickness of film able to fulfil the conditions necessary for optical mode formation. Here, we show that reflection zone plates and Bragg–Fresnel lenses can overcame the resolution limit of transmission zone plates and waveguides and make it possible to achieve spatial resolution into the sub-nanometer range, subject to source size limitations. Such optics can be used with existing third-generation synchrotron radiation sources, as well as with x-ray laser sources, e.g., free-electron lasers.

High spatial resolution X-ray microdiffraction applied to biomaterial studies and archeometry

The high spatial resolution X-ray microdiffraction by using X-ray optics can provide unique information on regions with very high gradients in physical quantities, as in the case of interfaces. Among the several available X-ray optics for synchrotron radiation producing high intensity micron and sub-micron beams, the X-ray waveguide (WG) can provide the smallest X-ray beam in one direction. Moreover, its applicability has been widened by an improved set-up installed at ID13 beamline at ESRF, where a new undulator is combined with an horizontally focusing mirror. In this work, we show different applications of waveguide-based microdiffraction, the first two regard biological problems and in particular the structural analysis of newly formed bone in ceramic scaffolds. The second application regards archeometry and in particular the sulphatation process and the thin gypsum crust formation on the surface of carbonate rocks (travertine, marbles), due to the exposure of the monuments at aggressive atmospheres. In the three cases, the local structural information derived thanks to the high spatial resolution demonstrates the power of the microdiffraction technique based on WG, and the possibility to apply this new methododogy in different scientific fields.

Front-coupling of a prefocused x-ray beam into a monomodal planar waveguide

A hard x-ray beam of photon energy E=12.5keV has been focused by a Kirkpatrick–Baez mirror system and coupled into the front side of a single-mode x-ray waveguide. The beam dimensions of 3.8×2.5μm2 in the focus of the mirror system have thus been reduced in one direction to 32nm, corresponding to the guiding layer thickness of the waveguide. At the same time the waveguide acts as a coherence filter and leads to a well-defined intensity distribution with steep tails in the near- and far-field regions. The total flux transmitted by the waveguide exceeded 108 photons/s while no significant contributions of radiation transmitted through the absorbing waveguide cladding have been observed.

Prediction of the transmission through thin-film waveguides for X-ray microscopy.

Thin-film slab waveguides can confine incident X-ray beams in one direction in guiding layers as thin as 10 nm. Consequently they can provide attractive beam dimensions for microscopy purposes. This report presents a simple model and analytical equations for the transmission calculation, which provide results consistent with the rigorous calculations based on recursion techniques. By using these results the waveguide transmission can be compared directly with other microscopy objectives. Ideally X-ray waveguides can filter the spatially coherent content out of an incident radiation beam with an efficiency of 1. The transmissions measured for state-of-the-art one- and two-dimensional waveguides are found to correspond to experimental efficiencies of 0.5 for each confinement direction. Waveguides with thinner guiding layers cannot be used efficiently in highly collimated beams; instead the beam divergence in unfocused beamlines at state-of-the-art synchrotron radiation sources may eventually have to be increased to the larger angular acceptance of these waveguides by use of other focusing optics.

White beam x-ray waveguide optics

We report a white beam x-ray waveguide (WG) experiment. A resonant beam coupler x-ray waveguide (RBC) is used simultaneously as a broad bandpass (or multibandpass) monochromator and as a beam compressor. We show that, depending on the geometrical properties of the WG, the exiting beam consists of a defined number of wavelengths which can be shifted by changing the angle of incidence of the white x-ray synchrotron beam. The characteristic far-field pattern is recorded as a function of exit angle and energy. This x-ray optical setup may be used to enhance the intensity of coherent x-ray WG beams since the full energetic acceptance of the WG mode is transmitted.

Can a Single-Wall Carbon Nanotube be an X-Ray Waveguide?

We study the transport efficiency for x-rays of single-wall carbonnanotubes (SWCNTs) in theory. Three effects, i.e. refraction,absorption and x-ray tunnelling loss, are considered. Our calculationshows that the SWCNT cannot be an x-ray waveguide due to the largex-ray tunnelling loss. If the tunnelling loss can be reduced effectively,a nanotube could be a waveguide.

X-ray characterization of Si microstructures with high spatial resolution

With the advent of the nanolithographic technologies, the demand of a local strain/stress analysis with submicrometer resolution is increasing. In particular, the aim of this work was the study of a patterned Si wafer subjected to preliminary process flow for 0.15 μm complementary metal oxide semiconductor generation. The main purpose was to follow the production of strain induced in the Si substrate during the shallow trench isolation process and subsequent formation of active stripes. The determination of structural parameters with very high spatial resolution in one dimension (300 nm) has been obtained using a scanning x-ray microscope whose main element is a waveguide for medium and high energy x-rays (8–30 keV). The experiment, which takes advantage of a nondispersive geometry, required neither a two-dimensional highly sensitive detector nor an insertion device beamline, but it has been carried out on bending magnet beamlines with relatively low flux.

Application of genetic algorithms for the optimization of X-ray waveguides

A genetic algorithm (GA) for the determination of the optimum thickness for X-ray waveguiding in thin-layered structures is proposed. We applied this GA for the characterization of Mo–Be–Mo waveguides, which were used recently for the microbeam formation, and we found that they can be improved. It is shown numerically that revision of the layers width leads to waveguides which should produce more intensive beam at the exit, because of a larger enhancement of the electric field intensity in the guiding layer. With a further extension of our GA model it is shown that the reported Mo cladding layers are the optimum for the operation of Be waveguides at 13 keV photon energy.

Propagation of X-rays in carbon nanotubes

In this paper, the possibility of exploiting carbon nanotubes as X-ray waveguides is explored through analytical and numerical solutions of Maxwell's equations. It is shown that nanotubes coated with various materials can theoretically sustain electromagnetic modes. The factors determining the modes that can be established are the tube diameter and the field penetration depth in the cladding. The latter is shown to vary from 6.4 nm in carbon to 3 nm in metals such as gold and platinum. Numerical simulations confirm that the inner tubes in multi-walled carbon nanotubes have a negligible impact on mode formation, thereby, allowing a large range of tube diameters to be exploited.

X-ray waveguides and thin macromolecular films

We report waveguide-enhanced X-ray experiments on thin macromolecular films, such as spin-coated polymer films, self-assembled polyelectrolyte layers, and lipid membranes, prepared on solid surfaces. Upon the deposition of an additional thin inorganic (metal) layer on top of the film (cap layer), X-ray waveguide structures can be obtained. Shining a highly brilliant X-ray beam onto the system, resonantly enhanced and precisely defined standing wave field distributions (modes) are excited. Different applications of waveguiding effects to the study of macromolecular films are discussed: (i) Waveguide-enhanced scattering, where the in-plane structure of the film is studied by grazing incidence diffraction under simultaneously excited modes, (ii) waveguide-enhanced X-ray fluorescence, and (iii) coherent scattering methods based on the coherence properties of the waveguide.

X-ray waveguides based on Bragg scattering of multilayers

We have analytically shown a possibility of creating hard x-ray waveguides using Bragg reflections of multilayer structures. The proposed waveguide consists of a guiding film embedded into a multilayer. Theoretical modeling indicates that a substantial x-ray intensity enhancement can be achieved inside the guiding film with the thickness of the order of 1 nm.

1D and 2D X-ray waveguides: Optics and applications

X-ray waveguides offer a novel approach for nanobeam production, which may become useful in coherent beam imaging and phase contrast projection microscopy. Internal field enhancement, coherency properties, and coupling efficiency of these devices have been measured and compared to theoretic predictions. The fundamentals of X-ray wave guide optics can be derived from a scalar wave equation. Up to now, X-ray waveguide optics have exclusively been one-dimensional (1D), while many applications demand two-dimensionally (2D) confined point beams. We have recently demonstrated the first proof of principle that X-ray waveguide effects can be generalized to 2D devices using e-beam defined lithographic nanostructures, delivering of a coherent hard x-ray beam with nanometer sized cross-section (69 nm × 33 nm)

X-ray refractive planar lenses in combination with planar X-ray waveguide

The optical system composed of an X-ray silicon planar refractive lens and an X-ray planar waveguide with air gap has been characterized with synchrotron radiation. The guiding structure consisted of two silicon plates, clamped by permanent magnets and divided by a spacer made of thin magnetic film. The system was studied in the energy range 8 - 17 keV on the Optics beamline BM05 at the ESRF (Grenoble). It was shown, that the number of resonant modes in the beam exiting from the waveguide could be controlled by planar lens orientation, so that minimal number of modes could be obtained. Refractive optics elements and X-ray planar waveguides are a good combination for synchrotron sources of last generation and free electron lasers.