Near-field Focusing Research Articles

A Dual-Beam Leaky-Wave Antenna Based on Squarely Modulated Reactance Surface

In this paper, a novel dual-beam leaky-wave antenna (LWA) based on squarely modulated reactance surface (SquMRS) is proposed. The equivalent transmission lines model is utilized to characterize the field distributions of surface wave guided by the SquMRS. The calculated dispersion characteristics of SquMRS are verified by the simulated results, and it is demonstrated that SquMRS exhibits a more flexible control of phase constant and attenuation constant compared with traditional sinusoidally modulated reactance surface (SinMRS), which means SquMRS has a great potential for near-field focusing and far-field beam shaping. On this basis, a versatile method, based on a superposition of individual modulation patterns, was used to generated two beams with almost identical gain at 8.5 GHz. The measured results show that the gains are 10 dBi and 8.2 dBi at θ1 = −30° and θ2 = 18°, respectively, and the radiation efficiency is 83%, which shows good agreement with the simulated results.

A Four-Corner-Fed Slotted Waveguide Sparse Array for Near-Field Focusing

This article presents a double-layer four-corner-fed slotted waveguide sparse array antenna designed for the near-field-focused applications in the X-band. Generally, it is challenging to realize the ideal phase distribution of the near-field focused array antenna, especially for the near-field focusing array antenna with a focal distance less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\lambda $ </tex-math></inline-formula> . We propose a four-corner-feed structure associated with appropriate waveguide dimensions to realize approximately ideal phase distribution for a focus at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\lambda $ </tex-math></inline-formula> . The sparse array based on the particle swarm optimization is adopted to suppress the sidelobes and the grating lobes on the focal plane caused by the residual phase errors. A test antenna is designed at the center frequency of 10 GHz, and is fabricated by Direct Metal Laser Sintering technique for demonstration. It is verified by the experiment that we achieve satisfying near-field focusing effects, especially in terms of the focal spot size and sidelobe levels on the focal plane 45 mm away from the antenna aperture.

Near-field leaky-wave focusing antenna with inhomogeneous rectangular waveguide

Near-field leaky-wave focusing antenna with inhomogeneous rectangular waveguide

Support Vector Regression for the Modeling and Synthesis of Near-Field Focused Antenna Arrays

The powerful support vector regression framework is proposed in a novel method for near-field focusing using antenna arrays. By using this machine-learning method, the set of weights required in the elements of an array can be calculated to achieve an assigned near-field distribution focused on one or more positions. The computational cost is concentrated in an initial training process so that the trained system is fast enough for applications where moving devices are involved. The increased learning capabilities of support vector machines allow using a reduced number of training samples. Thus, these training samples may be generated with a prototype or a convenient electromagnetic analysis tool, and hence realistic effects, such as coupling or the individual radiation patterns of the elements of the arrays, are accounted for. Illustrative examples are presented.

Beamforming Algorithm Architectures for Medical Ultrasound

Medical ultrasound scanners are amongst the most sophisticated signal processing machines in use today. The Beamformer is the brain of whole signal processing system of the scanner [1]. Beamforming allows message transmission or reception to be directed or spatially selective. It is used in receiving beamforming to concentrate the noise signals obtained in the region of concern as reflections from various tissue structures. This paper reviews the various receive beamformer architectures implemented in FPGA/ASIC for ultrasound imaging. Most of the receive beamformers are implemented using the standard technique Delay and Sum. Beamforming in ultrasound instruments for medical imaging has traditionally been implemented using analog delay lines. The concept of dynamic focusing in near field has resulted more complex analog delay structures and were replaced by digital structures. By the availability of high-speed analog to digital converters, and VLSI Technology improvements have now made real time implementation of digital beamformers feasible. The current innovations involve hybrid beamformers utilizing the pros of both analog and digital structures. This paper discusses the evolution of beamforming architectures from analog to digital environment and the current beamformer designs. The changes in beamformer designs in order to be compatible to high frequency probes and yield improved imaging performance, resource optimization, etc. are discussed.

Near-Field Finite-Zone Focused Radiation From Reflector Antenna With Continuously Tapered Ellipsoidal Surface Curvatures

In this paper, a design of reflector antennas to produce near-field focused (NFF) radiation is presented. Instead of focusing the radiation at a single focal point, which results in an ellipsoidal reflector, this new reflector antenna application concept designates a finite zone, especially a finite curved surface, for NFF illumination. This finite zone may consist of several subzones for multi-NFF radiation. The design concept employs equal path lengths of wave propagation from the feed’s phase center toward the focal zone via reflections from the reflector’s surface in a one-to-one correspondence, which results in a confluence of reflection points found from a set of ellipsoidal surfaces. Based on a definition of the mapping mechanism to extract the reflection points to form the reflector, the field can be properly focused in the target zone by the concept of geometric optics (GO). In this paper, the basic theoretical concept is first summarized with a design equation developed for easy implementation. The focus property is demonstrated by a time-domain physical optics (TD-PO) analysis of spherical wave illumination with a transient-step field behavior to show the focusing responses of electromagnetic scattering from the reflector. The focusing characteristics of the frequency domain (FD) are also shown by PO simulation at 20 GHz.

Terahertz Near-Field Focusing Using a 3-D Printed Cassegrain Configuration for Backscattered Side-Channel Detection

This paper presents the use of terahertz near-field focusing for backscatter side-channel detection. Near-field focusing is done by using a Cassegrain reflector configuration. The focuser is designed to produce the focused beam 28 cm away from the antenna aperture. The focusing is done in the near-field region by axially moving the subreflector from the focal point. It is observed that the subreflector position has to shift approximately 11 wavelengths along the axis to create the focus at the required location. The focused antenna gain is 46 dBi, while the 3 dB focus width and depth of the designed antenna is ~4 mm and 10 cm, respectively. It is found that the focal plane position is sensitive to the subreflector shifts and it is observed that 1 mm change in the subreflector position can shift the focal plane by ~2 cm. The simulations are compared with the measurement results of a fabricated prototype and good agreement is observed. The antenna is fabricated by using 3-D printing technology, which allows rapid prototyping. Finally, we have demonstrated the detection of backscatter side channel from the board placed at 28 cm away from the designed antenna. The received power level of the backscatter signal increases by 6 dB as compared to horn antenna.

The Design of the Cooke Triplet Antenna in Terahertz Band

A Cooke triplet antenna used for near-field focusing is designed by a quasi-optical method. To design the Cooke triplet, we propose the quasi-optical method to reduce the aberration spot radius in the design step. The ray tracing method, calculating only the coordinates in the image plane rather than the field intensity, is used without considering the diffraction effect. The diffraction effect is taken into consideration only in the verification step. We get a conclusion that the essence of the aberration optimization is to optimize the phase distribution in the diffraction integral formula. In this case, only optimizing the aberrations without considering the diffraction effect is acceptable in the design step. While in the verification step, the diffraction is used to calculate the resolution of the Cooke triplet antenna. The field of view of the Cooke triplet is −18°∼18° in simulation and −14°∼14° in experiment. The feed sources can be positioned vertically to the optical axis in the image plane without the focal surface. The resolution of the off-axis beam is only 17% lower than the resolution of the central beam whose resolution is less than 0.5 cm at the distance 0.37 m. The experiment agrees well with the simulation.

Photonic nanojet beam shaping by illumination polarization engineering

Photonic Nanojets (PNJs) have attracted considerable research attention in the fields of super-resolution optical microscopy, nano-photolithography, and single molecule sensors because of their sub-wavelength near-field focusing properties. In this paper, we thoroughly studied how the polarizations and amplitude profiles of the incident beams affect the shape, size, and location of the PNJs generated from the illuminated microspheres. Numerical results showed that the PNJs generated by microspheres were strongly modulated by the polarizations and amplitude profiles of the illumination beams. Therefore, PNJs can be engineered according to the requirements of a specific application by designing the polarizations and amplitude profiles of the illumination light. Various fascinating properties of the PNJs generated with different illumination schemes were demonstrated and their implications for potential applications were discussed as well.

Spin-orbit coupling controlled near-field propagation and focusing of Bloch surface wave.

Bloch surface wave (BSW) can be considered as the dielectric analogue of surface plasmon polariton (SPP) with less loss since it is sustained at the surface of a truncated dielectric multilayer. As dielectric materials show nearly no ohmic loss, BSW can propagates much farther compared to SPP, and thus is beneficial for planar optical devices. In this paper, we study the spin-orbital interaction between incident beam and BSW. We demonstrate that due to the spin-orbital coupling, the near-field properties of generated BSW can be controlled with a meta-antenna structure. The meta-antenna is composed of two gold nano-antennas oriented at 45° and 135° as a near-field coupler. By careful design of the meta-antenna, the generated BSW can be guided and focused depending on the chirality of the incident beam. Three examples of meta-antennas are demonstrated for chiral sensitive focusing, directional switching and asymmetric focusing. The proposed method can be applied as a design method for low-loss on-chip photonic devices.

Near-field mode and focusing optimization of geometric phase modulated surface plasmon polaritons focusing lens

Near-field mode and focusing optimization of geometric phase modulated surface plasmon polaritons focusing lens

Wireless Power Transfer Through Simultaneous Near-Field Focusing and Far-Field Synthesis

A technique for the synthesis of antenna arrays that can account, simultaneously, for near-field (NF) focusing on one or more spots and far-field (FF) specifications is proposed, which deals with wireless power transfer applications. The technique is based on the minimization of a properly defined cost function accounting for both NF and FF requirements at the same time and including a tradeoff parameter to balance each specification in the final result. By synthesizing the weights that have to be applied to the elements of the array fulfilling all the specifications, a wireless link between the antenna array and different devices can be established more efficiently since power radiated at undesired positions or directions can be reduced. Some illustrative examples are presented to show the ability of the method to solve problems with independent specifications for NF and FF distributions. A prototype of a $16\times 16$ planar array operating at 16 GHz has been realized and characterized; it is shown that it is able to, simultaneously, focus on two NF spots and at an assigned FF direction.

Research on near‐field focusing algorithm of reconfigurable reflect‐array

In this study, the near-field beam focusing algorithm of reconfigurable reflect array for security imaging application is proposed. The proposed near-field focusing algorithm is based on classic array method, which is usually adopted for far-field pattern calculation. In order to satisfy the near-field imaging application, the imaging distance is added to array method, and the improved formula can calculate the shifting phase of every array element precisely. In addition, due to discrete phase characteristic of reconfigurable reflect array, a phase choice method based on trigonometric function is used in authors’ design. Based on the proposed algorithm, the shifting phase of every element is calculated and obtained the simulated beam focusing patterns. Meanwhile, the calculated shifting phase is written to whole array by field-programmable gate array, the near-field beam scanning patterns is measured and the measured results are in good agreement with the simulated ones.

Two-Dimensional Near-Field Focusing Folded Reversely Fed Leaky-Wave Antenna Array With High Radiation Efficiency

This paper presents a 2-D near-field focusing (NFF) substrate-integrated-waveguide (SIW) leaky-wave antenna (LWA) array with high radiation efficiency. In most of the existing 2-D NFF antenna arrays, the required quadratic phase and balanced amplitude distribution are achieved by the use of the transverse topology with a complex feeding network or the simple radial topology with high polarization loss. Here, a folded reversely fed LWA with transverse polarization is proposed to achieve natural balanced radiation with a sample series-fed network. Meanwhile, the width of the single linear LWA is complementary with the reversely fed topology. In this case, the SIW with a stepped width is applied to achieve a quasi-uniform slot distribution and to avoid the spurious radiation. Then, all reversely fed linear LWAs are connected by the meander-line structure, which can simplify the feeding network and enhance the radiation efficiency. The limitation of the NFF LWA in radiation efficiency is broken. The 2-D focusing principle and the design process of the proposed antenna are presented in this paper. As an example, a 2-D focusing SIW LWA array operated at 35 GHz with an 80% radiation efficiency is fabricated and measured.

Wideband polarisation‐insensitive metasurface with tunable near‐field scattering focusing characteristic

A wideband polarisation-insensitive metasurface (MS) has been proposed for tunable near-field scattering focusing. The MS is composed of dual-polarised unit cells with tunable reflection phases covering a range of 360°. Designed with the optical path method, the proposed MS demonstrates precise tunable and polarisation-insensitive near-field focusing in a wide band from 11 to 13 GHz in both simulation and experimental results.

Groove-structured meta-surface for patterned sub-diffraction sound focusing

Confining acoustic fields in subwavelength volumes is of fundamental interest in wave-energy harvesting and high-resolution imaging. Phononic crystals have been shown to be capable of superfocusing but are highly limited by their very large dimensions. Acoustic metasurfaces can yield similar functionality with unit cells significantly smaller than the wavelength. However, they are studied mostly under effective medium theory and cannot manipulate evanescent waves directly to control near-field focusing. Here, we use a microscopic approach to study acoustic metasurfaces for subdiffraction focusing of reflected waves, which consist of an array of deep-subwavelength sized and spaced grooves. We further show that the focusing pattern can be tailored by the designer. To validate the effectiveness of our scheme, two representative metasurfaces are designed theoretically, proved numerically, and confirmed experimentally for subdiffraction sound focusing with different patterns. We hope that our approach can work as a general guideline to shape near-field signals in the broad field of acoustics.

Relaying of the local enhanced electric-field using stacked gold bowtie nanoantennas

We demonstrate the possibility of designing plasmonic structures that extend, and even further concentrate the field enhancement in the feed-gap region of single-layer Au bowtie nanoantennas (BNAs) hundreds of nanometers away from the initial metal-dielectric interface. The design is based on a stack of Au BNAs with progressively reduced gap distances sandwiched by thin dielectric layers. We find that this stacked BNA geometry also behaves as a near-field focusing lens of nanometer focal length. By merely controlling the number of BNAs in a stack and thickness of the accompanying dielectric layers, we show that the usual fast-damping field enhancement right above a 50 nm thick base BNA can be relayed and maintained at >104 over a distance of at least up to 200 nm above the base BNA surface. Our novel plasmonic structures offer an approach to design plasmonic nanostructures for applications in metamaterial engineering, SERS, and nonlinear optics.

Design of Near-Field Focused Metasurface for High-Efficient Wireless Power Transfer With Multifocus Characteristics

This paper presents a new design of near-field focused metasurface for high-efficiency wireless power transfer with multifocus characteristics. A general synthesis procedure is outlined to design metasurfaces with desired multiple foci in the near-field zone. The unit cell of printed tridipole is used to realize the focused metasurface which has a large range of phase shift and is adapted to near-field focusing applications. Different feeding and focusing designs of the printed tridipole metasurfaces are proposed and analyzed. Two kinds of metasurfaces with 20×20 elements are designed, fabricated, and measured at 5.8 GHz. One is the single-feed and single-focus case, and the other is single-feed and two-focus case. Two-dimensional planar near-field scanning technique is implemented to measure the E -field focusing characteristics. Full-wave simulations and experiments demonstrate that the proposed focusing metasurfaces have a high wireless power transfer efficiency of 70%. The relative bandwidth with 50% power focusing efficiency is about 16%. The measured results are in good agreement with the theoretical designs and simulated results, which verify the feasibility and correctness of the proposed approach in this paper.

Proactive Conformal Antenna Array for Near-Field Beam Focusing and Steering Based on Curved Substrate Integrated Waveguide

The quadratic phase distribution on the antenna aperture is necessary to produce a near-field-focused (NFF) beam. The generation, as well as the steering of the quadratic phase distribution, is difficult for a series-fed NFF array antenna, especially for a large-aperture application. This is because the phase control capability of the planar series-fed beam-forming network is limited. In this paper, a curved substrate integrated waveguide (SIW) is employed as the feeding carrier to enhance the amplitude and phase control capability. The spatial placement of the SIW slot array antenna can be sufficiently used to synthesize the desired NFF beam. As examples, two kinds of proactive conformal SIW NFF slot array antennas, working in the standing-wave mode and the leaky-wave mode, have been developed. Both of them have the ability to generate the high-quality NFF beam. In addition, the proactive conformal SIW leaky-wave slot array antenna has the stable focal height over a wide steerable range. After careful analysis and design, these two kinds of antenna arrays are fabricated and proactively conformal to 3-D-printed frameworks with the desired shape. The theoretical analysis and simulated results are verified by experimental results.

Investigation of sparse near‐field focusing array antenna with different topologies

The method to efficiently design a sparse near-field focusing sparse array antenna is introduced here. First, the calculation model and analysis method of three array topologies including the square grid, the circular grid, and the regular triangular grid are established. The evaluation criterion is the lowest sparsity under the condition that the focusing performance of the sparse array approaches to that of a referenced focusing array with the element spacing of a half of one wavelength. After comparing the focusing performance of these array antennas with different grids, the circular-grid array is the optimal one and selected to further discuss the influence of different topology parameters. The optimal sparse near-field focusing array should have the constant distance between two adjacent rings and increase three elements compared with the last ring. Finally, the method based on the proposed selection criteria is employed to design a sparse near-field focusing sparse array. The correctness of the method has been validated by the full-wave simulated results.