Showing papers on "Zone plate published in 2019"

Acoustic Focusing Enhancement In Fresnel Zone Plate Lenses.

Abstract: The development of flat acoustic lenses for different applications such as biomedical engineering is a topic of great interest. Flat lenses like Fresnel Zone Plates (FZPs) are capable of focusing energy beams without the need of concave or convex geometries, which are more difficult to manufacture. One of the possible applications of these type of lenses is tumor ablation through High Intensity Focused Ultrasound (HIFU) therapies with real time Magnetic Resonance Imaging (MRI) monitoring. In order to be MRI compatible, the FZP material cannot have electromagnetic interaction. In this work, a Phase-Reversal FZP (PR-FZP) made of Polylactic Acid (PLA) manufactured with a commercial 3D printer is proposed as a better, more efficient and MRI compatible alternative to conventional Soret FZPs. Phase-Reversal lenses, unlike traditional FZPs, take advantage of all the incident energy by adding phase compensation regions instead of pressure blocking regions. The manufactured PR-FZP achieves 21.9 dB of focal gain, which increases the gain compared to a Soret FZP of its same size by a factor of 4.0 dB. Both numerical and experimental results are presented, demonstrating the improved focusing capabilities of these types of lenses.

Nonlinear Metasurface Fresnel Zone Plates for Terahertz Generation and Manipulation.

Abstract: We introduce a nanoengineered nonlinear metasurface based optical element that acts as an emitting Fresnel zone plate of terahertz (THz) waves. We show that the nonlinear zone plate generates broad...

Design and Fabrication of a Custom-Dielectric Fresnel Multi-Zone Plate Lens Antenna Using Additive Manufacturing Techniques

Abstract: Dielectric flat plate lens antennas fabricated using 3D printing offer several attractive designs features including volumetric efficient geometries, beam-steering capabilities, and relative ease of fabrication. This paper seeks to synthesize and optimize a lens design for microwave applications utilizing additive manufacturing techniques to implement custom-tailored dielectric materials that overcome the inadequacies of commercially available dielectric resources. Starting with a multi-dielectric Fresnel zone plate lens geometry previously reported in this paper, techniques are implemented to achieve customized dielectric properties. A study of 3D printing materials is conducted to identify commercially available supplies that offer appropriate dielectric properties. Measurement techniques are reviewed to verify the accuracy of the material characteristics (both dielectric constant and loss tangent). Finally, three inhomogeneous concentric-ring lens designs are manufactured using the proposed methods and evaluated to assess the overall performance of the antenna gain patterns and beam-steering capabilities. The result is a low-profile antenna configuration, offering simple, and low-cost fabrication, which can be integrated into compact microwave wireless system applications.

Metasurface zone plate for light manipulation in vectorial regime

Abstract: Fresnel zone plates consisting of multiple concentric rings have been realized by tailoring amplitude, phase and polarization of light, but conventional Fresnel zone plates require totally different materials to control each property reducing light controllability. Here, a metasurface zone plate in which rings are composed of subwavelength antenna arrays is proposed to individually control amplitude, phase and polarization by the consistent material platform of metasurfaces. Hence, versatile degrees of freedom can be achieved to focus electromagnetic waves. We verify that dielectric metasurfaces can generate arbitrary poloarization states to cover the whole Poincare sphere in the visible regime, allowing light manipulation in the vectorial regime, which is not feasible by a conventional single liquid crystal film. Experimental demonstration of a polarization-modulated metasurface zone plate confirms the functional capability of dielectric metasurfaces exhibiting higher focusing efficiency than amplitude-modulated zone plates. Fresnel zone plates are capable of manipulating the amplitude, phase and polarization of light, but conventional designs require different materials for each functionality. Here, metasurface zone plates consisting of subwavelength antenna arrays, capable of efficient, multifunctional control from a single material, are presented.

Bessel terahertz imaging with enhanced contrast realized by silicon multi-phase diffractive optics.

Abstract: Bessel terahertz (THz) imaging employing a pair of thin silicon multi-phase diffractive optical elements is demonstrated in continuous wave mode at 0.6 THz. A proposed Bessel zone plate (BZP) design - discrete axicon containing 4 phase quantization levels - based on high-resistivity silicon and produced by laser ablation technology allowed to extend the focal depth up to 20 mm with minimal optical losses and refuse employment of bulky parabolic mirrors in the imaging setup. Compact THz imaging system in transmission geometry reveals a possibility to inspect objects of more than 10 mm thickness with enhanced contrast and increased resolution up to 0.6 of the wavelength by applying deconvolution algorithms.

Multifocal multi-value phase zone plate for 3D focusing

Abstract: We demonstrate a method for creating a three-dimensional (3D) array of focal spots by combination of a multi-focal diffractive lens and a two-dimensional multi-value phase grating. The multi-focal Fresnel-based lens is created by means of encoding special nonlinearities into the phase structure of a Fresnel zone plate and is represented as a mathematical superposition of this phase function with a refractive lens. The imposed nonlinearity type enables the creation of multiple focal spots with uniform intensity along the optical axis. We demonstrate the example of a 3D multi-value phase grating, which creates five focal planes with a $5 \times 5$5×5 transverse array of focal spots with equal energy distribution in each plane. Experimental results are included to verify the theoretical outcomes, where the phase pattern of a 3D multi-value phase grating is encoded onto a spatial light modulator.

Measuring the topological charge of an x-ray vortex using a triangular aperture

Abstract: In this paper, for the first time, we demonstrate that the topological charge of a hard x-ray vortex can be measured by a single diffraction pattern from a triangular aperture. The x-ray vortex was produced by a spiral Fresnel zone plate (SFZP) with topological charges of ±1. The triangular aperture was placed in the proximity of the focal plane of the SFZP. Explicit diffraction patterns, which agreed well with the calculated results, were observed and they were rotated by 180° by inverting the sign of the topological charge.

Theory of diffraction of vortex beams from structured apertures and generation of elegant elliptical vortex Hermite–Gaussian beams

Abstract: In this work, a comprehensive analytic study of the diffraction of vortex beams from structured apertures is presented. We formulate the near- and far-field diffraction of a vortex beam from an aperture having an arbitrary functionality in the Cartesian coordinates by two general and different approaches. We show that each of the resulting diffraction patterns can be determined by a number of successive derivatives of the 2D Fourier transform of the corresponding hypothetical aperture function or equally can be obtained by a summation of 2D Fourier transforms of the corresponding modified aperture function. We implement both introduced analytic approaches in predicting the diffraction of a vortex beam from an elliptic Gaussian aperture, an elliptic Gaussian phase mask, and a hyperbolic Gaussian phase mask in the near- and far-field regimes. It is shown that the predicted diffraction patterns by both these approaches are exactly the same. It is shown that the diffraction of a vortex beam from an elliptic Gaussian aperture at the far-field regime forms a light beam that belongs to a family of light beams we call elegant elliptical vortex Hermite–Gaussian beams. In addition, the diffractions of a vortex beam from a Fresnel zone plate in general form for the on- and off-axis situations are formulated, and sinusoidal and binary zone plates are investigated in detail. Our general analytic formula can be used for a large variety of apertures including off-center situations and asymmetrical cases.

Laser-processed diffractive lenses for the frequency range of 4.7 THz.

Abstract: The development of diffractive lenses for the upper terahertz (THz) frequency range above 1 THz was successfully demonstrated by employing a direct laser ablation (DLA) technology. Two types of samples such as the Soret zone plate lens and the multi-level phase-correcting Fresnel lens were fabricated of a metal foil and crystalline silicon, respectively. The focusing performance along the optical axis of a 4.745 THz quantum cascade laser beam with respect to the positioning angle of the sample was studied by using a real-time microbolometric camera. A binary-phase profile sample demonstrated the values of the focusing gain and focused beam size up to 25 dB and 0.15 mm (2.4λ), respectively. The increase of the phase quantization level to eight led to higher (up to 29 dB) focusing gain values without a measurable increase of optical losses. All the samples were tolerant to misalignment as large as 10 deg of oblique incidence with a focusing power drop no larger than 10%. The results pave the way for new applications of industry-ready DLA technology in the entire THz range.

Fast positioning for X-ray scanning microscopy by a combined motion of sample and beam-defining optics

Abstract: Scanning X-ray microscopy such as X-ray ptychography requires accurate and fast positioning of samples in the X-ray beam. Sample stages often have a high mobile mass as they may carry additional mechanics or mirrors for position measurements. The high mobile mass of a piezo stage can introduce vibrations in the setup that will lead to imaging quality deterioration. Sample stages also require a large travel range which results in a slow positioning step response and thus high positioning overhead. Moving lightweight X-ray optics, such as focusing Fresnel zone plates, instead of the sample can improve the situation but it may lead to undesired variations in the illumination probe which may result in reconstruction artifacts. This paper presents a combined approach in which a slow sample stage mechanism covers the long distance range for a large field of view, and a light-weight optics scanner with a small travel range creates a superimposed motion to achieve a fast step response. The step response in the ptychographic tomography instrument used was thereby improved by an order of magnitude, allowing for efficient measurement without loss of imaging quality.

Embossing Reactive Mesogens: A Facile Approach to Polarization-Independent Liquid Crystal Devices

Abstract: Reactive mesogens (RMs) have found their way into different branches of science and application. They can be processed as standard liquid crystals and can then be polymerised to stabilise their shape and anisotropic properties. Their birefringence is used in optical compensation films while the complex director field is exploited for actuators. However, creating complex shapes while maintaining good alignment is a challenge in these applications. In the present work, the embossing of reactive mesogens is introduced. Embossing is a fast and large-scale method, which allows to form a large variety of structured reactive mesogen devices. To create a polarisation-independent lens, two birefringent Fresnel zone plates (FZPs) are embossed. A lens is formed by assembling the FZPs in a twisted nematic cell and filling the cell with a nematic crystal index matched to the RM. The device can be switched from a non-focusing to a focusing state by applying a small voltage. After characterising the efficiency and beam properties, the method is used to fabricate a switchable multi-level Fresnel zone plate with optical efficiencies beyond 50%. Finally, manufactured polarisation-independent gratings and microlens arrays are presented using the method to illustrate the wide range of applicability.

Shaping coherent x-rays with binary optics

Abstract: Diffractive lenses fabricated by lithographic methods are one of the most popular image forming optics in the x-ray regime. Most commonly, binary diffractive optics, such as Fresnel zone plates, are used due to their ability to focus at high resolution and to manipulate the x-ray wavefront. We report here a binary zone plate design strategy to form arbitrary illuminations for coherent multiplexing, structured illumination, and wavefront shaping experiments. Given a desired illumination, we adjust the duty cycle, harmonic order, and zone placement to vary both the amplitude and phase of the wavefront at the lens. This enables the binary lithographic pattern to generate arbitrary structured illumination optimized for a variety of applications such as holography, interferometry, ptychography, imaging, and others.

Comprehensive focusing analysis of bi-segment spiral zone plate in producing a variety of structured light beams

Abstract: This study introduces and demonstrates the diffractive feature of a bi-segment spiral zone plate, so that each segment has its own width, topological charge, and radial phase shift. We show that the focusing behavior of this element depends strongly on these parameters. We demonstrate how these features can handle the focused intensity and structure of an incident plane beam. A variety of beam shapes and structures are generated at the focal plane. Theoretical and simulation results are verified by the corresponding experiments.

Terahertz focusing properties of polymeric zone plates characterized by a modified knife-edge technique

Abstract: The focusing properties of a series of polymeric zone-plate lenses are investigated around a target frequency of 1 THz. Their characterization is performed by means of terahertz (THz) time-domain spectroscopy, employing a modified knife-edge technique that compensates for asymmetries of the impinging THz beam shape of typical photoconductive antenna-based THz sources. The samples are fabricated by a three-axis milling technique on slabs of an ultralow-loss cyclo-olefin polymer. Three different zone plates are studied, a conventional binary zone plate, a conventional four-level zone plate, and a recently introduced double-sided zone plate consisting of the stack of two phase-reversal binary zone plates, which is simpler to fabricate and less sensitive to mechanical damage than multilevel zone plates. Experimental results, coupled with finite element simulations, demonstrate that the double-sided zone plate features a resolution increased by about 3λ with respect to the binary zone plate and comparable with that of the four-level zone plate. The double-sided zone plate has 40% lower focusing efficiency and approximately 7λ shorter depth of field compared to its four-level counterpart. Nevertheless, it outperforms conventional binary zone plates by 25% in power focusing efficiency and features a 10λ longer depth of field.

Planar GHz Ultrasonic Lens for Fourier Ultrasonics

Abstract: This paper presents GHz ultrasonic wave propagation through materials with embedded microfabricated planar lenses. The combination of lens, incoming images and output images at the output plane is important to compute Fourier Transforms using the wave propagation properties of ultrasonic waves [1]. Two Fresnel zone plate lenses are investigated, one made of thin multiple height steps, and another consisting of spatially varying densities of pillars made of different materials. The design space of lenses as a function of lens size, focal length, and transmittance is calculated. The simulation code is used to verify the effectiveness of lens design to obtain Fourier transforms and investigate the potential of electronically controlling the phase of pixel designs.

Fractal spiral zone plate with high-order harmonics suppression.

Abstract: We extend the concept of fractal spiral zone plates and define a new family of Cantor sequence spiral zone plates (CSSZPs) by removing the interference of high-order harmonics. In this typical design, apart from combining the spiral zone plates and Cantor fractal structure together, the desired physical properties have been realized by using a two-parameter modified sinusoidal apodization window along the azimuthal direction to eliminate the high-order harmonics. Numerical simulation reveals that the intensity of high diffraction orders of the CSSZPs can be effectively suppressed by at least 3 orders of magnitude, while the shapes of the sequences of focused optical vortices surrounding the first primary focal length are maintained, similar to those of the fractal spiral zone plates. The demonstration experiment, based on a spatial light modulator, has been also carried out to confirm the desired characteristics. This new kind of diffractive elements may offer potential alternatives for 3D optical tweezers, optical imaging, and lithography.

Design of Acoustical Bessel-Like Beam Formation by a Pupil Masked Soret Zone Plate Lens

Abstract: This work has been supported by the Spanish MINECO (TEC2015-70939-R) and was partially supported by the Tomsk Polytechnic University Competitiveness Enhancement Program.

Method of fabricating an array of diffractive optical elements by using a direct laser lithography

Abstract: Recently, aspheric lenses have been developed to correct various aberrations and to produce optical elements with various functions. In particular, efforts have focused on fabrication miniaturized and compact elements that make the use of flat, diffractive optical elements such as zone plates to replace conventional lenses. Direct laser lithography is one of the typical methods used to manufacture such diffraction optical elements. By using a high magnification objective lens, it generates a laser beam with a spot size of less than 1 μm at the focal point. To produce a pattern, such as a diffraction grating, a chromium-coated surface is controllably exposed to this light. In this study, the process conditions for fabricating a zone plate using a direct laser lithography were studied by using an air-bearing type linear XY stage which can easily make various patterns including a zone plate array. The previous studies using a polar coordinate lithographic system have an advantage for fabricating the circular type zone plate because the system uses a rotational stage. It, however, is not proper to fabricate an array type diffractive optics element. In this study, we used a rectangular type lithographic system by using an XY stage and tested the fabrication issues such as a runout error. The optical performance of the zone plate array fabricated by the suggested method was also verified by experimental evaluation.

Multifocal binary diffraction lens with arbitrary focal lengths and number of foci

Abstract: In this paper, a method is proposed to design a binary multifocal Fresnel zone plate (MFZP) with desirable focal lengths and number of foci. By performing simulations, the focusing characteristics of MFZPs with two, three, and four focal lengths are investigated. The obtained results show that the aspect ratio of focal lengths and the number of foci of MFZPs can be chosen freely. Furthermore, designed MFZPs are displayed on a digital micromirror device, and the focusing properties of the reflected laser beam are investigated. Experimental results support the validity of the method.

Dual-type fractal spiral zone plate for generating sequence of square optical vortices.

Abstract: We propose a technique for generating a sequence of co-axial zero axial irradiance with a so-called dual-type fractal spiral zone plate (DTFSZP). Based on the Fresnel diffraction theory, we simulated the focusing performance of this optical device. The results reveal that DTFSZP has the remarkable ability of generating a sequence of optical vortices with larger depth of focus and high lateral resolution. The central diffracted image rotates in the vicinity of the focal plane. Moreover, the focusing performance follows a modulo-4 transmutation rule. Such optics promises a complementary and versatile high-resolution non-destructive tool for particle manipulation and provides potential application in three-dimensional optical alignment systems.

Three-wave radial shearing interferometer.

Abstract: We propose a simple and compact three-wave radial shearing interferometer (TWRSI). This interferometer is constructed by a Gabor zone plate (GZP) and a charge-coupled device (CCD). The nominal plane wavefront under test is diffracted by the GZP into three orders (i.e., a parallel beam, a converging spherical beam, and a diverging spherical beam) leading to an interference pattern. In the imaging plane, three beams can be considered as being the copies of the incident beam, the compressed beam, and the expanded beam, respectively. A CCD detector located in the vicinity of the GZP records this interference pattern. Wavefront can be reconstructed by a single interferogram. The experiment result shows that the proposed method is correct and accurate. This TWRSI is compact, vibration insensitive, suitable for measurement of continuous and transient wavefront.

Design and performance of a new setup for spatially resolved transmission X-ray photoelectron spectroscopy at the Swiss Light Source

Abstract: The successful design, installation and operation of a high spatial resolution X-ray photoelectron spectrometer at the Swiss Light Source is presented. In this instrument, a Fresnel zone plate is used to focus an X-ray beam onto the sample and an electron analyzer positioned at 45° with respect to the incoming beam direction is used to collect photoelectrons from the backside of the sample. By raster scanning the sample, transmitted current, X-ray absorption and X-ray photoemission maps can be simultaneously acquired. This work demonstrates that chemical information can be extracted with micrometre resolution; the results suggest that a spatial resolution better than 100 nm can be achieved with this approach in future. This kind of photoelectron spectromicroscope will allow in situ measurements with high spatial resolution also under ambient pressure conditions (in the millibar range). Element-specific X-ray photoemission maps can be obtained before and while exposing the sample to gas/gas mixtures to show morphological and chemical changes of the surface.

Application of Fresnel Zone Plate Focused Beam to Optimized Sensor Design for Pulse-Echo Harmonic Generation Measurements.

Abstract: In nonlinear acoustic measurements involving reflection from the stress-free boundary, the pulse-echo method could not be used because such a boundary is known to destructively change the second harmonic generation (SHG) process. The use of a focusing acoustic beam, however, can improve SHG after reflection from the specimen boundary, and nonlinear pulse-echo methods can be implemented as a practical means of measuring the acoustic nonlinear parameter (β) of solid specimens. This paper investigates the optimal sensor design for pulse-echo SHG and β measurements using Fresnel zone plate (FZP) focused beams. The conceptual design of a sensor configuration uses separate transmission and reception, where a broadband receiver is located at the center and a four-element FZP transmitter is positioned outside the receiver to create a focused beam at the specified position in a solid sample. Comprehensive simulations are performed for focused beam fields analysis and to determine the optimal sensor design using various combinations of focal length, receiver size and frequency. It is shown that the optimally designed sensors for 1 cm thick aluminum can produce the second harmonic amplitude and the uncorrected nonlinear parameter corresponding to the through-transmission method. The sensitivity of the optimal sensors to the changes in the designed sound velocity is analyzed and compared between the odd- and even-type FZPs.

Optical Phase Properties of Small Numbers of Nanoslits and an Application for Higher-efficiency Fresnel Zone Plates

Abstract: We have studied the behavior of light in the intermediate regime between a single nanoslit and an infinite nanoslit array. We first calculated the optical characteristics of a small number of nanoslits using finite element numerical analysis. The phase variance of the proposed nanoslit model shows a gradual phase shift between a single nanoslit and ideal nanoslit array, which stabilizes before the total array length becomes ~0.5 λ. Next, we designed a transmission-enhanced Fresnel zone plate by applying the phase characteristics from the small-number nanoslit model. The virtual-point-source method suggests that the proposed Fresnel zone plate with phase-invariant nanoslits achieves 2.34x higher transmission efficiency than a conventional Fresnel zone plate. Our report describes the intermediate behaviors of a nanoslit array, which could also benefit subwavelength metallic structure research of metasurfaces.

Formation and Characterization of Hole Nanopattern on Photoresist Layer by Scanning Near-Field Optical Microscope.

Abstract: Patterning of lines of holes on a layer of positive photoresist SX AR-P 3500/6 (Allresist GmbH, Strausberg, Germany) spin-coated on a quartz substrate is carried out by using scanning near-field optical lithography. A green 532 nm-wavelength laser, focused on a backside of a nanoprobe of 90 nm diameter, is used as a light source. As a result, after optimization of parameters like laser power, exposure time, or sleep time, it is confirmed that it is possible to obtain a uniform nanopattern structure in the photoresist layer. In addition, the lines of holes are characterized by a uniform depth (71–87 nm) and relatively high aspect ratio ranging from 0.22 to 0.26. Numerical modelling performed with a rigorous method shows that such a structure can be potentially used as a phase zone plate.

High-Resolution Binary Zone Plate in Double-Sided Configuration for Terahertz Radiation Focusing

Abstract: A double-sided phase-reversal binary zone plate is numerically investigated and experimentally characterized at the frequency of 1 THz.The double-sided zone plate has a novel design based on a binary phase-reversal zone plate, in which the total zone thickness is split into two halves located at a distance optimized by means of numerical simulations.The samples are fabricated by a three-axis milling technique on slabs of an ultralow-loss cyclo-olefin polymer.The lenses’ characterization at the frequency of 1 THz is performed by means of a terahertz time-domain spectrometer, in terms of both focal length and spatial resolution.The focusing properties of the double-sided zone plate are compared with those of a conventional binary zone plate.The double-sided zone plate features $2.4\times $ smaller focal spot, $1.25\times $ higher focusing efficiency, and an approximately $10\lambda _{\mathbf {0}}$ longer depth of field than the conventional binary configuration.

A general n-fractal aperiodic zone plate

Abstract: We propose a general n-fractal aperiodic zone plate generating a beam with the first-order foci with more chosen positions. The construction method of the general n-fractal aperiodic zone plate can apply to many aperiodic zone plates, e.g. fractal, Thue-Morse and Fibonacci zone plates, and the general n-fractal aperiodic zone plates can generate more adjustable first-order foci. Parameters of the n-fractal aperiodic zone plate such as the substitution rule and the fractal order are examined and found not to affect the fractal distribution of the first-order foci. The more adjustable first-order foci generated by the general n-fractal aperiodic zone plate have been verified with both simulations and experiments.