Showing papers on "Fresnel zone published in 2018"

Sharp acoustic vortex focusing by Fresnel-spiral zone plates

Abstract: We report the optimal focusing of acoustic vortex beams by using flat lenses based on a Fresnel-spiral diffraction grating. The flat lenses are designed by spiral-shaped Fresnel zone plates composed of one or several arms. The constructive and destructive interferences of the diffracted waves by the spiral grating result in sharp acoustic vortex beams, following the focal laws obtained in analogy with the Fresnel zone plate lenses. In addition, we show that the number of arms determines the topological charge of the vortex, allowing the precise manipulation of the acoustic wave field by flat lenses. The experimental results in the ultrasonic regime show excellent agreement with the theory and full-wave numerical simulations. A comparison with beam focusing by Archimedean spirals also showing vortex focusing is given. The results of this work may have potential applications for particle trapping, ultrasound therapy, imaging, or underwater acoustic transmitters.

Optical Manipulation along an Optical Axis with a Polarization Sensitive Meta-Lens.

Abstract: The ability to manipulate small objects with focused laser beams opens a broad spectrum of opportunities in fundamental and applied studies, for which precise control over mechanical path and stability is required. Although conventional optical tweezers are based on refractive optics, the development of compact trapping devices that could be integrated within fluid cells is in high demand. Here, a plasmonic polarization-sensitive metasurface-based lens, embedded within a fluid, is demonstrated to provide several stable trapping centers along the optical axis. The position of a particle is controlled with the polarization of the incident light, interacting with plasmonic nanoscale patch antennas, organized within overlapping Fresnel zones of the lens. While standard diffractive optical elements face challenges in trapping objects in the axial direction outside the depth of focus, bifocal Fresnel meta-lens demonstrates the capability to manipulate a bead along a 4 μm line. An additional fluorescent module, ...

Lensless light-field imaging with Fresnel zone aperture: quasi-coherent coding

Abstract: We propose a new type of lensless camera enabling light-field imaging for focusing after image capture and show its feasibilities with some prototyping. The camera basically consists only of an image sensor and Fresnel zone aperture (FZA). Point sources making up the subjects to be captured cast overlapping shadows of the FZA on the sensor, which result in overlapping straight moire fringes due to multiplication of another virtual FZA in the computer. The fringes generate a captured image by two-dimensional fast Fourier transform. Refocusing is possible by adjusting the size of the virtual FZA. We found this imaging principle is quite analogous to a coherent hologram. Not only the functions of still cameras but also of video cameras are confirmed experimentally by using the prototyped cameras.

Geometric Power Fall-Off in Radar Sounding

Abstract: This paper reports the analysis of the geometric power fall-off of Fresnel zone scattering in radar sounding. Radar sounders can take advantage of strong coherent scattering from Fresnel zones at nadir which grow with sensor altitude. The strength of the signal and the actual rate of power fall-off, however, depend heavily on the surface properties. We first use the radar equation to separate geometric versus scattering mechanisms driving $R^{2}$ , $R^{3}$ , or $R^{4}$ fall-off. The Fresnel zone for planetary surfaces, where body curvature has an effect, is derived and its implications discussed. We show the impact of Gaussian and fractal surface roughness on the exponent of power fall-off and the transition from coherent to incoherent scattering. This is done in simulation and analytically to derive coherence loss functions. Finally, we study the effect of incoherent area fraction within the Fresnel zone. These results are intended to be used in radar link budgets, performance metrics, and scientific interpretation of sounding data.

GPS Multipath Analysis Using Fresnel Zones

Abstract: GNSS (Global Navigation Satellite Systems) multipath has been subject to scientific research for decades and although numerous methods and techniques have already been developed to mitigate this effect, it is still one of the accuracy-limiting factors in many GNSS applications. Since multipath is highly dependent on the individual antenna environment, there is still a need for new methods and further investigations to increase the understanding of this systematic effect. In this paper, the concept of Fresnel zones is applied to two different aspects of multipath. First, Fresnel zones are determined for the line-of-sight transmission between satellite and receiver. By comparing the boundary of the Fresnel zones to an obstruction adaptive elevation mask, potentially diffracted signals can be identified and excluded from the position estimation process. Both the percentage of epochs with fixed ambiguities and the positioning accuracy can be increased by the proposed method. Second, Fresnel zones are used to analyze the multipath induced by a horizontal and spatially-limited reflector. The comparison of simulated and real signal-to-noise (SNR) observations reveals a relationship between the percentage of the overlap of the Fresnel zone and reflector and the occurrence of multipath. It is found that an overlap of 50% is sufficient to induce multipath effects. This is of special interest, since this does not confirm theoretical assumptions of the multipath theory.

3D Nanofabrication of High-Resolution Multilayer Fresnel Zone Plates

Abstract: Focusing X-rays to single nanometer dimensions is impeded by the lack of high-quality, high-resolution optics. Challenges in fabricating high aspect ratio 3D nanostructures limit the quality and the resolution. Multilayer zone plates target this challenge by offering virtually unlimited and freely selectable aspect ratios. Here, a full-ceramic zone plate is fabricated via atomic layer deposition of multilayers over optical quality glass fibers and subsequent focused ion beam slicing. The quality of the multilayers is confirmed up to an aspect ratio of 500 with zones as thin as 25 nm. Focusing performance of the fabricated zone plate is tested toward the high-energy limit of a soft X-ray scanning transmission microscope, achieving a 15 nm half-pitch cut-off resolution. Sources of adverse influences are identified, and effective routes for improving the zone plate performance are elaborated, paving a clear path toward using multilayer zone plates in high-energy X-ray microscopy. Finally, a new fabrication concept is introduced for making zone plates with precisely tilted zones, targeting even higher resolutions.

Optimization-free approach for generating sub-diffraction quasi-non-diffracting beams.

Abstract: Sub-diffraction quasi-non-diffracting beams with sub-wavelength transverse size are attractive for applications such as optical nano-manipulation, optical nano-fabrication, optical high-density storage, and optical super-resolution microscopy. In this paper, we proposed an optimization-free design approach and demonstrated the possibility of generating sub-diffraction quasi-non-diffracting beams with sub-wavelength size for different polarizations by a binary-phase Fresnel planar lens. More importantly, the optimization-free method significantly simplifies the design procedure and the generation of sub-diffracting quasi-non-diffracting beams. Utilizing the concept of normalized angular spectrum compression, for wavelength λ0 = 632.8 nm, a binary-phase Fresnel planar lens was designed and fabricated. The experimental results show that the sub-diffraction transverse size and the non-diffracting propagation distances are 0.40λ0–0.54λ0 and 90λ0, 0.43λ0–0.54λ0 and 73λ0, and 0.34λ0–0.41λ0 and 80λ0 for the generated quasi-non-diffracting beams with circular, longitudinal, and azimuthal polarizations, respectively.

Analysis of Frequency-Dependent Line-of-Sight Probability in 3-D Environment

Abstract: Since mmWave signals are sensitive to blockages, prior channel models including the LoS probability function for ultra-high frequency band cannot be applied directly to analyze mmWave cellular networks. Compared with the empirical LoS probability model obtained from the computationally intensive simulations and measurements, the theoretical LoS probability model in mmWave cellular networks still remains unclear. According to the Huygens-Fresnel principle for the wave propagation, the LoS probability is closely related to the carrier frequency. In this letter, the 3-D frequency-dependent LoS probability model incorporated with the Fresnel zone is presented, and shows good agreement with the empirical models adopted in the 3GPP standard for various scenarios.

A Fresnel Diffraction Model Based Human Respiration Detection System Using COTS Wi-Fi Devices

Abstract: Recent work has revealed the sensing theory of human respiration outside the First Fresnel Zone (FFZ) using commodity Wi-Fi devices. However, there is still no theoretical model to guide human respiration detection when the subject locates in the FFZ. In our work [10], we propose a diffraction-based sensing model to investigate how to effectively sense human respiration in FFZ. We present this demo system to show human respiration sensing performance varies based on different human locations and postures. By deploying the respiration detection system using COTS Wi-Fi devices, we can observe that the respiration sensing results match the theoretical model well.

Extreme-ultraviolet refractive optics

Abstract: Refraction is a well-known optical phenomenon that alters the direction of light waves propagating through matter. Microscopes, lenses and prisms based on refraction are indispensable tools for controlling light beams at visible, infrared, ultraviolet and X-ray wavelengths1. In the past few decades, a range of extreme-ultraviolet and soft-X-ray sources has been developed in laboratory environments2–4 and at large-scale facilities5,6. But the strong absorption of extreme-ultraviolet radiation in matter hinders the development of refractive lenses and prisms in this spectral region, for which reflective mirrors and diffractive Fresnel zone plates7 are instead used for focusing. Here we demonstrate control over the refraction of extreme-ultraviolet radiation by using a gas jet with a density gradient across the profile of the extreme-ultraviolet beam. We produce a gas-phase prism that leads to a frequency-dependent deflection of the beam. The strong deflection near to atomic resonances is further used to develop a deformable refractive lens for extreme-ultraviolet radiation, with low absorption and a focal length that can be tuned by varying the gas pressure. Our results open up a route towards the transfer of refraction-based techniques, which are well established in other spectral regions, to the extreme-ultraviolet domain. A refractive lens and a refractive prism for extreme-ultraviolet radiation have been developed that use the deflection of the radiation in an inhomogeneous jet of atoms.

Multiple-plane image formation by Walsh zone plates.

Abstract: A radial Walsh filter is a phase binary diffractive optical element characterized by a set of concentric rings that take the phase values 0 or π, corresponding to the values + 1 or −1 of a given radial Walsh function. Therefore, a Walsh filter can be re-interpreted as an aperiodic multifocal zone plate, capable to produce images of multiple planes simultaneously in a single output plane of an image forming system. In this paper, we experimentally demonstrate for the first time the focusing capabilities of these structures. Additionally, we report the first achievement of images of multiple-plane objects in a single image plane with these aperiodic diffractive lenses.

Focusing Microwaves in the Fresnel Zone With a Cavity-Backed Holographic Metasurface

Abstract: We present the design and experimental demonstration of a cavity-backed, holographic metasurface capable of focusing microwaves in the Fresnel zone. The circular cavity consists of two stacked plates: microwaves are injected into the bottom plate via a coaxial connector, which forms the feed layer, and are coupled to the top holographic metasurface layer via an annular ring on the periphery of the cavity. This coupling results in an inward traveling cylindrical wave in the top layer, which serves as the reference wave for the hologram. A sparse array of slots, patterned into the upper plate, constitutes the hologram that produces the focal spot. To mitigate high sidelobe levels and improve performance, a tapered design, facilitated by varying the slot size, is also introduced. The proposed designs—which have a 10-cm diameter, operate at 20 GHz, and form a focal spot at a distance of 10 cm—are validated using full-wave simulations as well as measurements of fabricated samples. The proposed focusing metasurface may find application as a compact source for Fresnel zone wireless power transfer and remote sensing schemes.

Numerical and experimental study on multi-focal metallic Fresnel zone plates designed by the phase selection rule via virtual point sources

Abstract: We propose a novel design method for multi-focal metallic Fresnel zone plates (MFZPs), which exploits the phase selection rule by putting virtual point sources (VPSs) at the desired focal points distant to the MFZP plane. The phase distribution at the MFZP plane reciprocally formed by the VPSs was quantized in a binary manner based on the phase selection rule, thereby leading to a corresponding on-off amplitude pattern for the targeted MFZP. The resultant phase distribution was dependent on the complex amplitudes of the VPSs, so that they could be determined from the perspective of both multi-focal functionality and fabrication feasibility. As a typical example, we utilized the particle swarm optimization algorithm to determine them. Based on the proposed method, we designed and numerically analyzed two types of novel MFZPs—one for a monochromatic multi-focal application and the other for a multi-chromatic mono-focal application—verifying the effectiveness and validity of the proposed method. We also fabricated them onto Au-deposited glass substrates, using electron beam evaporation and a focused ion beam milling process. We experimentally characterized them and also verified that they successfully demonstrated their feasibilities. The former produced distinct hot spots at three different focal distances of 10, 15, and 20 μ m for monochromatic incidence at 650 nm, and the latter produced a single hot spot at a focal distance of 15 μ m for multi-chromatic incidence at 660, 532, and 473 nm. The experimental results were also in good agreement with their corresponding numerical results. We expect that both MFZPs will have various applications, such as laser micromachining, optical trapping, biomedical sensing, confocal collimation, achromatic optics, etc.

Turbulent Lidar: I−Design

Abstract: Two designs of a laser radar system based on the backscatter amplification effect (BSA) are suggested. The system is a micropulse aerosol lidar with two receiving channels, one of which records an increase in the lidar returns at the laser beam axis as the atmospheric turbulence intensifies. The second channel does not respond to the BSA and is required for calibration. The BSA effect manifests itself in a narrow spatial region around the laser beam axis; so, the receiver aperture should be small enough and comparable with the Fresnel zone. The creation of the turbulent lidar became possible with the advent of compact diode-pumped micropulsed lasers with high pulse repetition rates. The lidar is intended for continuous long-term unattended operation. It is eye-safe. Two designs of the turbulent lidar based on an afocal Mersenne telescope (mirror collimator) are suggested. BSA-2 and BSA-3 turbulent lidars are described. An algorithm is suggested for retrieval of the structure parameter of optical turbulence C 2 from lidar data based on the Vorob’ev’s approximation for a statistically homogeneous turbulent medium.

Petal-like zone plate: long depth bifocal diffractive lens and star-like beam generator.

Abstract: This study introduces and examines the diffraction properties of a so-called petal-like zone plate, which comprises Fresnel zones analogous to petals. We show that the focusing behavior of this novel type of zone plate depends on the number of petals included in the element. For a small value of the petal frequency, we observe star-like diffraction patterns at the focal plane, whose number of star arms equals the petal frequency of the element when the frequency is an odd integer and is twice as large as the petal frequency when it is an even number. In addition, we have shown that the star-like pattern rotates when it passes through the focus. Moreover, it is demonstrated that the element acts as a long depth bifocal diffractive lens for a large value of the petal frequency. The spacing between the foci is simply controlled by a so-called shifting parameter. At the same time, an annular beam is observed in the middle of the line joining the two foci together. Consequently, an axial bottle-like beam is produced around the focus, whose size could be simply monitored. Simulation results are followed and verified by experimental works.

More are better, but the details matter: combinations of multiple Fresnel zone plates for improved resolution and efficiency in X-ray microscopy

Abstract: Fresnel zone plates used for X-ray nanofocusing face high-aspect-ratio nanofabrication challenges in combining narrow transverse features (for high spatial resolution) along with extended optical modulation along the X-ray beam direction (to improve efficiency). The stacking of multiple Fresnel zone plates along the beam direction has already been shown to offer improved characteristics of resolution and efficiency when compared with thin single zone plates. Using multislice wave propagation simulation methods, here a number of new schemes for the stacking of multiple Fresnel zone plates are considered. These include consideration of optimal thickness and spacing in the axial direction, and methods to capture a fraction of the light otherwise diffracted into unwanted orders, and instead bring it into the desired first-order focus. The alignment tolerances for stacking multiple Fresnel zone plates are also considered.

Modified photon sieve as a high-performance bifocal and trifocal diffractive optical element.

Abstract: We demonstrate that by dividing a Fresnel zone plate into a few regions having different periods in the s=r2 coordinate, then replacing the clear zones by a given distribution of pinholes, a so-called modified photon sieve is constructed. The first feature of the element is to increase its diameter without worrying about its feature size as the limiting factor in the fabrication zone plate. Moreover, it is shown that the number of the zones included in each region is an important parameter that has a great impact on handling the number of foci. So, by choosing a suitable relation between the number of the Fresnel zones of the regions, one gets a high-efficiency unifocal or bifocal or even multifocal element depending on the number of the regions and zones. This technique is detailed by making unifocal, bifocal, as well as trifocal modified photon sieves and surveying their focusing properties. Simulation studies are followed by the corresponding experiments to verify them.

Analytical Fresnel imaging models for photon sieves.

Abstract: Photon sieves are a fairly new class of diffractive lenses that open unprecedented possibilities for high resolution imaging and spectroscopy, especially at short wavelengths such as UV and x-rays. In this paper, we model and analyze the image formation process of photon sieves using Fourier optics. We derive closed-form Fresnel imaging models that relate an input object to the image formed by a photon sieve system, both for coherent and incoherent illumination. These analytical models also provide a closed-form expression for the point-spread function of the system for both in-focus and out-of-focus cases. All the formulas are expressed in terms of Fourier transforms and convolutions, which enable easy interpretation as well as fast computation. The derived analytical models provide a unified framework to effectively develop new imaging modalities enabled by diffractive lenses and analyze their imaging capabilities for different design configurations, prior to physical production. To illustrate their utility and versatility, the derived formulas are applied to several important special cases such as photon sieves with circular holes and pixelated diffractive lenses generated by SLM-type devices. The analytical image formation models presented in this paper provide a generalizable and powerful means for effective analysis and simulation of any imaging system with a diffractive lens, including Fresnel zone plates, Fresnel phase plates, and other modified Fresnel lenses and mask-like patterns such as coded apertures.

Phenomenological Description of the Electromagnetic Field and Coherent Images in Radio Engineering and Optical Systems

Abstract: The basic principles of the scalar diffraction theory and the fundamental theorems of electromagnetic field calculation at any point in the volume from the field stated on the surface are analyzed It follows from the analysis that the concretization of spatially-distributed boundary conditions for real earth surfaces considerably complicates the solution of diffraction problems It is proposed to use the phenomenological description of electromagnetic fields in the domain of their registration Examples of restoration of coherent images from the received fields in the Fresnel zone for remote sensing problems are give The complex structure of the obtained coherent images requires their consideration not only in the form of Riemann integrals, but also as the Lebesgue, Stieltjes integrals, and the stochastic Ito integral

High-throughput synthesis of modified Fresnel zone plate arrays via ion beam lithography

Abstract: Fresnel zone plates (FZP) are diffractive photonic devices used for high-resolution imaging and lithography at short wavelengths. Their fabrication requires nano-machining capabilities with exceptional precision and strict tolerances such as those enabled by modern lithography methods. In particular, ion beam lithography (IBL) is a noteworthy method thanks to its robust direct writing/milling capability. IBL allows for rapid prototyping of high-resolution FZPs that can be used for high-resolution imaging at soft X-ray energies. Here, we discuss improvements in the process enabling us to write zones down to 15 nm in width, achieving an effective outermost zone width of 30 nm. With a 35% reduction in process time and an increase in resolution by 26% compared to our previous results, we were able to resolve 21 nm features of a test sample using the FZP. The new process conditions are then applied for fabrication of large arrays of high-resolution zone plates. Results show that relatively large areas can be decorated with nanostructured devices via IBL by using multipurpose SEM/FIB instruments with potential applications in FEL focusing, extreme UV and soft X-ray lithography and as wavefront sensing devices for beam diagnostics.

Multispectral metasurface hologram at millimeter wavelengths.

Abstract: We demonstrate a computer-generated metasurface hologram in which four distinct images are encoded at four different W-band (75–110 GHz) frequencies. The metasurface hologram consists of a planar array of resonant metamaterial elements excited by a collimated reference beam incident on the hologram at an oblique angle. Each of the images is encoded by a subset of metamaterial elements that are resonant at the specific excitation frequency and are spatially positioned to achieve a desired phase distribution in the plane in conjunction with the reference wave. The phase-only hologram is optimized using the Gerschberg–Saxton algorithm. The four well-defined images are produced at specific distances within the Fresnel zone of the aperture.

MRI Compatible Planar Material Acoustic Lenses

Abstract: Zone plate lenses are used in many areas of physics where planar geometry is advantageous in comparison with conventional curved lenses. There are several types of zone plate lenses, such as the well-known Fresnel zone plates (FZPs) or the more recent fractal and Fibonacci zone plates. The selection of the lens material plays a very important role in beam modulation control. This work presents a comparison between FZPs made from different materials in the ultrasonic range in order to use them as magnetic resonance imaging (MRI) compatible materials. Three different MRI compatible polymers are considered: Acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA) and polylactic acid (PLA). Numerical simulations based on finite elements method (FEM) and experimental results are shown. The focusing capabilities of brass lenses and polymer zone plate lenses are compared.

Spatial–Spectral Correspondence Relationship for Mono–Polychromatic Light Diffraction

Abstract: In this chapter, a spatial–spectral correspondence relationship is introduced, which can be applied to monochromatic light diffraction or propagation. The conditions that make this relationship valid are discussed and its mathematical forms in the near and far field are given. Many interesting results based on this approach are presented, including the Fresnel zone spectra and Talbot spectra. Other effects or applications such as the spectral shift amplification and lattice spectroscopy are also analyzed. Another related phenomenon called the spectral switch is also discussed. This is the discontinuous jump for the diffracted spectral maximum shift when continuously varying the detected position or system parameters. Three different mechanisms generating the spectral switch and the way to use them for digital data transmission are introduced. It is believed that this correspondence relationship will play an influential role in transforming monochromatic results in the spatial domain into chromatic ones in the spectral domain.

On the use of phase correction rings on Fresnel zone plates with ultrasound piston emitters

Abstract: In this work, the distortion of the Fresnel Zone Plate focusing profile generated by a piston emitter in ultrasound applications is significantly reduced through the use of phase correction rings, which compensate the effect of the piston emitter radiation diagram. Both simulation and experimental results demonstrate the improvement achieved with this design method over the conventional case.

A Large Planar Holographic Refiectarray for Fresnel-Zone Microwave Wireless Power Transfer at 5.8 GHz

Abstract: We present a 5.8 GHz Fresnel-zone microwave wireless power transfer experiment leveraging a large planar holographic metasurface reflectarray to form a focused spot at a distance of 6.5 m. The 1.5 $\mathbf{m}^{2}$ holographic metasurface is fabricated from 15 panels having total dimensions of $\mathbf{122}\times \mathbf{127}$ cm, and is comprised of over 4,000 reflective patch resonators on the top surface of a 3.0 mm thick FR-4 substrate. A constrained hologram approach is used to discretize the desired hologram and approximate the desired focal spot given the Lorentzian coupled amplitude-phase response of the patch resonators. When the metasurface is illuminated by a 20 dB standard-gain horn 1.8 $\mathbf{m}$ away, it produces a spot with a 3 dB beam waist (FWHM) of approximately 50 cm. The experimentally measured beam profile matches the simulated beam profile to ± 1 dB within the beam, and we estimate almost 40% of the transmitted power was incident onto the receiver at the focus point.