Fresnel zone

About: Fresnel zone is a research topic. Over the lifetime, 2337 publications have been published within this topic receiving 37650 citations.

Multifocus off-axis zone plates for x-ray free-electron laser experiments

Abstract: X-ray free-electron lasers (XFELs) are paving the way towards new experiments in many scientific fields, such as ultrafast science, nonlinear spectroscopy, and coherent imaging. However, the strong intensity fluctuations inherent to the lasing process in these sources often lead to problems in signal normalization. In order to address this challenge, we designed, fabricated, and characterized diffractive x-ray optics that combine the focusing properties of a Fresnel zone plate with the beam-splitting capability of a grating in a single diffractive optical element. The possibility to split the incident beam into identical copies allows for direct shot-to-shot normalization of the sample signal, thereby greatly enhancing the signal-to-noise ratio in experiments with XFEL radiation. Here we propose two schemes for the design of such diffractive x-ray optical elements for splitting and focusing an incoming beam into up to three foci by merging a grating with a focusing zone plate. By varying the duty cycle of the grating or the relative shift of the Fresnel zone plate structure, we are able to tune the relative intensities of the different diffraction orders to achieve the desired splitting ratios. Experimental confirmation of the design is provided with soft x-ray light (540 eV) and shows a good agreement with calculations, confirming the suitability of this approach for XFEL experiments.

New type of x-ray source for lensless laboratory nano-CT with 50-nm resolution

Abstract: Most X-ray systems are limited in spatial resolution by the x-ray source performance. In laboratory sources, x-rays are generated by the interaction of an electron beam with a metal target. Bulk target sources produce a spot size in the micron range. Thin layer targets allow a spot size improvement down to hundreds of nanometers, but with a significant flux reduction. Until now a spatial resolution under 100 nm could only be achieved by imaging with Fresnel zone plates with limited depth of focus, typically - several microns. This is acceptable for imaging of flat objects, but it creates a problem for tomography, which requires all parts of a bulk object to be in focus. To overcome the limitations, we invented an x-ray source with a new type of target. Because x-ray cameras can only collect photons from a small angle, the new emitter is physically shaped in such way that the camera can see it as a small dot, but it has a big length along the direction perpendicular to the camera creating a significant flux without compromising the resolution. Evaluation shows that structures down to 50 nm can be distinguished while maintaining a significant x-ray flux and infinite depth of focus required for nano-tomographical reconstruction.

Fabrication of Fresnel zone plates by ion-beam lithography and application as objective lenses in extreme ultraviolet microscopy at 13 nm wavelength

Abstract: Fresnel zone plates are used for imaging at extreme ultraviolet and soft x-ray wavelengths. Fabricating these zone plates is challenging due to small structure sizes (<150 nm) and complex nanostructuring processes. Fabrication techniques such as electron-beam lithography followed by etching and electroplating processes have been developed over the years. We are reporting on the development of a technique incorporating focused gallium ion-beam lithography to fabricate Fresnel zone plates with 120 nm outermost structure size in a process that combines pattern exposure and structure transfer in one single step. The fabricated zone plates were successfully applied in a microscopic setup at λ=13 nm wavelength.

Forward acoustic scattering by moving objects: Theory and experiment

Abstract: The normal mode model for scattering in shallow water is employed to investigate the forward scattering with a target crossing the source-receiver axial line. An experiment was conducted in a littoral environment to analyze forward scattering by a slowly moving object. The theoretical and experimental results show that the sound field aberration takes minimum values if the object is located mid-point along the source-receiver line, whereas it attains its maximum if the object is close to the source or receiver. The total field is either enhanced or suppressed if the object crosses different Fresnel zones. In addition, the duration of shadow-induced aberration is dependent on the width of the first Fresnel zone, which is longest at the mid-point of the source-receiver line.

Switchable polarisation-independent blue phase liquid crystal Fresnel lens based on phase-separated composite films

Abstract: A simple method for fabricating a polarisation independent blue-phase liquid crystal Fresnel lens (BPLCFL) is demonstrated by utilising the photo-polymerisation-induced phase separation. The BPLC/polymer binary Fresnel zones is obtained well by periodic UV illumination with phase separation of the BPLC molecules and UV-curable pre-polymer mixture. The diffraction efficiency can be controlled when applying a uniform electric field which modulates the phase difference between even and odd Fresnel zones. Experimental results show that the maximum diffraction efficiency reaches 24.3%, which is close to the measured diffraction efficiency of the used Fresnel zone-plate mask of 25%. We also characterise the tunable lens performance at different applied voltages.