Showing papers on "Zone plate 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.

Ultrathin Planar Cavity Metasurfaces

Abstract: An ultrathin planar cavity metasurface is proposed based on ultrathin film interference and its practicability for light manipulation in visible region is experimentally demonstrated. Phase of reflected light is modulated by finely adjusting the thickness of amorphous silicon (a-Si) by a few nanometers on an aluminum (Al) substrate via nontrivial phase shifts at the interfaces and interference of multireflections generated from the planar cavity. A phase shift of π, the basic requirement for two-level phase metasurface systems, can be accomplished with an 8 nm thick difference. For proof of concept, gradient metasurfaces for beam deflection, Fresnel zone plate metalens for light focusing, and metaholograms for image reconstruction are presented, demonstrating polarization-independent and broadband characteristics. This novel mechanism for phase modulation with ultrathin planar cavity provides diverse routes to construct advanced flat optical devices with versatile applications.

Enhancing the power of high power microwaves by using zone plate and investigations for the position of virtual cathode inside the drift tube

Abstract: Having the device simplicity and high output-power capacity, the capability of the virtual cathode oscillator (vircator) for emitting high power microwaves is demonstrated. We have investigated the axial-typed vircator in our pulsed power generator, “Chundoong” (Max 600 kV, 88 kA, and 60 ns), and simulated the vircator using the 3-D particle-in-cell simulation code called “MAGIC.” We try to find out the position of the virtual cathode inside the drift tube and enhance the power of microwaves by focusing them at the focus point using the ring-typed zone plate with a focal length of 18.8 cm. The dominant frequency is obtained as 3.5 GHz measured by fast Fourier transform, which is in good agreement with simulation frequency. It is found that the mean position of the virtual cathode is almost the same as the A-K gap distance of 10 mm, in which the virtual cathode oscillates from 7.9 mm to 12.1 mm behind the meshed anode, as verified by the simulation results. The maximum output power without the zone plate is obtained as 0.66 GW with the efficiency of 27%, which is maximized up to 1.22 GW with the efficiency of 51% at the focus point by using the zone plate. The microwave emission mode from the vircator is the TM01 mode based on the simulation results. The zone plate contributes significantly in enhancing the power efficiency and determination of the position of the virtual cathode. These observations might be used in developing a high efficient microwave source by using a zone plate under the decision of the position of the virtual cathode.

Bifocal Fresnel Lens Based on the Polarization-Sensitive Metasurface

Abstract: Thin structured surfaces allow flexible control over the propagation of electromagnetic waves. Focusing and polarization state analysis are among functions, required for effective manipulation of radiation. Here, a polarization-sensitive Fresnel zone plate lens is proposed and experimentally demonstrated for gigahertz spectral range. Two spatially separated focal spots for orthogonal polarizations are obtained by designing metasurface pattern, made of overlapping tightly packed cross- and rod-shaped antennas with a strong polarization selectivity. The optimized subwavelength pattern allows multiplexing two different lenses with low polarization crosstalk on the same substrate and provides a control over focal spots of the lens only by changing the polarization state of the incident wave. More than a wavelength separation distance between the focal spots was demonstrated for a broad spectral range, covering half a decade in frequency. The proposed concept could be straightforwardly extended for terahertz and visible spectra, where polarization-sensitive elements utilize localized plasmon resonance phenomenon.

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.

Fibonacci terahertz imaging by silicon diffractive optics.

Abstract: Fibonacci or bifocal terahertz (THz) imaging is demonstrated experimentally employing a silicon diffractive zone plate in continuous wave mode. Images simultaneously recorded in two different planes are exhibited at 0.6 THz frequency with the spatial resolution of wavelength. Multifocus imaging operation of the Fibonacci lens is compared with a performance of the conventional silicon phase zone plate. Spatial profiles and focal depth features are discussed varying the frequency from 0.3 to 0.6 THz. Good agreement between experimental results and simulation data is revealed.

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.

Compact Folded Fresnel Zone Plate Lens Antenna for mm-Wave Communications

Abstract: A low-profile single-polarized folded Fresnel zone plate (FZP) lens antenna is presented for 5G millimeter-wave point-to-point communications in V-band. This antenna has two benefits: 1) the compactness of folded reflector structures made of a trans-reflector and a twist-reflector, and 2) the simplicity of FZP lenses. The idea of the folded structure is to triple the electrical path compared to nonfolded configurations. The combination of all these benefits leads to a very low-profile, high-directive, broadband, and low-cost antenna, as confirmed by measurements. A measured peak gain of 32.7 dBi is obtained at 60 GHz, with almost flat gain curve from 57 to 64 GHz, with a half-power beamwidth of 3°, compliant with ETSI V-band class 2 standard.

Dual-Band Fresnel Zone Plate Antenna With Independently Steerable Beams

Abstract: Fresnel zone plate (FZP) antennas can implement steerable beams based on reconfigurable binary-amplitude diffractive patterns. Because of this unique characteristic, FZP antennas have concise architectures compared with phased array systems. So far, FZP antennas can only support a single frequency band. This is because only one diffractive pattern can exist on the aperture at a given time. In this communication, we demonstrate a dual-band FZP antenna with two independent coexistent diffractive patterns. Implemented based on multiple pairs of electrically induced opacity and transparency, such diffractive patterns can be independently configured by binary direct current (dc) voltages. Simulation and experimental results validated the theoretical analysis. The proposed method can be extended to obtain triple-band FZP antennas, playing important roles in a wide range of potential applications.

Focusing of Terahertz Radiation With Laser-Ablated Antireflective Structures

Abstract: Numerical simulations and experimental characterization of laser-ablated focusing antireflective and phase-shifting structures for terahertz frequencies are presented. More than 10% shift of reflectance minimum to lower frequencies was predicted by simulations for relatively coarse structures with the period of 100 $\mu$ m in comparison with that of a substantially smaller period and with results of the model used for the design of antireflective surfaces in the terahertz range. Such a shift of the resonance frequency can be employed to optimize the thickness of antireflective layers simultaneously obtaining additional means of more precise control of layer properties due to ablation of larger structures. Nearly 90% transmittance of silicon wafers within 0.5–0.6 THz frequencies was confirmed experimentally. Optical path differences equivalent to a half period at 0.53 THz, suitable for applications in high-efficiency zone plates, were demonstrated with high transmittance simultaneously. Possibilities of delay adjustment up to one wavelength were illustrated by numerical simulations. A focusing binary zone plate for 0.6 THz was produced employing phase-shift differences of the dual-function antireflective layer. Its close to diffraction-limited focusing performance was evaluated, further confirming sufficient uniformity of the structured layer.

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.

X-ray ptychography using randomized zone plates.

Abstract: We have developed a randomized grating condenser zone plate (GCZP) that provides a µm-scale probe for use in x-ray ptychography. This delivers a significantly better x-ray throughput than probes defined by pinhole apertures, while providing a clearly-defined level of phase diversity to the illumination on the sample, and helping to reduce the dynamic range of the detected signal by spreading the zero-order light over an extended area of the detector. The first use of this novel x-ray optical element has been demonstrated successfully for both amplitude and phase contrast imaging using soft x-rays on the TwinMic beamline at the Elettra synchrotron.

Towards Millimeter-wavelength: Transmission-Mode Fresnel-Zone Plate Lens Antennas using Plastic Material Porosity Control in Homogeneous Medium.

Abstract: We present two transmission-mode dielectric Fresnel-Zone Plate Lens (FZPL) antennas for use within the V-band spectrum. The proposed FZPs are realized via pure plastic material using two different additive manufacturing processes. The proposed FZP lenses are designed with half (λ/2) and quarter (λ/4) phase correction rings at 60-GHz with 30λ0 diameter, where λ0 is the free-space wavelength. The permittivity effect for lens sub-zones is controlled by material porosity in cube-shaped structures. The 3D printed zone plate lenses are built using additive manufacturing plastic materials with a thickness of λ0 and constant relative permittivities equal to 2.76 and 3.6. Different types of antenna with cos n -like radiation patterns as lens illuminators are analyzed on the vertical plane of the flat lenses to have a high efficiency over the considered operating band. Simulations and experimental measurements show a reasonably close match, therefore allowing for a reliable predictability.

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.

Multifocal diffractive lens generating several fixed foci at different design wavelengths.

Abstract: We propose a method for designing multifocal diffractive lenses generating prescribed sets of foci with fixed positions at several different wavelengths. The method is based on minimizing the difference between the complex amplitudes of the beams generated by the lens microrelief at the design wavelengths, and the functions of the complex transmission of multifocal lenses calculated for these wavelengths. As an example, a zone plate generating three fixed foci at three different wavelengths was designed, fabricated, and experimentally investigated. The proof-of-concept experimental results confirm the formation of foci with fixed positions at the design wavelengths. The obtained results may find applications in the design and fabrication of novel multifocal contact and intraocular lenses with reduced chromatic effects.

High-efficiency arrays of any desired optical beams using modified grating-based elements

Abstract: High efficiency grating-based diffractive elements were previously proposed in order to generate desired arrays of various optical beams. The elements enable us to generate a diversity of arrays of optical beams in desired shapes, so that the arrays produced by the method are homogeneous, uniform and lacking of unwanted higher diffraction orders. In this technique a modulated grating is used to restructure Fresnel zone plate or zone plate-based elements to get modified grating-based elements. To examine the method, a variety of rectangular arrays of optical spots and annular beams are created by the ordinary and modified elements. It is shown that the arrays generated by the modified elements are much more uniform than the same arrays produced by the ordinary ones. In addition to the uniformity of the arrays, the unwanted higher diffraction orders are also totally eliminated. This article exploits simulation and experimental studies in order to demonstrate the functionality of the proposed technique as a multi functional high-efficiency beam shaping.

A scanning transmission X-ray microscope at the Pohang Light Source.

Abstract: A scanning transmission X-ray microscope is operational at the 10A beamline at the Pohang Light Source. The 10A beamline provides soft X-rays in the photon energy range 100–2000 eV using an elliptically polarized undulator. The practically usable photon energy range of the scanning transmission X-ray microscopy (STXM) setup is from ∼150 to ∼1600 eV. With a zone plate of 25 nm outermost zone width, the diffraction-limited space resolution, ∼30 nm, is achieved in the photon energy range up to ∼850 eV. In transmission mode for thin samples, STXM provides the element, chemical state and magnetic moment specific distributions, based on absorption spectroscopy. A soft X-ray fluorescence measurement setup has been implemented in order to provide the elemental distribution of thicker samples as well as chemical state information with a space resolution of ∼50 nm. A ptychography setup has been implemented in order to improve the space resolution down to 10 nm. Hardware setups and application activities of the STXM are presented.

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.

A Millimeter-Wave Fresnel Zone Plate Lens Design Using Perforated 3D Printing Material

Abstract: This paper presents a novel Fresnel Zone Plate Lens (FZPL) using perforated 3D printing material at 60 GHz$\lambda_\mathrm {0} = \mathrm {5.45 mm}$. A gradient refractive index condition is controlled using the air-hole approach in the dielectric slab. This structure employs on conical horn antenna as a beam launcher for focusing applications. The proposed FZPL lens is a flat type lens with half phase correction properties. A recommended prototype planar lens is performed on the 3mm thickness of a plastic material with a constant relative permittivity. Throughout this paper, the lens antenna design process at millimeter wave frequency band has been demonstrated for V-band applications.

Azimuthal-segmented linear zone plate: 1D beam structurer and topological charge detector

Abstract: Unique linear structured light beams carrying fractional vortex dipoles in the shape of either a transverse elliptical beam or a transverse bottle-like beam are presented and studied. These beams are generated using a novel diffractive element produced by a given combination of two phase-shifted spiral linear zone plates with equal but opposite helicities. The impact of the topological charge upon the beams is surveyed and demonstrated to be that the size of the beams is quietly controlled by the charge. As an interesting application, we demonstrate that the element may be simply used to measure the topological charge of a vortex optical beam. On the other hand, when the element is illuminated by a vortex beam, the focused pattern is modulated so the number and direction of the modulated fringes easily reveal the sign and modulus of the topological charge. The experimental results verify that the simulation works well.

Terahertz Switching Focuser Based on Thin Film Vanadium Dioxide Zone Plate

Abstract: In this paper, we propose a switchable focuser device based on a Fresnel zone plate (FZP) structure for terahertz (THz) applications. Each FZP contains seven rings, etched in thin VO2 film with the designed focal lengths of 50 and 100 mm for 3.7-THz frequency. Temperature-induced VO2 phase transition leads to the change in dielectric susceptibility of the material, which allows one to switch on and off the focusing properties of the device. The devices were tested with radiation of 3.1 and 3.7 THz emitted by quantum cascade lasers. Experimental results were compared with numerical simulations. In this article, we compare the FZP based on VO2 films with different properties and show that a thicker VO2 film reveals higher focusing efficiency, while a thinner one reveals a higher modulation ratio for the peak intensity at the focal point of FZP. We demonstrate experimentally the near-diffraction-limited size of the beam in the focal point of the device. Switching between two phase states of the VO2 films results in up to the 38-fold change of intensity in the focal point.

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.

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.

Fresnel zone plate fabricated using a polyvinyl chloride gel

Abstract: Based on creep-induced deformation of a polyvinyl chloride (PVC) gel in an electric field, a Fresnel zone plate (FZP) is fabricated. The employed electrode is constructed with a series of concentric zones, and the soft PVC gel coated on the electrode forms a flat membrane. By applying a DC voltage to the electrode, electrons are injected to the PVC gel. Due to the generated electrostatic force, the surface of the PVC membrane is undulated. The configuration of the PVC membrane resembles the pattern of the electrode. As a result, the PVC membrane functions as an FZP. The results show that incident light passing through the FZP can be diffracted, producing a focused spot due to constructive interferences. The maximum diffraction efficiency can reach ∼32.5 % , and the focal length is switchable. The advantages of this FZP are the ease of manufacturability, compactness, large size, and optical isotropy.

X-ray spectroscopy with variable line spacing based on reflection zone plate optics

Abstract: X-ray spectroscopy is a method, ideally suited for investigating the electronic structure of matter, which has been enabled by the rapid developments in light sources and instruments The x-ray fluorescence lines of life-relevant elements such as carbon, nitrogen, and oxygen are located in the soft x-ray regime and call for suitable spectrometer devices In this Letter, we present a high-resolution spectrum of liquid water, recorded with a soft x-ray spectrometer based on a reflection zone plate (RZP) design The RZP-based spectrometer with meridional variation of line space density from 2953 to 3757 l/mm offers extremely high detection efficiency and, at the same time, medium energy resolution We can reproduce the well-known splitting of liquid water in the lone pair regime with 10 s acquisition time

Multiple beam steering using dynamic zone plates on a micromirror array

Abstract: Three-dimensional (3-D) laser beam steering of focal position has been demonstrated using a single optical device—a DMD micromirror array. Laser beam focus position is controlled using dynamically adjustable zone plates. These zone plates take the form of elliptical Fresnel zone plates or other variations such as binary Gabor zone plates. Active beam pointing and control can be realized without the need for a pair of galvanometer mirrors and a focusing lens. Focusing efficiencies into an off-axis diffraction order of a few percent are typically seen and continuity between neighboring orders increases the effective field of regard. Writing multiple zone plate patterns to the DMD enables multiple focused spots to be generated and controlled independently.