Showing papers on "Fresnel zone published in 2003"
TL;DR: An achromatic Fresnel optical system for EUV or X-ray radiation that combines a Fresnel zone plate with a refractive lens with opposite chromatic aberration is reported, which can deliver a resolution comparable to the highest resolution in the entire electromagnetic spectrum but with an improvement of two or more orders of magnitude in spectral bandwidth.
Abstract: Advances in extreme-ultraviolet (EUV) and X-ray optics are providing powerful new capabilities in high-resolution imaging and trace-element analysis of microscopic specimens1, and the potential for fabricating devices of smaller critical dimensions in next-generation integrated circuit lithography2. However, achieving the highest resolution with such optics usually requires the illuminating EUV or X-ray beam to be highly monochromatic. It would therefore be highly desirable to have large-field-of-view, sub-100-nm resolution optics that are achromatic to a significant degree, allowing more light to be utilized from broader bandwidth sources such as laser-produced plasmas. Here we report an achromatic Fresnel optical system for EUV or X-ray radiation that combines a Fresnel zone plate with a refractive lens with opposite chromatic aberration. We use the large anomalous dispersion property of the refractive lens material near an absorption edge to make its fabrication practical. The resulting structure can deliver a resolution comparable to that of the Fresnel zone plates that have achieved the highest resolution (25 nm; ref. 3) in the entire electromagnetic spectrum, but with an improvement of two or more orders of magnitude in spectral bandwidth.
178 citations
[...]
TL;DR: It is shown that the axial irradiance exhibited by the FZP has self-similarity properties that can be correlated to those of the diffracting aperture.
Abstract: Fractal zone plates (FZPs), i.e., zone plates with a fractal structure, are described. The focusing properties of this new type of zone plate are compared with those of conventional Fresnel zone plates. It is shown that the axial irradiance exhibited by the FZP has self-similarity properties that can be correlated to those of the diffracting aperture.
172 citations
TL;DR: In this article, the authors present a brief review of time reversal theory in a remote sensing regime and a robust procedure for estimating ae from the signals received at the array, which is referred to as super-resolution of the time reversal process in random media.
Abstract: In time reversal, an array of transducers receives the signal emitted by a localized source, time reverses it and re-emits it into the medium. The emitted waves back-propagate to the source and tend to focus near it. In a homogeneous medium, the cross-range resolution of the refocused field at the source location is λ0L/a, where λ0 is the carrier wavelength, L is the range and a is the array aperture. The refocusing spot size in a homogeneous medium is independent of the bandwidth of the pulse, but broad-band can help in reducing spurious Fresnel zones. In a noisy (random) medium, the cross-range resolution is improved beyond the homogeneous diffraction limit because the array can capture waves that move away from it at the source, but get scattered onto it by the inhomogeneities. We refer to this phenomenon as super-resolution of the time reversal process in random media. Super-resolution implies in particular that, because of multipathing, the array appears to have an effective aperture ae that is greater than a. Since ae depends on the scattering medium, it is not known. In this paper we present a brief review of time reversal theory in a remote sensing regime and a robust procedure for estimating ae from the signals received at the array. Knowing ae permits assessing quantitatively super-resolution in time reversal for applications in spatially localized communications with reduced interference. We also review interferometric imaging and its relation to time reversal and to matched field imaging. We show that ae quantifies in an explicit way the loss of resolution in interferometric array imaging.
144 citations
TL;DR: A switchable Fresnel zone plate lens is demonstrated using a polymer-stabilized liquid crystal that can be operated below 10 volts with fast response time and works well for linearly polarized light.
Abstract: A switchable Fresnel zone plate lens is demonstrated using a polymer-stabilized liquid crystal. The fabrication process is relatively simple and the device can be operated below 10 volts with fast response time. Such a device works well for a linearly polarized light.
127 citations
TL;DR: In this article, the phase contrast in the x-ray images is about five times higher than the pure absorption contrast, which is a significant improvement over the pure absorbance contrast.
Abstract: We report on x-ray microscopy of advanced microelectronic devices imaged in Zernike-type phase contrast mode at 4 keV photon energy. Fresnel zone plates were used as high resolution x-ray objectives providing 60 nm spatial resolution. Integrated circuit copper interconnect structures were imaged in positive as well as negative phase contrast. In both cases the phase contrast in the x-ray images is about five times higher than the pure absorption contrast.
85 citations
TL;DR: A method of synthesizing computer-generated holograms of real existing objects is proposed that is based on a series of projection images of an incoherently illuminated object recorded from different perspectives, resulting in a Fresnel hologram calculated from the three-dimensional Fourier spectrum.
Abstract: A method of synthesizing computer-generated holograms of real existing objects is proposed that is based on a series of projection images of an incoherently illuminated object recorded from different perspectives. In accordance with the principles of computer tomography, the three-dimensional Fourier spectrum of the object is calculated by use of several projection images. A method of calculating a Fresnel hologram from the three-dimensional Fourier spectrum is proposed. Experimental results in the form of a computer simulation and optical reconstruction are presented.
83 citations
TL;DR: Two different methods thatuse Twin-CGHs for absolute testing of aspheric surfaces are described, and a mutual comparison of the two results and a comparison with the established N-position rotation test are given.
Abstract: A complete absolute interferometric test of axially symmetric aspheres is presented. The method is based on a specially designed computer-generated hologram (CGH) that reconstructs an aspherical wave as well as a spherical auxiliary wave. Since both phase functions have the same symmetry and their pattern is simultaneously encoded, we call this type of multiplex hologram a Twin-CGH. The spherical wave is used for calibration. The aberrations of the spherical auxiliary wave are measured absolutely with either a spherical mirror or an absolute test for Fresnel zone plates. Thus the two types of aberration inherent in the CGH can be identified and separated from each other. The errors of the spherical wave can be transferred to those of the aspherical wave. Two different methods that use Twin-CGHs for absolute testing of aspheric surfaces are described. Test procedures are explained, equations are derived, and experimental results are presented. A mutual comparison of the two results and a comparison with the established N-position rotation test are given.
64 citations
TL;DR: In this paper, a comprehensive numerical study of 3D acoustic wave propagation in weakly heterogeneous random media is presented, where the authors compare the predictions of linearized ray theory and 3D Born-Frechet (banana-doughnut) kernel theory.
Abstract: SUMMARY We present the results of a comprehensive numerical study of 3-D acoustic wave propagation in weakly heterogeneous random media. Finite-frequency traveltimes are measured by cross- correlation of a large suite of synthetic seismograms with the analytical pulse shape representing the response of the background homogeneous medium. The resulting 'ground-truth' traveltimes are systematically compared with the predictions of linearized ray theory and 3-D Born-Frechet (banana-doughnut) kernel theory. Ray-theoretical traveltimes can deviate markedly from the measured cross-correlation traveltimes whenever the characteristic scalelength of the 3-D heterogeneity is shorter than half of the maximum Fresnel zone width along the ray path, i.e. whenever a < 0.5(λL) 1/2 , where a is the heterogeneity correlation distance, λ is the dominant wavelength of the probing wave, and L is the propagation distance. Banana-doughnut theory has a considerably larger range of validity, at least down to a ≈ 0.1(λL) 1/2 in sufficiently weakly heterogeneous media, because it accounts explicitly for diffractive wave front healing and other finite-frequency wave propagation effects. In this paper, we conduct a comprehensive numerical investiga- tion of the validity of both linearized ray theory and 3-D Born- Frechet kernel theory in weakly heterogeneous random media. We use a pseudospectral method to solve the 3-D acoustic wave equation in a suite of Gaussian and exponentially correlated random media, characterized by their root-mean-square slowness variation and their correlation scalelength. Finite-frequency traveltime shifts are mea- sured at a variety of source-receiver distances, by cross-correlation of the numerically computed synthetic seismograms with the cor- responding analytical response of the background homogeneous medium. This study extends the analyses of Nolet & Dahlen (2000) and Hung et al. (2001), who investigated the wave front healing effects downstream of an isolated, slow or fast, spherically sym- metric slowness anomaly using the parabolic approximation and the pseudospectral method, respectively. In a statistically homoge- neous random medium, the diffractive healing of wave front corru- gations produced by near-source slowness anomalies is continually being augmented by new corrugations produced by more distant anomalies, as the wave propagates away from the source. Because of this, a ray-theoretical skeptic might argue that conclusions based upon the study of a 'lonely bowling ball' are not pertinent to seis- mic wave propagation in the Earth's mantle. Spetzler & Snieder's (2001b) study of picked traveltimes of plane waves in 2-D random media shows that, for their case, Rytov scattering theory predicts traveltimes more accurately than ray theory. Similarly, we use our 'ground-truth' numerical results to place empirical constraints upon the validity of both linearized ray theory and 3-D Born-Frechet ker- nel theory in spatially extended 3-D random heterogeneous media.
59 citations
TL;DR: Through numerical experiments, it is confirmed that a Gaussian focal spot with a beam width of 7.7 nm can be produced by a modified Fresnel zone plate with a minimum structure size of 30 nm.
Abstract: A modified Fresnel zone plate that can produce an approximate Gaussian focal spot is proposed for the focusing and imaging of soft x rays and extreme ultraviolet radiation. The selection conditions for the positions and the widths of the concentric open rings are analytically presented. The focal spot size can be much smaller than the width of the narrowest open ring, and the sidelobes and the higher orders can be effectively suppressed. Through numerical experiments, we confirm that a Gaussian focal spot with a beam width of 7.7 nm can be produced by a modified Fresnel zone plate with a minimum structure size of 30 nm.
48 citations
TL;DR: In this article, a simpler fiber design alternative to photonic crystal fibers is proposed for high-peak-power propagation with reduced nonlinear interactions, and multiple foci are observed in the far field.
Abstract: Propagation of light at 1.5 µm with peak intensity in an air hole is achieved within an air–silica-structured Fresnel waveguide. Thus a simpler fiber design alternative to photonic crystal fibers is possible for high-peak-power propagation with reduced nonlinear interactions. Multiple foci are observed in the far field.
45 citations
TL;DR: A new technique for precise focal-length measurement with a hologram is presented and offers the additional benefit of easy characterization by use of the same interferometer employed in examining the test optic.
Abstract: A new technique for precise focal-length measurements with a hologram is presented. This technique is widely applicable and is particularly useful for measuring large, slow lenses. In diffraction, the Fresnel-zone plate hologram emulates the reflective properties of a convex spherical mirror for use during transmission null tests of an optic by use of a phase-shifting interferometer. The hologram is written lithographically and therefore offers a higher degree of precision at a lower cost than its spherical mirror counterpart. A hologram offers the additional benefit of easy characterization by use of the same interferometer employed in examining the test optic. Better than ±0.01% precision is achieved during measurement of a 9-m focal-length lens by use of a 150-mm aperture interferometer.
TL;DR: Shimoji et al. as mentioned in this paper derived the location of the boundary surface and the magnitude of the errors in field amplitude that can be expected as a result of applying the Fresnel and Fraunhofer approximations.
Abstract: Evaluation of the electromagnetic fields diffracted from plane apertures are, in the general case, highly problematic. Fortunately the exploitation of the Fresnel and more restricted Fraunhofer approximations can greatly simplify evaluation. In particular, the use of the fast Fourier transform algorithm when the Fraunhofer approximation is valid greatly increases the speed of computation. However, for specific applications it is often unclear which approximation is appropriate and the degree of accuracy that will be obtained. We build here on earlier work (Shimoji M 1995 Proc. 27th Southeastern Symp. on System Theory (Starkville, MS, March 1995) (Los Alamitos, CA: IEEE Computer Society Press) pp 520–4) that showed that for diffraction from a circular aperture and for a specific phase error, there is a specific curved boundary surface between the Fresnel and Fraunhofer regions. We derive the location of the boundary surface and the magnitude of the errors in field amplitude that can be expected as a result of applying the Fresnel and Fraunhofer approximations. These expressions are exact for a circular aperture and are extended to give the minimum limit on the domain of validity of the Fresnel approximation for plane arbitrary apertures.
Patent•
12 Sep 2003
TL;DR: In this paper, a method for improving illumination analysis in seismic data having steps of: selecting a model of the medium 54, a source 20 and a receiver 24, a reflecting surface 62, finding a reflection point 70 and the associated Fresnel zone according to Fermat principle using ray tracing, calculating the illumination in the Fresnel Zone using the wave equation, repeating the illumination calculation for all frequencies all points on the reflecting surface and all source and the receiver pairs.
Abstract: A method for improving illumination analysis in seismic data having steps of: selecting a model of the medium 54, a source 20 and a receiver 24, a reflecting surface 62, finding a reflection point 70 and the associated Fresnel zone according to Fermat principle using ray tracing, calculating the illumination in the Fresnel zone using the wave equation, repeating the illumination calculation for all frequencies all points on the reflecting surface 62 and all source and the receiver pairs. The resulting illumination image may be used to assess the quality of or improve the seismic image of the same medium.
TL;DR: Axial irradiance distribution arising from the diffraction of a uniform, converging, spherical wave at a circular aperture is studied on the basis of scalar boundary-diffraction wave theory.
Abstract: Axial irradiance distribution arising from the diffraction of a uniform, converging, spherical wave at a circular aperture is studied on the basis of scalar boundary-diffraction wave theory. The combined effects of Fresnel number and angular aperture on the focal shift are evaluated, and the validity of the results is checked against the Kirchhoff boundary conditions.
TL;DR: A comparison of the diffraction patterns in the focal region between pulsed and continuous-wave illumination shows that the focal shape produced by a zone plate can be significantly altered when an ultrashort pulse is shorter than 100 fs.
Abstract: The focal distribution produced by a zone plate under ultrashort pulsed laser illumination is investigated under the Fresnel approximation. A comparison of the diffraction patterns in the focal region between pulsed and continuous-wave illumination shows that the focal shape produced by a zone plate can be significantly altered when an ultrashort pulse is shorter than 100 fs. In particular, the focal width in the axial and the transverse directions is increased by approximately 5% and 85%, respectively, from continuous-wave illumination to 10-fs pulsed illumination.
TL;DR: In this paper, a new method of spatial coherence profilometry is demon-strated, where the surface profile is measured by shifting the spatial degree of coherence gradually in its own space of existence, and modulating its phase angle.
Abstract: A new method of spatial coherence profilometry is demon- strated. The surface profile is measured by shifting the spatial degree of coherence gradually in its own space of existence, and modulating its phase angle. In each point of the sample we analyze the change of light intensity versus the phase of a Fresnel zone pattern used as the intensity distribution of an incoherent quasimonochromatic source. The tilt of the surface is measured by gradually shifting the Fresnel zone plate on its transverse plane. This shift of the light source rotates the spatial degree of coherence around the coordinate origin until the condition of maximum interference visibility is fulfilled. The method works without any mechani- cal movement and a quasimonochromatic light illuminates the interfero- metric system. Experimental demonstration of the new method is pre- sented. © 2003 Society of Photo-Optical Instrumentation Engineers.
21 Jun 2003-Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment
TL;DR: In this paper, a beam profile monitor using Fresnel zone plates (FZPs) and the measurement of an electron-beam size with this monitor in the KEK-Accelerator Test Facility (ATF) damping ring was presented.
Abstract: We present a non-destructive and real-time beam profile monitor using Fresnel zone plates (FZPs) and the measurement of an electron-beam size with this monitor in the KEK–Accelerator Test Facility (ATF) damping ring. The monitor system has the structure of a long-distance X-ray microscope, where two FZPs constitute an X-ray imaging optics. The synchrotron radiation from the electron beam at the bending magnet is monochromatized by a crystal monochromator and the transverse electron beam image is twenty times magnified by the two FZPs and detected on an X-ray CCD camera. The expected spatial resolution for the selected photon energy of 3.235 keV is sufficiently high to measure the horizontal and vertical beam sizes of the ATF damping ring. With the beam profile monitor, we succeeded in obtaining a clear electron-beam image and measuring the extremely small beam size less than 10 μm. The measured magnification of the X-ray imaging optics in the monitor system was in good agreement with the design value.
TL;DR: In this article, two new optical particle shape sensors are introduced, placed directly in the near-field of the particle projection (Fresnel region), no lenses or additional optical components are required to obtain particle images.
Abstract: Two new optical particle shape sensors are introduced. By placing them directly in the near-field of the particle projection (Fresnel region), no lenses or additional optical components are required to obtain particle images. Besides size information, accurate shape information is also obtained. Simulations show that distortion by diffraction is limited and can be reduced even further. Both static and dynamic measurements have been performed, which show that the sensors work as predicted by theory. The sensors have been developed to be applied in a microfluidic cytometer.
TL;DR: In this paper, an opaque self supporting flat piece of heat resistant metal let the solar light shine through a few thousand holes properly designed in positions and diameters in order to obtain constructive interferences at some focus.
Abstract: We elaborate about obtaining images of the solar disc and of the solar corona at discrete wavelengths along the EUV emission solar spectrum on board the Solar Orbiter spacecraft. Refractive optics cannot be used. The thermal load is twenty five times higher than on a near Earth orbit. As on one side the efficiency of a stenopeic device is too low, and as on the other side mirrors exposed directly to the light and to the particles emitted by the Sun may severely degrade during time, we investigated using the EUV analog of a Fresnel lens, i.e. a photon sieve.
An opaque self supporting flat piece of heat resistant metal let the solar light shine through a large numbers of a few thousand holes properly designed in positions and diameters in order to obtain constructive interferences at some focus.
We report about practical experiments in the visible.
23 Mar 2003
TL;DR: Propagation of light at 1.5mm along an air hole is achieved within an air-silica structured Fresnel waveguide where the holes are distributed along the Fresnel zones of the fibre.
Abstract: Propagation of light at 1.5mm along an air hole is achieved within an air-silica structured Fresnel waveguide where the holes are distributed along the Fresnel zones of the fibre. Fresnel lensing is reported.
TL;DR: New formulas for the Rayleigh/Fresnel distance in the case of nonlinear diffraction of weak shock waves by a large, finite aperture are deduced from the present study.
Abstract: Fresnel diffraction at a straight edge is revisited for nonlinear acoustics. Considering the penumbra region as a diffraction boundary layer governed by the KZ equation and its associated jump relations for shocks, similarity laws are established for the diffraction of a step shock, an “N” wave, or a periodic sawtooth wave. Compared to the linear case described by the well-known Fresnel functions, it is shown that weak shock waves penetrate more deeply into the shadow zone than linear waves. The thickness of the penumbra increases as a power of the propagation distance, power 1 for a step shock, or 34 for an N wave, as opposed to power 12 for a periodic sawtooth wave or a linear wave. This is explained considering the frequency spectrum of the waveform and its nonlinear evolution along the propagation, and is confirmed by direct numerical simulations of the KZ equation. New formulas for the Rayleigh/Fresnel distance in the case of nonlinear diffraction of weak shock waves by a large, finite aperture are d...
22 Jun 2003
TL;DR: The Finite Difference Time Domain (FDTD) method as mentioned in this paper is a full wave electromagnetic calculation method that is most efficiently applied to electrically large problems is the FDTD method that offers several advantages over the commonly used ray and PE methods, so that the approximation and assumptions needed for application of PE and ray methods do not limit its accuracy.
Abstract: The full wave electromagnetic calculation method that is most efficiently applied to electrically large problems is the Finite Difference Time Domain (FDTD) method. FDTD offers several advantages over the commonly used ray and PE methods. It is full wave, so that the approximations and assumptions needed for application of PE and ray methods do not limit its accuracy. It call include all of the pertinent information available for the propagation path, including terrain profile, terrain permittivity and conductivity, vegetation features, and atmospheric conditions. The FDTD mesh, assuming a uniform atmosphere, need be only long enough (the dimension along the propagation path) to contain the dispersed pulse, and high enough to include the terrain profile and a few Fresnel zones above the highest elevation.
TL;DR: In this article, the guiding of the continuous, short-pulse, coherent, partially coherent and incoherent waves is presented as the diffraction and interference of several X-ray beams produced in free space by the Fresnel source of the waveguide.
Abstract: The optical guiding of X-ray radiation by capillary waveguides is analysed by using the Fresnel-Kirchhoff diffraction theory and the method of images. The guiding of the continuous, short-pulse, coherent, partially coherent and incoherent waves is presented as the diffraction and interference of several X-ray beams produced in free space by the Fresnel source of the waveguide. Using the Fresnel source properties, the necessary conditions for influence of the diffraction and interference on the output characteristics of the capillary X-ray optics in the near- and far-field diffraction zones are derived. The experimental data and computer simulations presented confirm the wave optics behaviour of X-rays under such conditions.
TL;DR: In this paper, the structure of the Fresnel fringes of a triple-gaussian-slit light interferometer is investigated both theoretically and experimentally, and it is shown that the structure is similar to the fractional Talbot images of diffraction gratings.
TL;DR: In this article, a spatial resolution of 100 nm has been achieved in scanning microscopy using highly coherent X-ray beam at a 248 m-long beamline 20XU and F2P' focusing optics.
Abstract: Development of microfocusing optics for scanning microscopy and image forming optics for imaging microscopy are now in progress in SPring-8 by using varieties of optical devices: refractive lens, total reflection mirrors. Fresnel zone plates, and sputtered-sliced Fresnel zone plates. Spatial resolution of 100 nm has been achieved in scanning microscopy using highly coherent X-ray beam at a 248 m-long beamline 20XU and F2P' focusing optics. Recent results on development of optical systems are described.
TL;DR: Improvements of an algorithm based on digital terrain data that combines the basic empirical propagation curves of Okumura et al. (1968) with terrain adaptive propagation curves derived from Fresnel zone clearance analyses of several terrain subsections is discussed.
Abstract: Due to their low computational requirements, two-dimensional semiempirical wave propagation algorithms are still indispensable tools for terrestrial mobile communications and radio broadcasting coverage predictions in the very high and ultrahigh frequency bands. In this paper, we discuss various improvements of an algorithm based on digital terrain data. The algorithm combines the basic empirical propagation curves of Okumura et al. (1968) with terrain adaptive propagation curves derived from Fresnel zone clearance analyses of several terrain subsections. Also, a multiple knife-edge diffraction algorithm with approximate evaluation of the Kirchhoff diffraction integrals is applied when the line of sight is obstructed. Prediction quality is further improved by a robust algorithm for determination of effective transmitter antenna heights, and land usage along the terrain profile different from "open terrain" is considered by a multilevel Fresnel zone blockage evaluation. Prediction errors with standard deviations of about 6 to 7 dB were found for a great variety of measurements in flat and moderately undulating terrain. Transmitter stations with very high antennas (more than 200 m) caused larger standard deviations of the prediction errors. However, improvements of the predictions could be achieved in these cases by evaluating street orientations with respect to the direct path.
TL;DR: In this paper, a multilevel diffractive phase element on the top of the fiber by nanolitography was designed to redistribute the diffractive electromagnetic field on a selected area by a suitable phase modulation of the light, in analogy with what happens for Fresnel zone plates lenses.
Abstract: The present research work is devoted to the realisation of efficient fiber-wave-guide optical coupling between single-mode fiber and rectangular wave guide by fabricating a multilevel diffractive phase element on the top of the fiber by nanolitography. This optical arrangement is able to redistribute the diffractive electromagnetic field on a selected area by a suitable phase modulation of the light, in analogy with what happens for Fresnel Zone Plates Lens. The design of diffractive optical elements has been realized using our own algorthim and code. The out-coming laser beam exiting from the fiber has a gaussian transversal field in contrast to single-mode wave-guide which has an asymmetric transversal field distribution in X and Y direction. Phase modulation has been accomplished by multilevel profiling a polymeric material coated on the top of the fiber by means of a specific fabrication process including e-beam lithography and chemical etching. Focalization experiments for a fiber-waveguide coupling with a 20 microns diameter diffractive element were made with the use of 1550 nm wavelength laser are also discussed.
01 Sep 2003
TL;DR: In this paper, the authors present experimental results and analysis on the application of a new variable in the design of zone plate antennas, which is the choice of phase reference used in the definition of the Fresnel zones from which zone plates are constructed.
Abstract: I present experimental results and analysis on the application of a new variable in the design of zone plate antennas. This variable is the choice of phase reference used in the definition of the Fresnel zones from which zone plates are constructed. The standard zone plate construction assumes a specific choice for this reference phase which, however, can be chosen to have any value between 0 o and 360 o . When the reference phase is varied, antenna pattern measurements reveal small systematic changes in the main lobe gain accompanied by large improvements in the overall sidelobe level and position in both the E- and H-planes. Measurements also show that through variation of the reference phase, the phase of the main lobe can be varied controllably through 360 o . I conclude that reference phase is a useful new tool available to
11 Jul 2003
TL;DR: In this paper, a truncated Gaussian amplitude distribution with a 10 dB taper at the edge of the aperture provides the optimum aperture efficiency, as well as low sidelobe levels.
Abstract: During the past several years many new results have been obtained for Fresnel zone plate antennas having a focal length to diameter ratio (F/D) near unity. Although zone plates have been used for many years at optical wavelengths, the typical configuration has employed F/D values from 20 to 100, sometimes greater than that. This results in a small angle (a few degrees or less) at the focal region, whereas the focal angles for F/D near unity (between 0.5 and 2.0) range from 90° to 28.1°. The small-angle optical conditions permitted making approximate analytical solutions which do not apply to the large-angle case. The recent results for large-angle applications will be summarized for microwave and millimeter-wave examples, although they are valid at any wavelength. In addition, a description will be given of optimization of feed methods (typically corrugated horn antennas) to improve overall efficiency (including aperture efficiency and diffraction efficiency), while also improving (lowering) first-sidelobe levels. This involves solving the case of tapered amplitude illumination across the zone plate aperture, whereas most of the previous investigations have assumed uniform illumination. A truncated Gaussian amplitude distribution with a 10 dB taper at the edge of the aperture provides the optimum aperture efficiency, as well as low sidelobe levels. The utilization of zone plate antennas at terahertz frequencies will also be described. At these frequencies the thin structure of the planar zone plate offers much lower loss than that of a conventional lens. All transparent materials increase in attenuation (loss tangent) as one moves from microwave frequencies to the terahertz range, and conventional lenses have high attenuation. This makes the use of the zone plate lens a preferred choice, for certain configurations.
TL;DR: In this paper, a thin-film Bragg-Fresnel lens (BFL) was developed for diffractive focusing of hard x-rays into submicron to nanometer spots for scanning x-ray spectromicroscopy.
Abstract: A thin-film Bragg–Fresnel lens (BFL) was developed for diffractive focusing of hard x-rays into submicron to nanometer spots for scanning x-ray spectromicroscopy. The lens is made of metal-layer Fresnel zones deposited on an x-ray reflective substrate. The use of a high-density lens structure reduces the thickness of the lens and simplifies the fabrication process. Linear and elliptical lenses made of a 200-nm-thick Au film were fabricated using e-beam lithography and a metal deposition process. The focusing capabilities of the Au layer BFLs were demonstrated at the Advanced Photon Source.