Showing papers on "Fresnel zone published in 2008"

Hard x-ray microscopy with Fresnel zone plates reaches 40 nm Rayleigh resolution.

Abstract: Substantial improvements in the nanofabrication and characteristics of gold Fresnel zone plates yielded unprecedented resolution levels in hard-x-ray microscopy. Tests performed on a variety of specimens with 8–10keV photons demonstrated a first-order lateral resolution below 40nm based on the Rayleigh criterion. Combined with the use of a phase contrast technique, this makes it possible to view features in the 30nm range; good-quality images can be obtained at video rate, down to 50ms∕frame. The important repercussions on materials science, nanotechnology, and the life sciences are discussed.

Some simple rules for contrast, signal-to-noise and resolution in in-line x-ray phase-contrast imaging.

Abstract: Simple analytical expressions are derived for the spatial resolution, contrast and signal-to-noise in X-ray projection images of a generic phase edge. The obtained expressions take into account the maximum phase shift generated by the sample and the sharpness of the edge, as well as such parameters of the imaging set-up as the wavelength spectrum and the size of the incoherent source, the source-to-object and object-to-detector distances and the detector resolution. Different asymptotic behavior of the expressions in the cases of large and small Fresnel numbers is demonstrated. The analytical expressions are compared with the results of numerical simulations using Kirchhoff diffraction theory, as well as with experimental X-ray measurements.

Full-field hard x-ray microscopy below 30 nm: a challenging nanofabrication achievement

Abstract: The fabrication of devices to focus hard x-rays is one of the most difficult—and important—challenges in nanotechnology. Here we show that Fresnel zone plates combining 30 nm external zones and a high aspect ratio finally bring hard x-ray microscopy beyond the 30 nm Rayleigh spatial resolution level and measurable spatial frequencies down to 20–23 nm feature size. After presenting the overall nanofabrication process and the characterization test results, we discuss the potential research impact of these resolution levels.

Horizontal resolution in a non-destructive shallow GPR survey: An experimental evaluation

Abstract: This paper summarizes various approaches to determine the spatial resolution (SR) in non-destructive ground-penetrating radar (GPR) studies and compares the results obtained in these approaches to experimental data acquired in two different media. In one of the experimental measurements, we used 0.4 mS/m conductivity water, which is a low-attenuating medium. In the other case we used sand. Due to the water content in the sand, we obtained velocities similar to the usual velocities in concrete. Experimental radar data showed two different zones: first, the zone in which the recorded anomalies indicate the presence of more than one target; and second, the zone in which the recorded anomalies are related to the existing targets and the interference effects are minimized. We associate the first zone with the antenna's ability to detect two close targets buried at the same depth, and we associate the second zone with the path of the antenna, which is usually estimated as the first Fresnel zone. All of these estimations were carried out using the nominal frequency of the antennas and the dominant frequency obtained from the spectra of the reflected waves.

High-fidelity numerical realization of multiple-step Fresnel propagation for the reconstruction of digital holograms

Abstract: A cascaded Fresnel algorithm for the flexible reconstruction of digital holograms is proposed. Since the fast-Fourier-transform-based numerical realization of the Fresnel integral shows a dependency of its pixel resolution and its computation window size on the propagation distance different from that of the corresponding physical system, the computation window can be smaller than the actual physical diffraction field in the intermediate plane. Consequently, distortions in the final reconstruction may occur. A method is proposed to eliminate such distortion. The validity of this method is shown by both numerical simulations and experimental results.

Phase contrast soft x-ray microscopy using Zernike zone plates.

Abstract: Soft x-ray Zernike phase contrast microscopy was implemented using a “Zernike zone plate” (ZZP) without the use of a separate phase filter in the back focal plane. The ZZP is a single optic that integrates the appropriate ±π/2 radians phase shift through selective zone placement shifts in a Fresnel zone plate. Imaging using a regular zone plate, positive ZZP, and negative ZZP was performed at a wavelength of λ=2.163 nm. Contrast enhancement with the positive ZZP and contrast reversal with the negative ZZP were observed.

The application of Fresnel zone plate based projection in optofluidic microscopy.

Abstract: Optofluidic microscopy (OFM) is a novel technique for low-cost, high-resolution on-chip microscopy imaging. In this paper we report the use of the Fresnel zone plate (FZP) based projection in OFM as a cost-effective and compact means for projecting the transmission through an OFM's aperture array onto a sensor grid. We demonstrate this approach by employing a FZP (diameter = 255 µm, focal length = 800 µm) that has been patterned onto a glass slide to project the transmission from an array of apertures (diameter = 1 µm, separation = 10 µm) onto a CMOS sensor. We are able to resolve the contributions from 44 apertures on the sensor under the illumination from a HeNe laser (wavelength = 633 nm). The imaging quality of the FZP determines the effective field-of-view (related to the number of resolvable transmissions from apertures) but not the image resolution of such an OFM system -- a key distinction from conventional microscope systems. We demonstrate the capability of the integrated system by flowing the protist Euglena gracilis across the aperture array microfluidically and performing OFM imaging of the samples.

Planar infrared binary phase reflectarray.

Abstract: A reflective, binary phase reflectarray is demonstrated in the infrared, at a wavelength of 10.6 μm. The unique aspect of this work, at this frequency band, is that the specific desired phase shift is achieved using an array of subwavelength metallic patches on top of a ground-plane-backed dielectric stand-off layer. This is an alternative to the usual method of constructing a reflective Fresnel zone plate by means of a given thickness of dielectric. This initial demonstration of the reflectarray approach at infrared is significant in that there is inherent flexibility to create a range of phase shifts by varying the dimensions of the patches. This will allow for a multilevel phase distribution, or even a continuous variation of phase, across an optical surface with only two-dimensional lithography, avoiding the need for dielectric height variations.

High-resolution X-ray imaging—a powerful nondestructive technique for applications in semiconductor industry

Abstract: The availability of high-brilliance X-ray sources, high-precision X-ray focusing optics and very efficient CCD area detectors has contributed essentially to the development of transmission X-ray microscopy (TXM) and X-ray computed tomography (XCT) with sub-50 nm resolution. Particularly, the fabrication of high aspect ratio Fresnel zone plates with zone widths approaching 15 nm has contributed to the enormous improvement in spatial resolution during the previous years. Currently, Fresnel zone plates give the ability to reach spatial resolutions of 15 to 20 nm in the soft and of about 30 to 50 nm in the hard X-ray energy range. X-ray microscopes with rotating anode X-ray sources that can be installed in an analytical lab next to a semiconductor fab have been developed recently. These unique TXM/XCT systems provide an important new capability of nondestructive 3D imaging of internal circuit structures without destructive sample preparation such as cross sectioning. These lab systems can be used for failure localization in micro- and nanoelectronic structures and devices, e.g., to visualize voids and residuals in on-chip metal interconnects without physical modification of the chip. Synchrotron radiation experiments have been used to study new processes and materials that have to be introduced into the semiconductor industry. The potential of TXM using synchrotron radiation in the soft X-ray energy range is shown for the nondestructive in situ imaging of void evolution in embedded on-chip copper interconnect structures during electromigration and for the imaging of different types of insulating thin films between the on-chip interconnects (spectromicroscopy).

Near-field behavior of zone-plate-like plasmonic nanostructures

Abstract: The near-field behavior of a new plasmonic structure, the plasmonic micro-zone-plate (PMZP), is presented. The PMZP can realize superfocusing at a working distance on the micrometer scale and a resolving power beyond the diffraction limit. Compared with conventional Fresnel zone plates (CFZPs), its unique characteristics of a significantly elongated depth of focus (DOF) and focal length will make autofocusing easier for the relevant optical systems. These characteristics imply that it is possible to realize a free feedback control system for autofocusing systems in which probe scanning is performed with a constant working distance from the probe to the sample surface, provided that the flatness variation of the sample substrate is within the DOF. Moreover, unlike the CFZPs, there is no series of focal points appearing for beam propagation in the near-field region with a propagation distance ranging from λ to 8λ or even longer. In addition, transmission properties in the near-field region are investigated by means of a computational simulation based on a finite-difference time-domain numerical algorithm. Peak transmission wavelength shifts were observed while the metal film thickness was changed. Focusing characteristics were analyzed for different numerical apertures of the PMZPs.

Super-Fresnel resolution of plasma inhomogeneities by electromagnetic sounding

Abstract: The application of the double weighted Fourier transform (DWFT) method is suggested for extracting the linear integral of the electron density in the inhomogeneous plasma from microwave or IR sounding data. The virtue of DWFT is its ability to localize the measured linear integral in the area, narrow as compared with the radius of the Fresnel zone, that is to realize super-Fresnel resolution. The advantages of the DWFT method are illustrated, firstly, by numerical simulation of a wave propagation through the Gaussian inhomogeneity in conditions of weak scattering, when a small angle Born approximation is applicable, and secondly, by theoretical estimates of resolution in conditions of strong scattering. The method under discussion promises to be helpful for studying the fine structure of turbulent plasma.

Photoacoustic imaging using an ultrasonic Fresnel zone plate transducer

Abstract: A photoacoustic (PA) imaging system based on an ultrasonic Fresnel zone plate (FZP) transducer is developed for the purpose of imaging biological tissue. This FZP transducer has a two-zone negative zone plate piezoelectric material pattern, and an optical fibre is integrated with the transducer on the symmetric axis of the zone plates to deliver laser pulses to the sample. The focal characteristic of the FZP transducer is analysed by theoretical prediction and experimental measurement, and the measured results are in good agreement with the predicted results. The limited-field back-projection deconvolution algorithm combined with the coherence-factor weighting technique is used to reconstruct the optical absorption distribution. The experiments were performed with phantoms and the blood vessels of chicken embryo chorioallantoic membrane. The results demonstrate that PA imaging using the FZP transducer has the ability to image biological tissue and has potential application in monitoring neovascularization in tumour angiogenesis.

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

Abstract: An in-plane Fresnel zone plate for focusing surface plasmon polariton (SPP) fields has been designed, fabricated and tested. Diffractive SPP fields from each Fresnel zone constructively interfere and focus at the designed focal point.

Fabrication of high-aspect-ratio Fresnel zone plates by e-beam lithography and electroplating

Abstract: The fabrication of gold Fresnel zone plates, by a combination of e-beam lithography and electrodeposition, with a 30 nm outermost zone width and a 450 nm-thick structure is described. The e-beam lithography process was implemented with a careful evaluation of applied dosage, tests of different bake-out temperatures and durations for the photoresist, and the use of a developer without methylisobutylketone. Electrodeposition with a pulsed current mode and with a specially designed apparatus produced the desired high-aspect-ratio nanostructures. The fabricated zone plates were examined by electron microscopy and their performances were assessed using a transmission X-ray microscope. The results specifically demonstrated an image resolution of 40 nm.

Phase modulation of surface plasmon polaritons by surface relief dielectric structures

Abstract: Phase modulation of surface plasmon polaritons (SPPs) is studied as a function of geometric length and thickness of surface relief dielectric structures on a metal film. The results indicate that the SPPs are analogous to conventional free-space light waves in terms of phase modulation by geometric sizes and refractive indices. An SPP Fresnel zoneplate is considered as an example to employ the phase modulation method for focusing enhancement at the focal point. It is found that the phase modulation optimized zoneplate has obtained 130% higher electric field intensity at the focal point than that of an amplitude-modulated zoneplate.

Analysis of extreme ultraviolet microscopy images of patterned nanostructures based on a correlation method

Abstract: A method to analyze extreme ultraviolet microscopy images of nanostructures that allows for the simultaneous determination of an object’s feature size and image resolution is presented. It is based on the correlation between the image and a set of templates of known resolution generated from the original image using Gaussian filters. The analysis was applied to images obtained with a Fresnel zone plate microscope that uses a 13.2 nm wavelength laser light for illumination. The object’s feature size and the resolution obtained with this method are shown to be in very good agreement with independent measurements of both magnitudes. © 2008 Optical Society of America OCIS codes: 100.2000, 100.2960, 110.0180.

Infrared holography using a microbolometer array

Abstract: We describe a series of experiments to demonstrate holography at far-infrared wavelengths using an uncooled microbolometer array. Simple interference patterns and Fresnel zone holograms are recorded with a 10 W cw CO(2) laser illumination in a Mach-Zehnder interferometer setup. A sparse-sampling method is used to sample the hologram at a rate dependent on the bandwidth of the object wavefront rather than the carrier frequency. The samples are then used to reconstruct the complex object wavefront in the hologram plane, which is Fresnel backpropagated for image reconstruction. Uncooled microbolometer arrays are most commonly used in passive mode to image the thermal-blackbody radiation. Their technology has matured to include the wavelength range of far-infrared to submillimeter radiation. The use of microbolometers with active illumination for holography, as described in this paper, suggests their interesting future applications.

Method for determining adequacy of seismic data coverage of a subsurface area being surveyed

Abstract: A method for assessing data coverage in a three dimensional marine seismic survey includes determining at least one Fresnel zone for at least one of a plurality of seismic data traces. A contribution is determined for each of the seismic data traces to each one of a set of bins in a defined pattern. Each contribution is based on the Fresnel zone associated with each seismic data trace. The contributions from all seismic data traces contributing to each bin are summed. The summed contribution for each bin are stored or displayed and the summed contributions in each bin are compared to a selected threshold to determine coverage.

Wave tracing: Ray tracing for the propagation of band-limited signals: Part 1 — Theory

Abstract: Many seismic imaging techniques require computing traveltimes and travel paths. Methods to compute raypaths are usually based on high-frequency approximations. In situations such as head waves, these raypaths minimize traveltime but are not paths along which most of the energy travels. We have developed a new approach to computing raypaths, using a modification of ray bending that we call wave tracing; it computes raypaths and traveltimes that are more consistent with the paths and times for the band-limited signals in real data than the paths and times obtained using high-frequency approximations. Wave tracing shortens the raypath while keeping the raypath within the Fresnel zone for a characteristic frequency of the signal.

A new Fresnel zone antenna with beam focused in the Fresnel region

Abstract: A new Fresnel Zone (FZ) Antenna to focus the microwave power in the radiation near field region is presented. FZ Antennas, in comparison with array antennas or reflector antennas, conventional focused antennas, take the advantages of design simplicity as well as lower sidelobe levels (SLL) in both axial and transverse directions. Simulation results and comparisons made between the new structure and aperture antennas based on the quadratic phase distribution show a reduction of sidelobe levels in both axial and transverse directions.

Optimization of polygonal Fresnel zone plates

Abstract: The performance of Fresnel zone plates having polygonal boundaries between zones has been studied for polygons with a low number of sides. An optimized polygonal shape has been analytically defined by minimizing the mismatched area between the circular and the polygonal pseudo-Fresnel zones. The square polygon has been analyzed in more depth than the others because of its simple symmetry. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 536–541, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23125

A Time-Efficient Near-Field Scattering Method Applied to Radio-Coverage Simulation in Urban Microcellular Environments

Abstract: An accurate and time-efficient method for the calculation of the scattered electromagnetic (EM) field from rectangular plates (walls) in the near and Fresnel zone areas is presented in this letter. The proposed method, which is based on the analytical method of physical optics, is subsequently applied for the purpose of time-acceleration of radio-coverage prediction codes in urban microcellular systems, which incorporate near\Fresnel field calculations. Comparisons of the results obtained through this method with the corresponding results based on different EM techniques, as well as the incorporation of this method into a radiocoverage simulation program, recently published in the literature by the authors' research group, prove both the excellent accuracy of the proposed method and the significant reduction of the computation time.

Elastic Kirchhoff migration of vectorial wave-fields

Abstract: Based on Kuo and Dai’s vectorial wave-field extrapolation equations, we derive new Kirchhoff migration equations by introducing unit vectors which represent the ray directions at the imaging points of the reflected P- and PS converted-waves. Furthermore, using the slope of the events on shot records and a ray racing procedure, mirror-image reflection points are found and the reflection data are smeared along the Fresnel zone. The migration method proposed in this paper solves two troublesome imaging problems caused by limited receiving aperture and migration artifacts resulting from wave propagation at the velocities of non original wave type. The migration method is applied successfully with model data, demonstrating that the new method is effective and correct.

Variable focus lens system for eyeglasses

Abstract: A variable focus lens system that can be used in eyeglasses is disclosed. The system consists of superimposed first and second transparent thin lenses with surfaces divided into Fresnel zones and zone boundaries. The lenses are relatively slidable for focus adjustment. The Fresnel zone boundary surfaces are made parallel to the user's optical line of sight to reduce visual obstructions, and the zone boundaries may be restricted to areas near the lens periphery to further reduce visual obstructions. An opaque light absorbing coating may be applied to the zone boundaries to reduce light scattering. A fixed prescription lens, having the user's distance and astigmatism corrections, may be superimposed on the other two lenses.

Fresnel lenses based on dye-doped liquid crystals

Abstract: We demonstrated transmissive- and reflective-type Fresnel lenses based on dye-doped liquid crystal using photoalignment technique. The former is a polarization-independent and electrically tunable. The maximum diffraction efficiency reaches 37%, which approaches the theoretical limit ~ 41 %. Such a lens functions as a half-wave plate, and this feature could be well preserved under the applied voltage. The reflective-type Fresnel lens is based on dye-doped cholesteric liquid crystals (DDCLC). 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, and rewritable. Notably, both of the transmissive- and reflective-type Fresnel lenses are switchable between focusing and defocusing states, upon application of a voltage. In addition, these devices are simple to fabricate, and have fast switching responses between focusing and defocusing state.

The response of microwave reflectometry under generalized configuration

Abstract: The response of microwave reflectometry for various configurations is studied theoretically. Based on the Kirchhoff integral (i.e. physical optics), analytic formulae for the measured electric field are obtained. Phase and power sensitivities for weak fluctuations are derived. It was found that the effective spot size, which is a combination of the real spot size and the Fresnel zone, of the launching and the receiving beams is an important parameter, determining the sensitivities. Fringe jump and phase runaway phenomena are attributed to two parameters: one is proportional to the normalized wavenumber of the fluctuations, and the other is proportional to the plasma shift (i.e. misalignment). In the case of frozen turbulence, a complex power spectrum of the measured field becomes a universal quantity, because of easy correction of the configuration effect. Using this feature, an ideal complex amplitude signal (i.e. free from diffraction effects) can be reconstructed.

Focusing properties of two types of diffractive photonic crystal lens

Abstract: We designed novel two type of binary 2D subwavelength (wavelength was λ= 10 mm) focus diffractive photonic crystal lens and calculated the diffraction of plane TE-wave by use FDTD-method (our program in C++). It has been shown that diffractive photonic crystal lens designs have not an unique solution. It has been also shown the diffractive lens with equal holes in Fresnel zones has better focusing characteristics. Lens diameter was 5 times more than her width and full width half maximum diameter of focal spot was 0.48λ.

Fabrication of Fresnel zone plates with high aspect ratio by soft X-ray lithography

Abstract: High focusing efficiency Fresnel zone plates for hard X-ray imaging is fabricated by electron beam lithography, soft X-ray lithography, and gold electroplating techniques. Using the electron beam lithography, Fresnel zone plates which has an outermost zone width of 100 nm and thickness of 250 nm has been fabricated. Fresnel zone plates with outermost zone width of 150 nm and thickness of 660 nm has been fabricated by using soft X-ray lithography.

A Study of Fresnel Scattered Fields for Ellipsoidal and Elliptic-Disk-Shaped Scatterers

Abstract: The study of scattered fields from nonspherical scatterers is becoming an important subject particularly in the area of theoretical modeling of microwave backscatter from vegetation. The generalized Rayleigh-Gans approximation has been widely used in the calculation of scattered fields from scatterers where at least one of its dimensions is comparably smaller than the wavelength. In such calculations, far-field approximations are used, which are not accurate when the observation points are in the Fresnel and near-field regions of the scatterer. Hence, the effects of Fresnel phase and amplitude corrections for the scattered field of circular disks, needles, and cylinders have been examined and shown to be significant in the calculation of closely spaced scatterers. However, the effects of Fresnel corrections in the scattered fields from general ellipsoids and elliptic disks have not yet been studied. In this paper, the scattered fields from general ellipsoids and elliptic disks are formulated based on the generalized Rayleigh-Gans approximation for cases with and without Fresnel corrections. Theoretical analysis shows that the Fresnel effects are important particularly at larger frequencies and at the null locations in the plots of the backscattering cross section. These effects become more important as the ellipticity of the scatterer increases. Comparisons with measurement data demonstrate that the calculated results with Fresnel corrections provide a better match compared with those without.