Showing papers on "Zone plate published in 2021"

Additively Manufactured Millimeter-Wave Dual-Band Single-Polarization Shared Aperture Fresnel Zone Plate Metalens Antenna

Abstract: Fresnel zone plate (FZP) lens antenna, consisting of a set of alternative transparent and opaque concentric rings arranged on curvilinear or flat surfaces, have been widely used in various fields for sensing and communications. Nevertheless, the state-of-art FZP lens antennas are limited to a single band due to the frequency-dependent feature, which hinders their use in multi-band applications. In this work, a shared aperture dual-band FZP metalens antenna is proposed by merging two single-band FZP metalens antenna operating at distinct frequency bands seamlessly into one. Instead of using conventional metallic conductors, double-screen metagrids are devised in this work to form the concentric rings. Because the metagrids show distinct transmission/reflection properties at different frequencies, the performance of one set of concentric rings operating at the one band will not be affected by the other operating at the different band. In addition, to compensate for the phase shift introduced by the metagrids, an additional dielectric ring layer is added atop the FZP taking advantage of additive manufacturing. Thus, the radiation performance of the dual-band FZP lens antenna is comparable to that of each single FZP metalens antenna. For proof-of-concept, an antenna prototype operating at the dual band, 75 and 120 GHz with a frequency ratio of 1.6, is fabricated using an integrated additively manufactured electronics (AME) technique. The measured peak gains of 20.3 and 21.9 dBi are achieved at 75 and 120 GHz, respectively.

Dielectric metasurface zone plate for the generation of focusing vortex beams

Abstract: Vortex beams carrying orbital angular momentum have important applications in high dimensional optical information processing, manipulations of tiny particles, super-resolution imaging and so on. Among various optical components, metasurface represents an ideal platform for realizing vortex beams with multiple optical functionalities due to its strong ability in manipulating the phase, polarization and amplitude of light. A metasurface combing the functions of a lens and a vortex beam generator can greatly shrink the size of many optical systems. Here, we alternatively propose a new metasurface design based on the concept of a Fresnel zone plate to generate, focus the vortex beams, and perform on-axis interference between different vortex beams. These functions are experimentally demonstrated through encoding the spiral phase profiles into the odd and even zones of a dielectric metasurface. The proposed vortex beam generation strategy employs the advantages of both the Fresnel zone plate and the metasurface, and may open new routes for high-dimensional optical information processing.

High-energy x-ray nanotomography introducing an apodization Fresnel zone plate objective lens

Abstract: In this study, high-energy x-ray nanotomography (nano-computed tomography, nano-CT) based on full-field x-ray microscopy was developed. Fine two-dimensional and three-dimensional (3D) structures with linewidths of 75 nm–100 nm were successfully resolved in the x-ray energy range of 15 keV–37.7 keV. The effective field of view was ∼60 µm, and the typical measurement time for one tomographic scan was 30 min–60 min. The optical system was established at the 250-m-long beamline 20XU of SPring-8 to realize greater than 100× magnification images. An apodization Fresnel zone plate (A-FZP), specifically developed for high-energy x-ray imaging, was used as the objective lens. The design of the A-FZP for high-energy imaging is discussed, and its diffraction efficiency distribution is evaluated. The spatial resolutions of this system at energies of 15 keV, 20 keV, 30 keV, and 37.7 keV were examined using a test object, and the measured values are shown to be in good agreement with theoretical values. High-energy x-ray nano-CT in combination with x-ray micro-CT is applied for 3D multiscale imaging. The entire bodies of bulky samples, ∼1 mm in diameter, were measured with the micro-CT, and the nano-CT was used for nondestructive observation of regions of interest. Examples of multiscale CT measurements involving carbon steel, mouse bones, and a meteorite are discussed.

Lithographic pattern quality enhancement of DMD lithography with spatiotemporal modulated technology.

Abstract: In this paper, we propose spatiotemporal modulation projection lithography (STPL) technology, which is a spatiotemporal modulation technology applied to the conventional digital micromirror device (DMD) projection lithography system. Through coordinating the micro-movement of the piezoelectric stage, the flexible pattern generation of DMD, and the exposure time, the proposed STPL enables us to fabricate a microstructure with smooth edges, accurate linewidth, and accurate line position. Further application on fabricating a diffraction lens has been implemented. The edge sawtooth of the Fresnel zone plate fabricated by using the STPL is reduced to 0.3 µm, the error between the actual measured linewidth and the ideal linewidth is only within ±0.1µm, and the focal length is 15 mm, which is basically consistent with the designed focal length. These results indicated that STPL can serve a significant role in the micromanufacturing field for achieving high-fidelity microdevices.

Study of focusing parameters of wavelength-scale binary phase Fresnel zone plate

Abstract: Binary Fresnel zone plates (FZP) are among the most commonly used focusing elements of 2D-planar optical circuits in micro- and nano-photonics. When the diameter and focal distance of a FZP are reduced to the wavelength dimensions, the parameters of the focal area experience strong influence by FZP constructive design (material, thickness, depth of zone relief). By means of the numerical simulations, the near-field diffraction of monochromatic optical wave on a wavelength-scale binary phase plate is investigated. We found a range of optimal depths of zone plate grooves etching as well as substrate thicknesses providing the best focusing of the incident circularly polarized optical wave in terms of maximum field intensity and minimum size of the focal spot. A certain improvement of these focus parameters can be achieved by filling the zone grooves with a dielectric having a specific refractive index contrast against the FZP substrate. Additionally, the concept of a super-focus binary phase plate with a solid immersion layer (SIL) in the form of a truncated cone made of the ZP substrate material is proposed. Similar to conventional SIL device, this flat SIL-FZP can focus a circularly polarized optical radiation into a subdiffraction spot with a full-width of the order λ/2n (n is FZP refraction index).

High-resolution x-ray radiography with Fresnel zone plates on the University of Rochester’s OMEGA Laser Systems

Abstract: Experiments performed at the Laboratory for Laser Energetics with a continuous-wave (cw) x-ray source and on the OMEGA and OMEGA EP Laser Systems [Boehly et al., Opt. Commun. 133, 495 (1997) and Waxer et al., Opt. Photonics News 16, 30 (2005)] have utilized a Fresnel zone plate (FZP) to obtain x-ray images with a spatial resolution as small as ∼1.5 μm. Such FZP images were obtained with a charge-coupled device or a framing camera at energies ranging from 4.5 keV to 6.7 keV using x-ray line emission from both the cw source and high-intensity, laser-beam–illuminated metal foils. In all cases, the resolution test results are determined from patterns and grids backlit by these sources. The resolutions obtained are shown to be due to a combination of the spectral content of the x-ray sources and detector resolution limited by the magnification of the images (14× to 22×). High-speed framing cameras were used to obtain FZP images with frame times as short as ∼30 ps. Double-shell implosions on OMEGA were backlit by laser-irradiated Fe foils, thus obtaining a framing-camera–limited, FZP-image resolution of ∼3 μm–4 μm.

X-ray imaging of Rayleigh–Taylor instabilities using Fresnel zone plate at the National Ignition Facility

Abstract: Being able to provide high-resolution x-ray radiography is crucial in order to study hydrodynamic instabilities in the high-energy density regime at the National Ignition Facility (NIF). Current capabilities limit us to about 20 μm resolution using pinholes, but recent studies have demonstrated the high-resolution capability of the Fresnel zone plate optics at the NIF, measuring 2.3 μm resolution. Using a zinc Heα line at 9 keV as a backlighter, we obtained a radiograph of Rayleigh–Taylor instabilities with a measured resolution of under 3 μm. Two images were taken with a time integrated detector and were time gated by a laser pulse duration of 600 ps, and a third image was taken with a framing camera with a 100 ps time gate on the same shot and on the same line of sight. The limiting factors on image quality for these two cases are the motion blur and the signal to noise ratio, respectively. We also suggest solutions to increase the image quality.

Particle in cell simulation for the power enhancement by forming the second virtual cathode in an axial vircator

Abstract: A floating zone plate (ZP) was installed in the axial vircator for the formation of VC2 by using the wall charge of drifting electrons. The material of the opaque zones was changed to aluminum, copper, and glass to observe the effects. The main frequency of the microwave increased when VC2 is formed. At new frequencies, the opaque zones with aluminum and copper behave as an obstacle for the propagation of microwave, while the glass is suitable for the power enhancement. These results have significant importance for the implementation of real experiments to form VC2 by wall charge.

Zoom liquid lens employing a multifocal Fresnel zone plate.

Abstract: We propose a zoom liquid lens employing a multifocal Fresnel zone plate. The proposed lens has two optical surfaces: liquid-liquid interface and Fresnel zone plate. The Fresnel zone plate is designed to have a multifocal point and an increased depth of focus. Therefore, the proposed lens has two obvious advantages. Due to increased depth of focus, the proposed lens can realize zooming using only one tunable liquid-liquid interface, which is not available for conventional liquid lens. Thus, it is possible to remove conventional zooming mechanisms from cameras. Besides, the focal length tuning range is also increased, and a lens system based on the proposed lens can simultaneously collect two images with different magnifications. We present the design, fabrication and characterization of the proposed lens. The shortest positive and negative focal length are ∼17.5mm and ∼-34.5mm and the diameter is 5mm. The zoom ratio of the proposed lens reaches ∼1.48×. Our results confirm that the proposed lens has widespread applications in imaging system.

Super-Variable Focusing Vortex Beam Generators Based on Spiral Zone Plate Etched on Optical Fiber Facet

Abstract: In this article, an all-fiber super-variable focusing vortex beam generator is proposed and demonstrated, the focal point of which can be drastically relocated by varying the wavelength of the incident light. The all-fiber generator is fabricated by etching a spiral zone plate (SZP) on the top of the composite fiber structure composed of a single mode fiber and a quarter pitch graded index fiber using focused ion beam lithography. Numerical simulation and experimental measurements have been applied in investigation of all-fiber output light field characteristics, such as the focal spot, focal length, and vortex topological charges. Results show that these parameters can be controlled by flexibly adjusting the design parameters of SZP microstructure. In particular, the position of the donut-shaped focus spot can be dynamically tuned in a wide range by varying the incident laser wavelength with the focal spot profile remains constant. The all-fiber generator can be potentially applied in particle manipulation, high capacity communication, and fiber optic endoscopic imaging, all of which require ultra-compact, multi-dimension, and flexible focusing schemes.

Soft x-ray imaging spectroscopy with micrometer resolution

Abstract: Soft x-ray spectroscopy is invaluable for gaining insight into quantum materials. However, it is typically conducted in a spatially averaging way, making it blind to inhomogeneity in samples. Here, we demonstrate how we couple imaging to x-ray absorption spectroscopy and resonant inelastic x-ray scattering. Accordingly, we use a 2D detector and an off-axis Fresnel zone plate that images the sample in one spatial dimension and provides spectroscopic information in the other dimension. With our setup, we envision to enable a more detailed understanding of how the behavior of microscopic domains determines the functionality of quantum materials.

Axial intensity distribution of a micro-Fresnel zone plate at an arbitrary numerical aperture

Abstract: The axial focus number (the number of focal spots along the axial direction) and focus intensity of a micro-Fresnel zone plate (FZP) are analyzed from deep ultraviolet to infrared using the Fourier decomposition, the vectorial angular spectrum (VAS) theory, and the three-dimensional finite-difference time-domain (FDTD) method. For a low-numerical aperture (NA) micro-FZP (NA 0.3), the axial high-order foci are suppressed and there is one single focus. A fast, precise, and cost-efficient additive manufacturing method, i.e. two-photon polymerization, is used to fabricate high-NA phase-type micro-FZPs. The experiment has validated the phenomenon of linear negative focal shift of a high-NA micro-FZP. This property can be particularly applied in precise measurement of micro-displacement, film thickness, micro/nano step height, and wavelength.

Femtosecond laser fabrication of diffractive optics for spatial and spectral imaging at synchrotron infrared beamlines

Abstract: Infrared (IR) microspectroscopy is a powerful molecular fingerprinting tool widely used for the identification of structural and functional composition of biological and chemical samples. The IR microspectroscopy beamline at the Australian Synchrotron can be operated either with a single-point narrow-band mercury cadmium telluride (MCT) detector or a focal plane array (FPA) imaging detector with 64 × 64 pixels. For the implementation of indirect nonscanning imaging technology, the system was operated with the FPA detector. In this study, we propose an indirect IR imaging technique based on the principles of correlation optics using diffractive optical elements such as random pinhole array (RPA) and Fresnel zone plate (FZP). The spatial and spectral variations of point spread functions (PSFs) of the RPA and FZP were simulated for the synchrotron configuration. Intensity responses for 2D objects were simulated using the same simulation conditions and reconstructed using Lucy-Richardson algorithm. Fabrication of diffractive elements for IR wavelengths is often a challenging task as the IR transparent material substrates, such as barium fluoride and calcium fluoride, are highly susceptible to thermal shocks and brittle by nature. The diffractive elements were fabricated by ablating directly on a 100 nm thick gold coated substrate using femtosecond laser pulses. The simulation results and the fabrication outcomes demonstrate the feasibility of indirect imaging at the synchrotron IR beamline.

Optical scanning holography with a polarization directed flat lens.

Abstract: Recently, an optical scanning holographic system with a polarization directed flat lens was proposed to realize coaxial scanning holography (CSH). The advantage of CSH is its small form factor and the stability. However, the diffraction efficiency of the polarization directed flat lens cannot be 100%, and thus there is always zeroth order light in the scanning beam. The imperfect diffraction property of the polarization directed flat lens results in an incomplete scanning Fresnel zone plate. Consequently, the reconstructed image is blurred and noisy. In this paper, we compared different methods, including the back propagation, the phase correlation, and inverse filtering, for the hologram reconstruction. It is demonstrated that inverse filtering is the only method that can retrieve the high-frequency component of the hologram. However, additional noise also arises with the use of inverse filtering. Therefore, the imaging performance of CSH by using a polarization directed flat lens is inherently worse than that of conventional OSH.

High tolerance detour-phase graphene-oxide flat lens

Abstract: Flat lenses thinner than a wavelength promise to replace conventional refractive lenses in miniaturized optical systems. However, Fresnel zone plate flat lens designs require dense annuli, which significantly challenges nanofabrication resolution. Herein, we propose a new implementation of detour phase graphene flat lens with flexible annular number and width. Several graphene metalenses demonstrated that with a flexible selection of the line density and width, the metalenses can achieve the same focal length without significant distortions. This will significantly weaken the requirement of the nanofabrication system which is important for the development of large-scale flat lenses in industry applications.

Off-axis multilayer zone plate with 16 nm × 28 nm focus for high-resolution X-ray beam induced current imaging

Abstract: Using multilayer zone plates (MZPs) as two-dimensional optics, focal spot sizes of less than 10 nm can be achieved, as we show here with a focus of 8.4 nm × 9.6 nm, but the need for order-sorting apertures prohibits practical working distances. To overcome this issue, here an off-axis illumination of a circular MZP is introduced to trade off between working distance and focal spot size. By this, the working distance between order-sorting aperture and sample can be more than doubled. Exploiting a 2D focus of 16 nm × 28 nm, real-space 2D mapping of local electric fields and charge carrier recombination using X-ray beam induced current in a single InP nanowire is demonstrated. Simulations show that a dedicated off-axis MZP can reach sub-10 nm focusing combined with reasonable working distances and low background, which could be used for in operando imaging of composition, carrier collection and strain in nanostructured devices.

Demonstration of single-frame coherent X-ray diffraction imaging using triangular aperture: Towards dynamic nanoimaging of extended objects

Abstract: Coherent diffraction imaging (CDI) is a powerful method for visualizing the structure of an object with a high spatial resolution that exceeds the performance limits of the lens. Single-frame CDI in the X-ray region has potential use for probing dynamic phenomena with a high spatiotemporal resolution. Here, we experimentally demonstrate a general method for single-frame X-ray CDI using a triangular aperture and a Fresnel zone plate. Using 5 keV synchrotron radiation X-rays, we reconstructed the object image of the locally illuminated area with a spatial resolution of higher than 50 nm and an exposure time of more than 0.1 s without prior information about the sample. After a 10 s exposure, a resolution of 17 nm was achieved. The present method opens new frontiers in the study of dynamics at the nanoscale by using next-generation synchrotron radiation X-rays/free-electron lasers as light sources.

Tailorable polygon-like beams generated by modified spiral petal-like zone plates

Abstract: A modified spiral petal-like zone plate (MSPZP) is proposed to generate the polygon-like beam with the tailorable shape, size and topological charge. The size and shape of the polygon-like beam are adjustalbe by the frequency N and the exponent m, whose product represents the number of sides of the polygon. Moreover, the size of the polygon-like beam can be also tailorable by the original radius r0. When the exponent m is equal to 2, the distances between the vertices of the polygon-like beams and the central point are equal to r0. In addition, the topological charges of the polygon-like beam can be identified by the number of fringes generated by the corresponding interference phases of MSPZPs. The method of constructing the MSPZP is illustrated. Furthermore, it is also proved in the simulations and experiments that the tailorable polygon-like beam can be generated by the MSPZP. The proposed zone plates are applicable to trap particles along the different polygon-like curves and have potential applications in optical communication.

Dual energy X-ray beam ptycho-fluorescence imaging.

Abstract: X-ray ptychography and X-ray fluorescence are complementary nanoscale imaging techniques, providing structural and elemental information, respectively. Both methods acquire data by scanning a localized beam across the sample. X-ray ptychography processes the transmission signal of a coherent illumination interacting with the sample, to produce images with a resolution finer than the illumination spot and step size. By enlarging both the spot and the step size, the technique can cover extended regions efficiently. X-ray fluorescence records the emitted spectra as the sample is scanned through the localized beam and its spatial resolution is limited by the spot and step size. The requisites for fast ptychography and high-resolution fluorescence appear incompatible. Here, a novel scheme that mitigates the difference in requirements is proposed. The method makes use of two probes of different sizes at the sample, generated by using two different energies for the probes and chromatic focusing optics. The different probe sizes allow to reduce the number of acquisition steps for the joint fluorescence–ptychography scan compared with a standard single beam scan, while imaging the same field of view. The new method is demonstrated experimentally using two undulator harmonics, a Fresnel zone plate and an energy discriminating photon counting detector.

Alumide as a Nonmagnetic Fresnel Zone Lens at 30 GHz

Abstract: A nonmagnetic Fresnel Zone Plate Lens (FZPL) antenna with high dielectric plastic porosity controlling is proposed in this paper. Compared to conventional Fresnel plates, the proposed structure has a single homogeneous platform for transmitting mode role, making the gradient refractive index lens antenna a perfect candidate for high gain applications. A non-magnetic plastic has achieved the intended GRIN subzones homogeneously. Each desired permittivity for half-wave plate subzones is estimated with a capacitive estimation on Brakora analysis. Half-wave zone type of FZP is selected to utilize based on optic rules to realize this estimation. A Cos41-like conical horn is used as a central beam-launcher to excite the lens diaphragms and was implemented and measured at 30 GHz. The measured scattering parameters for ZP shows a better than −15 dB return loss and validate our modeling with 5 mm thickness and $3\lambda_{0}$ lateral dimension.

High frequency focal transducer with a Fresnel zone plate for intravascular ultrasound

Abstract: The diameter of an intravascular ultrasound (IVUS) catheter is always less than 1 mm, because it must be inserted into a blood vessel to obtain ultrasound images. Owing to this requisite small size, it is difficult to perform geometric focusing on the surface of an IVUS transducer to improve the spatial resolution of the image. This study proposes a high frequency transducer with a Fresnel zone plate (FZP) for intravascular ultrasound imaging. Through theoretical calculations, the parameters and structure of the transducer are optimized for high-frequency ultrasound. The acoustic beam is simulated using COMSOL software. The aperture size of the ultrasound element is 0.778 × 0.9 mm2. Transducers with or without the FZP layer are designed and fabricated in this study. The center frequency and –6 dB bandwidth of the FZP transducer are 52.5 MHz and 42%, respectively. Meanwhile, the center frequency and –6 dB bandwidth of the plane-shape transducer are 51.3 MHz and 58%, respectively. Wire phantom and porcine artery imaging experiments were performed to evaluate the performance of the designed transducers. The spatial resolution of the FZP transducer is 46.8 μm axially and 183.6 μm laterally, and the resolution of the plane-shape transducer is 44.3 μm axially and 313.5 μm laterally. The results demonstrate that the FZP transducer provides superior lateral imaging resolution for IVUS applications.

Sub-micrometer direct laser writing using an optimized binary-amplitude zone plate lens.

Abstract: Direct laser writing (DLW) is a versatile and reliable lithography method widely used in many micro and nano fabrication areas. However, the resolution of DLW is limited by the optical diffraction limit. Many methods have been proposed to improve the lithography resolution, but with either high cost or increasing the complexity of the system. Here, we propose a high numerical aperture binary-amplitude-type zone plate lens that can achieve a sub-wavelength focal spot with a large depth of focus and long working distance. The critical dimension of such a lens is set at micrometer scale for ease of fabrication. By integrating the as-designed planar lens into a DLW system, we experimentally demonstrate less than 300 nm lithography resolution with exposure depth larger than 500 nm. Our results show the possibility of writing sub-micrometer scale structures with the integration of a planar lens into the DLW system, which enables miniaturization and compactness of lithography instruments for many applications.

Lithium fluoride detectors for high spatial resolution imaging of tabletop XUV from high harmonic generation in gases

Abstract: As high harmonic generation (HHG) sources have proliferated, the need for high resolution, affordable extreme ultraviolet (XUV) detectors has become ubiquitous. We studied lithium fluoride (LiF) crystals, traditionally used for x rays, as a detector for a tabletop XUV source from high harmonic generation in gases. The LiF response curve showed a dynamic range of 103 (not saturated), with a minimum threshold fluence of 66µJ/cm2. Imaging tests were performed revealing sub-micrometer spatial resolution. We successfully recorded a scan of the focal plane of a Fresnel zone plate with high dynamic range. With this study, we showed that LiF can be used as a detector for HHG XUV, with a high potential to do near field imaging of complex objects such as the focus of structured light.