Showing papers on "Fresnel zone published in 2007"

X-ray computed tomography in Zernike phase contrast mode at 8 keV with 50-nm resolution using Cu rotating anode X-ray source

Abstract: High-resolution X-ray computed tomography (XCT) enables nondestructive 3D imaging of complex structures, regardless of their state of crystallinity. This work describes a sub-50 nm resolution XCT system operating at 8 keV in absorption and Zemike phase contrast modes based on a commercially available Cu rotating anode laboratory X-ray source. The system utilizes a high efficiency reflective capillary condenser lens and high-resolution Fresnel zone plates with an outermost zone width of 35 nm and 700 nm structure height resulting in a spatial resolution better than 50 nm currently. Imaging a fragment of the solid oxide fuel cells (SOFC) with 50 nm resolution is presented as an application example of the XCT technique in materials science and nanotechnology.

White-light imaging with fractal zone plates.

Abstract: We report the achievement of the first images to our knowledge obtained with a fractal zone plates (FraZPs). FraZPs are diffractive lenses characterized by the fractal structure of their foci. This property predicts an improved performance of FraZPs as image forming devices with an extended depth of field and predicts a reduced chromatic aberration under white-light illumination. These theoretical predictions are confirmed experimentally in this work. We show that the polychromatic modulation transfer function of a FraZP affected by defocus is about two times better than one corresponding to a Fresnel zone plate.

Scanning holographic microscopy with resolution exceeding the Rayleigh limit of the objective by superposition of off-axis holograms

Abstract: We present what we believe to be a new application of scanning holographic microscopy to superresolution. Spatial resolution exceeding the Rayleigh limit of the objective is obtained by digital coherent addition of the reconstructions of several off-axis Fresnel holograms. Superresolution by holographic superposition and synthetic aperture has a long history, which is briefly reviewed. The method is demonstrated experimentally by combining three off-axis holograms of fluorescent beads showing a transverse resolution gain of nearly a factor of 2.

Fourier plasmonics: Diffractive focusing of in-plane surface plasmon polariton waves

Abstract: An in-plane Fresnel zone plate (FZP) for focusing surface plasmon polariton (SPP) fields has been designed, fabricated, and tested. The fabricated device consists of 400nm tall by 5μm wide amorphous Si-based SPP FZP on an Al film integrated with a pair of two-dimensional nanohole arrays for excitation of the incident and detection of the diffracted SPP fields. Diffracted SPP fields from each Fresnel zone constructively interfere at the expected focal point to produce focusing with threefold intensity enhancement. Temporal and spatial characteristics of the focused SPP fields are studied with time-resolved spatial-heterodyne imaging technique. Good agreement with average power measurements is demonstrated. Diffractive focusing of SPP fields, based on Fourier plasmonics, represents an approach to SPP in-plane microscopy.

Extraordinary transmission and left-handed propagation in miniaturized stacks of doubly periodic subwavelength hole arrays

Abstract: Metallic plates embedded between dielectric slabs and perforated by rectangular arrays of subwavelength holes with a dense periodicity in one of the directions support extraordinary transmission (ET) phenomena, viz. strong peaks in the transmittance frequency dependence. Stacks of such perforated plates support ET phenomena with propagation along the stack axis that is characterized by the left handed behavior. The incorporation of the dielectric materials and dense periodicity allows significantly reducing the illuminated area of the perforated plate required experimentally to observe the ET phenomena as compared to the areas required in the case of free standing rectangular hole arrays. This facilitates the experimental investigation of ET under excitation in the Fresnel zone of Gaussian beams.

Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals

Abstract: This work demonstrates a reflective Fresnel zone plate based on dye-doped cholesteric liquid crystals (DDCLC) using the photo-induced realignment technique. Illumination of a DDCLC film with a laser beam through a Fresnel-zone-plate mask yields a reflective lens with binary-amplitude structures - planar and focal conic textures, which reflect and scatter probed light, respectively. The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7%, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6%). The lens is thermally erasable, rewritable and switchable between focusing and defocusing states, upon application of a voltage.

Volume Fresnel zone plates fabricated by femtosecond laser direct writing

Abstract: In this letter, volume Fresnel zone plates fabricated inside fused silica using femtosecond laser direct writing are demonstrated. A volume zone plate consists of a number of layers of Fresnel zone plates designed to focus light together. Results indicate that volume Fresnel zone plates increase the overall diffraction efficiency significantly. Phase zone plates yield considerably higher diffraction efficiency due to a number of reasons including negligible light absorption, robustness against fabrication phase errors, and minimum interference between different zone plates in a volume.

A Full Electromagnetic Analysis of Grooved-Dielectric Fresnel Zone Plate Antennas for Microwave and Millimeter-Wave Applications

Abstract: Grooved-dielectric, phase-correcting, Fresnel zone plate antennas are analyzed using the body-of-revolution finite-difference time-domain (BOR-FDTD) method. Parametric studies of the focusing ability of these antennas are performed to examine the effects of the focal length, diameter, number of zones, and the thickness of the lens, as well as the number of phase corrections per zone. The results of these studies are presented as design graphs and are used to lend insight into the focusing mechanism of phase-correcting zone plates. The BOR-FDTD analysis is validated by comparison with previous measurements.

A field comparison of Fresnel zone and ray-based GPR attenuation-difference tomography for time-lapse imaging of electrically anomalous tracer or contaminant plumes

Abstract: Ground-penetrating radar GPR attenuation-difference tomographyisausefultoolforimagingthemigrationofelectrically anomalous tracer or contaminant plumes. Attenuation-difference tomography uses the difference in the trace amplitudes of tomographic data sets collected at different times to image the distribution of bulk-conductivity changes within the medium. Themostcommonapproachforcomputingthetomographicsensitivities uses ray theory, which is well understood and leads to efficient computations. However, ray theory requires the assumption that waves propagate at infinite frequency, and thus sensitivities are distributed along a line between the source and receiver. The infinite-frequency assumption in ray theory leads to a significant loss of resolution both spatially and in terms of amplitude of the recovered image. We use scattering theory to approximate the sensitivity of electromagnetic EM wave amplitude to changes in bulk conductivity within the medium. These sensitivities occupy the first Fresnel zone, account for the finite frequency nature of propagating EM waves, and are valid whenvelocityvariationswithinthemediumdonotcausesignificant ray bending. We evaluate the scattering theory sensitivities by imaging a bromide tracer plume as it migrates through a coarse alluvial aquifer over two successive days. The scattering theory tomograms display a significant improvement in resolution over the ray-based counterparts, as shown by a direct comparison of the tomograms and also by a comparison of the verticalfluidconductivitydistributionmeasuredinamonitoringwell, located within the tomographic plane. By improving resolution, the scattering theory sensitivities increase the utility of GPR attenuation-difference tomography for monitoring the movement of electrically anomalous plumes. In addition, the improved accuracy of information gathered through attenuation-difference tomography using scattering theory is a positive step toward futuredevelopmentsinusingGPRdatatohelpcharacterizethedistributionofhydrogeologicproperties.

Wave-optical simulation of hard-x-ray nanofocusing by precisely figured elliptical mirrors

Abstract: Computer simulations of nanofocusing by elliptical mirrors are presented wherein the diffraction and propagation of coherent hard x rays are predicted using wave-optical calculations. Surface height data acquired via microstitching interferometry were used to calculate the complex pupil function of a mirror, taking into account the Fresnel reflectivity and treating the surface topography as an aberration to a perfect elliptical mirror. The reflected wave-field amplitude and phase downstream of the mirror were obtained by numerically evaluating the Fresnel-Kirchhoff diffraction integral. Simulated intensity profiles and contours (isophotes) around the focal plane are presented for coherent illumination by a 15 keV point source, which indicate nearly diffraction-limited focusing at the 40 nm level. The effect of high spatial frequency microroughness on nanofocusing was investigated by low-pass filtering the Fourier spectrum of the residual height profile. Simulations using the filtered metrology data confirmed that roughness length scales shorter than 0.1 mm have a minor effect on the focal spot size and intensity.

Incorporating the Fresnel Zone Theory in Ray Tracing for Propagation Modelling of Fixed Wireless Access Channels

Abstract: A propagation model which combines the Fresnel zone theory and the ray tracing techniques is presented in this paper. The aim of this novel approach is a more accurate representation of the fixed wireless access channel. Deterministic models do not consider power loss when the radio waves propagate close to roof tops. However, according to the theory of the Fresnel zones, even when there is line-of-sight (LOS), the loss can be as much as 6 dB due to a single obstacle just below the LOS. In the model presented here, the path of each ray is found by a three-dimensional (3D) ray tracing algorithm and the electromagnetic field of each ray is calculated according to geometric optics (GO) and the uniform theory of diffraction (UTD). The first Fresnel zone of each ray is also found and examined and the losses due to its partial blockage are taken into account. Simulation results show significant differences in the power predictions when the blockage of the Fresnel zones is considered.

W-Band Fresnel Zone Plate Reflector for Helicopter Collision Avoidance Radar

Abstract: A high gain Fresnel zone plate reflector is presented for helicopter collision avoidance radar at 94 GHz. The antenna system consists of a compact, airtightness and low return loss primary source illuminating a printed reflector with Fresnel plate zone phase correction. In order to increase the overall efficiency, the reflector combines 8 correcting zones in its center and 4 on the border. Reflected phase of over 360deg is achieved by means of circular ring or combined circular and ring patches. The reflector is fabricated using standard photolithographic techniques. The primary source consists of a small FSS made by a double slot array antenna pasted on the aperture of a standard WR-10 waveguide. Antenna measurements were conducted using a compact base arrangement. The antenna performs 37.8 dBi at the center frequency of 94 GHz with a maximum value of the return loss value not exceeding -25 dB. The frequency bandwidth at -3 dB in gain and return loss is of 9%

Quasioptical Polarizer Based on Self-Complementary Sub-Wavelength Hole Arrays

Abstract: In this letter, extraordinary transmission in sub-wavelength hole arrays is employed in combination with self-complementary Babinet concepts to design a novel class of quasioptical polarizer for millimeter wave frequencies. After a proper design, the device has been miniaturized in such a way that operation under Fresnel zone of a Gaussian beam has been demonstrated. Experimental validation opens up the way to new possibilities for polarizer design.

Prediction of Urban GNSS Availability and Signal Degradation Using Virtual Reality City Models

Abstract: A software toolkit has been developed by UCL/Spirent Communications that can harvest geometric data from computerised models of urban environments. This data can then be used to predict satellite availability and receiver position error due to multipath. Generally, methods that exist to predict satellite availability use line of sight (LOS) propagation models only. This paper identifies reasons as to why the LOS only approach is inappropriate. A simulator has been developed to predict satellite availability via a range of sophisticated propagation modes, including diffraction and reflection. This simulator has been designed and written such that it will ultimately be used by low resource devices. An overview of the simulator and the simulation process is first provided. The definition of an available signal is then discussed. Description is given of a model developed to determine the signals arriving at an antenna via diffracted propagation. Further to this, description is given of a separate propagation model that assesses the level of signal obstruction within a theoretical volume known as a Fresnel zone. It is within this zone that the majority of the energy for transmission between transmitter (GNSS satellite) and receiver is contained. The third signal propagation model provides an estimate of signals that are received by reflected paths. A demonstration of how this tool can be used is provided, and simulation results are shown. These simulated results are then compared with real-world observations in order to test and validate the three signal propagation models. A discussion of the results is provided before conclusions and details of continuing work are given.

Two-step hard X-ray focusing combining Fresnel zone plate and single-bounce ellipsoidal capillary

Abstract: A two-step focusing set-up combining a Fresnel zone plate with an ellipsoidal capillary is presented. It is shown that, in addition to the anticipated gain in flux, the employment of the prefocusing micro-optic makes optimal use of the elliptical shape of the capillary by almost eliminating aberrations. A small cross section of the prefocused beam allows a tiny fraction of the capillary surface to be selected, thus reducing the influence of slope errors. An X-ray beam with a 15 keV energy was focused down to a spot size as small as 250 nm, demonstrating the best value that has been achieved up to now for single-bounce capillaries. The use of an ellipsoidal capillary as a micromirror under off-axis illumination by microfocusing optics may open up new opportunities in nanofocusing developments.

Influence of geometry on polychromatic speckle contrast.

Abstract: Understanding speckle behavior is very important in speckle metrology application. The contrast of a polychromatic speckle depends not only on surface roughness and the coherence length of a light source, as shown in previous works, but also on optical geometry. We applied the Fresnel approach of diffraction theory for the free-space geometry and derived a simple analytical relationship between contrast, coherence length, size of illuminated spot, and distances between source, object, and observation plane. The effect of contrast reduction is found to be significant for low-coherence light sources.

Focusing high energy X‐rays with stacked Fresnel zone plates

Abstract: Stacking technique was developed in order to increase focusing efficiency of Fresnel zone plates at high energies. Two identical Si chips each of which containing Fresnel zone plates were used for stacking. Alignment of the chips was achieved by on-line observation of the moire pattern from the two zone plates. The formation of moire patterns was studied theoretically and experimentally at different experimental conditions. To provide the desired stability Si-chips with zone plates were bonded together with slow solidification speed epoxy glue. Technique of angular alignment in order to compensate a linear displacement in the process of gluing was proposed. Two sets of stacked FZPs were produced and experimentally tested to focus 15 and 50 keV X-rays. Gain in the efficiency by factor 2.5 was demonstrated at 15 keV. Focal spot of 1.8 µm vertically and 14 µm horizontally with 35% efficiency was measured at 50 keV. Forecast for the stacking of nanofocusing Fresnel zone plates was discussed.

Laser direct writing of volume modified Fresnel zone plates

Abstract: Volume zone plates consisting of a number of Fresnel zone plate layers are designed and fabricated inside a fused silica by the femtosecond laser direct writing method. Light diffracted from different layers of a volume zone plate constructively interfere at the focal spot to increase diffraction efficiency. The technique is experimentally verified to be effective for both low-numerical-aperture (NA) and high-NA zone plates, resulting in a significant increase in overall diffraction efficiency. The spatial resolution of a high NA volume zone plate is 500 nm.

Laboratory studies of petal-shaped occulters

Abstract: We present laboratory studies of scaled occulting starshades for the New Worlds Observer (NWO). A deep reactive ion etched silicon starshade has been fabricated by NIST, designed to cover the same number of Fresnel zones as in the proposed mission. The broadband shadow is mapped with a photometer in a dark vacuum tunnel fed by a heliostat at HAO. CCD images provide direct contrast measurements of different features around the starshade. Preliminary measurements reach 5x10 -6 suppression in the center of the shadow at the focal plane. The two-dimensional structure of the starshade diffraction pattern is compared to that produced by the Fresnel integral.

Improved zoning rule for designing square Fresnel zone plate lenses

Abstract: An improved zoning rule is presented for designing a square Fresnel zone plate lens (FZPL). This new rule results in a higher gain when the FZPL is used as an antenna element or can enhance the focusing properties of the square FZPL when used to collimate an incident plane wave. The derivation of this improved zoning rule is presented along with simulated results for some typical cases. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 276–278, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22108

On the width of a ray

Abstract: Consistent with earlier work by Kravtsov and Orlov, a simple general expression for the width of a Fresnel zone δrF in a smooth inhomogeneous environment is derived; this is the diffractive contribution to the width of a ray. In a stratified environment at long range, the general Fresnel zone width expression is shown to reduce approximately to one that is proportional to ∣α∣r(R−r)∕σ where α is the ray stability parameter, σ is the acoustic frequency, r is the range from the source to the field point of interest, and R is the source to receiver range. In a stratified environment on which a weak small-scale perturbation is superimposed, deterministic rays in the background environment that connect fixed end points break up into bundles of micromultipaths at moderate to long range and a second, scattering-induced, contribution δrs to the width of a ray must be considered. It is shown that δrs is proportional to ∣α∣r(R−r) and argued that in a micromultipathing environment the total effective width of a backg...

Influence of the Interface Fresnel zone on the reflected P-wave amplitude modelling

Abstract: SUMMARY The aim of the paper is to emphasize the importance of accounting for the Fresnel volume and for the Interface Fresnel zone (IFZ) for calculating the amplitude of the P wave emanating from a point source and recorded at a receiver after its specular reflection on a smooth homogeneous interface between elastic media. For this purpose, by considering the problem of interest as a problem of diffraction by the IFZ, that is, the physically relevant part of the interface which actually affects the reflected wavefield, we have developed a method which combines the Angular Spectrum Approach (ASA) with the IFZ concept to get the 3-D analytical solution. The variation in the reflected P-wave amplitude evaluated with the ASA, as a function of the incidence angle, is compared with the plane wave (PW) reflection coefficient and with the exact solution provided by the 3-D code OASES, for one solid/solid configuration and two dominant frequencies of the source. For subcritical incidence angles the geometrical spreading compensation is mostly quite sufficient to reduce the point-source amplitudes to the PW amplitudes. On the contrary, for specific regions of incidence angles for which the geometrical spreading compensation is not sufficient anymore, that is, near the critical region and in the post-critical domain, the ASA combined with the IFZ concept yields better results than the PW theory whatever the dominant frequency of the source, which suggests that the additional application of the IFZ concept is necessary to obtain the reflected P-wave amplitude. Nevertheless, as the ASA combined with the IFZ has been used only for evaluating the contribution of the reflected wavefield at the receiver, its predictions fail when the interference between the reflected wave and the head wave becomes predominant.

Fabrication of x-ray zone plates by surface-plasma chemical vapor deposition

Abstract: A new cost-efficient sputter-slice technology for hard x-ray (10-30 keV) Fresnel zone plates fabrication, imposing no limitation to aspect ratio, is proposed. By means of a plasma chemical process, SiO(2)/Si(1-x)Ge(x)O(2) glassy film multilayer structures are deposited on a lateral surface of a silica rod, outermost layers being as thin as 100 nm. It has been shown by numerical simulation that for x=0.2 germanium fraction, 100-300 microm zone plate thickness and the number of zones of about 1000, first order diffraction efficiency as high as 20%-30% at the energy of approximately 20 keV can be achieved.

Focused ion beam fabrication procedures of x-ray micro Fresnel zone plates

Abstract: In this paper the use of focused ion beam (FIB) mask-less lithography is presented as a novel and simple way to fabricate x-ray Fresnel zone plates (FZPs), and a prototype of a FIB-made 100 nm resolution FZP with 38 zones is described. Considerations for future developments—the maximum aspect ratio achievable by FIB lithography in nickel, a way to produce zones with a parabolic transverse profile, as theoretically required for the highest efficiency in focusing—are also reported.

Effective Fresnel number and the focal shifts of focused partially coherent beams.

Abstract: We define the effective Fresnel number of the cylindrical lens illuminated by a plane wave or Schell-model beams. On the basis of the concept of the effective Fresnel number, the focusing properties of the cylindrical lens illuminated by the Schell-model beam are investigated in a simple way. It is shown that the relative focal shift can be evaluated by an analytical formulation, which is expressed as a function of the effective Fresnel number. To evaluate our approach, we make the comparison between the results obtained by our method and the numerical calculation based on the diffraction integral. The results indicate that we can simply and exactly evaluate the focal shifts with our method.

Studies on the light permeance characteristic of a Fresnel lens group applied in high concentration solar energy

Abstract: A key point to limit wide applications of solar energy is the exploitation cost. With the advantages of compact volume, less weight, small focal length and low cost Fresnel lenses are suitable for solar radiation concentration and system modularization. The optical efficiency of transmitted solar radiation is improved compared to thick ordinary lenses and the solar tracking accuracy required by a Fresnel lens group is lower than for other reflection schemes. In this study, a modular device composed of a two-stage Fresnel lens is presented and its performance is investigated. Based on the analysis of rays passing through the wedge structure of the lens, the optical efficiency is obtained under different incidental angles. Facular point image qualities of a Fresnel lens are compared using simulation results. As for the Fresnel lens 100 mm in diameter, 220 mm in focal length, when the incident angle is kept within 1°, the focal point will not move out of the receiver domain with the dimensions of 10 mm × 10 mm and the relationship between incident angle against receiver dimension is obtained. Optical efficiency measurements have been carried out on a practical Fresnel lens group which includes another Fresnel lens 30 mm in diameter with the small-size secondary lenses integrated into an intermediate panel combined with the first Fresnel lens plane to improve the performance of the integral battery system at high concentration ratio. According to experimental studies, the positions of the focal spot varying with the incident angles of solar radiation are recorded continuously. It has been found that 50–60% of the collected solar radiation can pass through the Fresnel lens group under a condition of over 1000 suns, and the deviation range of the focal spot fundamentally agrees with the result of the simulation.

Stacked Fresnel Zone Plates for High Energy X-rays

Abstract: A stacking technique was developed in order to increase focusing efficiency of Fresnel zone plates (FZP) at high energies. Two identical Si chips each of which containing 9 FZPs were used for stacking. Alignment of the chips was achieved by on‐line observation of the moire pattern. The formation of moire patterns was studied theoretically and experimentally at different experimental conditions. To provide the desired stability Si‐chips were bonded together with slow solidification speed epoxy glue. A technique of angular alignment in order to compensate a linear displacement in the process of gluing was proposed. Two sets of stacked FZPs were experimentally tested to focus 15 and 50 keV x rays. The gain in the efficiency by factor 2.5 was demonstrated at 15 keV. The focal spot of 1.8 μm vertically and 14 μm horizontally with 35% efficiency was measured at 50 keV. Forecast for the stacking of nanofocusing FZPs was discussed.