Near-field Focusing Research Articles

Photonic hook shaping achieved by the micro-nano fiber array based Janus cylindrical metalens

Abstract Near-field focusing of an electromagnetic wave on the assembly of hexagonally asymmetric arranged close-contact nanofibers into a cylindrical Janus metalens is considered for different fiber sizes and optical properties. We propose combining the meta-material concept with a photonic hook based on the finite-difference time-domain technique. Simulation results show that the cylindrical meta-photonic Janus structure produces the focal region of enhanced optical intensity, which exists in the spatial form of a localized structured light known as a photonic hook. A detailed study of all key parameters of a photonic hook depending on the Janus metalens topology and optical properties of the fiber filling is carried out. The structure conditions have been determined for the beam waist of a photonic hook that is less than the diffraction limit. This Janus cylindrical metalens may contribute to the versatile control of photonic hook generation in the applications of bio-photonics and optical microscopes.

Two- and Three-Dimensional Ag-Au Nanoalloying: Heterogeneous Building Blocks Enhancing Near-Field Focusing.

In this study, we report a strategy for fabricating binary surface-enhanced Raman scattering (SERS) substrates composed of plasmonic Pt@Ag and Pt@Au truncated-octahedral (TOh) dual-rim nanoframes (DNFs) functioning as a "nanoalloy." Pt TOh frameworks act as a scaffold to develop nanoarchitectures with surface decoration using plasmonically active materials (i.e., Au or Ag), resulting in identical sizes and shapes for the two distinct plasmonic elements, facilitating the fabrication of a "nanoalloy" blend of two shape-complex building blocks. The structural complexity from the dual-rim on (111) facets, combined with the mirror charge effect (i.e., enhanced polarization between Ag and Au) at the interface of heterogeneous components, significantly amplifies SERS activity. We carefully investigated near-field focusing of binary SERS substrates through single-particle and bulk SERS measurements corroborated by finite element method (FEM) calculations. Crucially, we developed free-standing superpowders (SPs) in which each heterogeneous building block formed micron-sized supercrystals with adjustable component ratios. These plasmonic SPs were applied to contaminated areas for analyte detection, demonstrating their potential for practical SERS applications.

Plasmonic Nanotrenches with 1 nm Nanogaps for Surface-Enhanced Raman Scattering-Based Screening of His-Tagged Proteins.

This study represents a highly sensitive and selective approach to protein screening using surface-enhanced Raman scattering (SERS) facilitated by octahedral Au nanotrenches (OANTs). OANTs are a novel class of nanoparticles characterized by narrow, trench-like excavations indented into the eight facets of a Au octahedron. This unique configuration maximizes electromagnetic near-field focusing as the gap distance decreases to ∼1 nm. Owing to geometrical characteristics of the OANTs, near-field focusing can be maximized through the confinement and reflectance of light trapped within the trenches. We used Ni ions and molecular linkers to confer selective binding affinity for His-tagged proteins on the surfaces of the OANTs for SERS-based protein screening. Remarkably, SERS-based protein screening with the surface-modified OANTs yielded outstanding screening capabilities: 100% sensitivity and 100% selectivity in distinguishing His-tagged human serum albumin (HSA) from native HSA. This highlights the significantly enhanced protein screening capabilities achieved through the synergistic combination of SERS and the OANTs.

Recent Progress in the Synthesis of 3D Complex Plasmonic Intragap Nanostructures and Their Applications in Surface-Enhanced Raman Scattering.

Plasmonic intragap nanostructures (PINs) have garnered intensive attention in Raman-related analysis due to their exceptional ability to enhance light-matter interactions. Although diverse synthetic strategies have been employed to create these nanostructures, the emphasis has largely been on PINs with simple configurations, which often fall short in achieving effective near-field focusing. Three-dimensional (3D) complex PINs, distinguished by their intricate networks of internal gaps and voids, are emerging as superior structures for effective light trapping. These structures facilitate the generation of hot spots and hot zones that are essential for enhanced near-field focusing. Nevertheless, the synthesis techniques for these complex structures and their specific impacts on near-field focusing are not well-documented. This review discusses the recent advancements in the synthesis of 3D complex PINs and their applications in surface-enhanced Raman scattering (SERS). We begin by describing the foundational methods for fabricating simple PINs, followed by a discussion on the rational design strategies aimed at developing 3D complex PINs with superior near-field focusing capabilities. We also evaluate the SERS performance of various 3D complex PINs, emphasizing their advanced sensing capabilities. Lastly, we explore the future perspective of 3D complex PINs in SERS applications.

The Focusing Properties of a Modular All-Metal Lens in the Near-Field Region.

This article proposes a modular and passive all-metal lens to achieve near-field focusing with adjustable focus. The proposed lens consists of all-metal units with the phase coverage range exceeding 360°, and the arrangement of units is guided by the phase compensation method. Specifically, using the strategy of module unit synthesis, the arrangement of lens units under different focuses can be assembled arbitrarily, which reduces the production costs by 39.3% and improves the freedom of lens design. The simulation and experimental results show that the lens exhibits excellent focusing properties and freely changes the position of the expected focus (0.30 m-0.75 m). Therefore, the modular all-metal lens designed in this article has the characteristics of high transparency and a high degree of freedom, which can provide low-cost and lightweight solutions for various applications in the field of antennas, such as near-field target detection, microwave imaging, biomedicine, and so on.

Tailoring aberration-free photonic nanojets through the illumination of dielectric cylinders using cylindrical vector beams.

This study explores the manipulation of photonic nanojets (PNJs) via axial illumination of cylindrical dielectric particles with cylindrical vector beams (CVBs). The edge diffraction effect of cylindrical particles is harnessed to achieve the near-field focusing of CVBs, minimizing the spherical aberration's impact on the quality of the PNJ. By discussing how beam width, refractive index, and particle length affect PNJs under radially polarized incidence, a simple and effective approach is demonstrated to generate rod-like PNJs with uniform transmission distances and super-diffraction-limited PNJs with pure longitudinal polarization. Azimuthal polarization, on the other hand, generates tube-like PNJs. These PNJs maintain their performance across scale. Combining edge diffraction with CVBs offers innovative PNJ modulation schemes, paving the way for potential applications in particle trapping, super-resolution imaging, photo-lithography, and advancing mesotronics and related fields.

Microstrip array antenna for precise near-field focusing

ABSTRACT An array of microstrip antennas is introduced to achieve precise near-field focusing. A precise focusing approach of synthesising weights for array excitations is employed in the proposed near-field-focused (NFF) array, consisting of 3 × 3 subarrays. The usage of this approach allows the beam angle of every subarray to point to the focal spot precisely at the working frequency of 10 GHz. By introducing this working mechanism, a good trade-off has been achieved in the performance. The design can form a small focal width, about 6.6 mm, with a small location error of the focal spot of 8%. Meanwhile, the sidelobe level and the aperture remain small. To establish the effectiveness of this approach, we have designed, simulated, and measured this work.

Four-channel near-field focusing metasurface lens based on frequency-polarization multiplexing

In response to the issue of traditional near-field focusing metasurfaces having a single working frequency band and limited focusing functionality, this paper proposes a four-channel focusing metasurface based on frequency-polarization multiplexing. Utilizing the principles of anisotropy and metasurface resonant phase, a metasurface unit composed of two sets of orthogonal resonators was designed. The designed units have independent transmission amplitude and phase at 15 GHz and 23 GHz, with the transmission phase of the two sets of resonators encoded in 2-bit and 1-bit based on phase differences of 90° and 180°, respectively. Based on the units, combined with the focusing principle of metasurface lenses and the principle of phase superposition, a four-channel near-field focusing metasurface lens was designed. The lens is composed of 20 × 20 units, measuring 200 × 200 mm2. Numerical simulations and experimental results demonstrate that the designed metasurface lens achieves single-point and dual-points focusing at 15 GHz under x- and y-polarized wave incidence, with the focus at the z = 70 mm plane and a maximum focusing efficiency of 48.5%. At 23 GHz under x- and y-polarized wave incidence, metasurface lens focuses electromagnetic waves at different positions, with the focus at the z = 80 mm plane and a maximum focusing efficiency of 27.8%. The metasurface lens designed by this method is expected to find applications in fields such as Medical Sensing, Microwave Imaging, and Integrated Optics.

Low-coupling reflective metasurfaces for accurate near-field focusing

Metasurface is a 2D metamaterial which manipulates electromagnetic wavefront by carefully designing the transmissive or reflective responses of the planar subwavelength structures. Despite various emerging new functionalities, one of the limitations of metasurfaces in practical applications is the lack of control accuracy of its local amplitude and phase responses. This is in part caused by the discrepancy between the periodic EM simulated responses of unit cells and the actual non-periodic metasurfaces when functionality-determined amplitude and phase gradients are introduced. Under such conditions, the focusing capabilities of metasurfaces can be greatly affected. In this paper, we address this problem by introducing a slotted ground plane metasurface design which reduces the above-mentioned discrepancy by limiting the mutual coupling effects of a unit cell with its surrounding elements. An offset-fed near-field focusing reflective metasurface is designed and fabricated to verify the performance of the proposed design. Both the EM simulated and measurement results demonstrate the advantages of the proposed design in terms of sidelobe level and transfer efficiency.

Design of a polarization-multiplexed, high-resolution, near-field focusing metasurface lens.

To meet the requirements of integrated and high-resolution focusing devices for passive millimeter-wave (PMMW) imaging systems, a polarization-multiplexed high-resolution near-field focusing metasurface lens is proposed. Metasurface units consist of two dielectric layers and three metal layers and are designed with a multiarm windmill structure. This design allows the units to independently control the electromagnetic response of incident x-polarized and y-polarized waves while maintaining a thickness of only 0.16λ (2mm). The metasurface lens that can achieve dual-channel near-field focusing was designed by combining the focusing principle of the metasurface lens and phase superposition principle based on the above design. The lens consists of 30×30 units and has a size of 120×120m m 2. According to the simulation results, the lens is able to focus the y-polarized waves of 24GHz at z=50m m plane with a focal spot size of 0.68λ (8.5mm), and the focusing beam efficiency is 35.2%. Similarly, the x-polarized waves of 24GHz are focused at z=70m m plane with a focal spot size of 0.72λ (9mm), and the focusing beam efficiency is 40.7%. The proposed metasurface lens is promising for applications in PMMW imaging systems, medical sensors, automotive millimeter-wave radar, and other related fields, owing to the characteristics of high resolution, compact size, and multifunctionality.

Octahedron in a Cubic Nanoframe: Strong Near-Field Focusing and Surface-Enhanced Raman Scattering.

Here, we describe the synthesis of a plasmonic particle-in-a-frame architecture in which a solid Au octahedron is enclosed by a Au cubic nanoframe. The octahedra are positioned inside and surrounded by outer Au cubic nanoframes, creating intra-nanogaps within a single entity. Six sharp vertexes in the Au octahedra point toward the open (100) facets of the cubic nanoframes. This allows not only efficient interactions with the surroundings but also tip-enhanced electromagnetic near-field focusing at the sharp tips of the octahedra, combined with intraparticle coupling. The solid core-frame shell structure enhances near-field focusing, giving rise to a heightened concentration of "hot spots". This effect enables highly sensitive detection of 2-naphthalenethiol and thiram, indicating these substrates for use in surface-enhanced Raman spectroscopy-related applications.

Engineering Assembly of Plasmonic Virus-Like Gold SERS Nanoprobe Guided by Intelligent Dual-Machine Nanodevice for High-Performance Analysis of Tetracycline.

Accurate detection of trace tetracyclines (TCs) in complex matrices is of great significance for food and environmental safety monitoring. However, traditional recognition and amplification tools exhibit poor specificity and sensitivity. Herein, a novel dual-machine linkage nanodevice (DMLD) is proposed for the first time to achieve high-performance analysis of TC, with a padlock aptamer component as the initiation command center, nucleic acid-encoded multispike virus-like Au nanoparticles (nMVANs) as the signal indicator, and cascade walkers circuit as the processor. The existence of spike vertices and interspike nanogaps in MVANs enables intense electromagnetic near-field focusing, allowing distinct surface-enhanced Raman scattering (SERS) activity. Moreover, through the sequential activation between multistage walker catalytic circuits, the DLMD system converts the limited TC recognition into massive engineering assemblies of SERS probes guided by DNA amplicons, resulting in synergistic enhancement of bulk plasmonic hotspot entities. The continuously guaranteed target recognition and progressively promoted signal enhancement ensure highly specific amplification analysis of TC, with a detection limit as low as 7.94 × 10-16g mL-1. Furthermore, the reliable recoveries in real samples confirm the practicability of the proposed sensing platform, highlighting the enormous potential of intelligent nanomachines for analyzing the trace hazards in the environment and food.

Fabrication of an Optoplasmonic Raft with Improved SERS Performance Detecting Methamphetamine through Bubble Enrichment.

In this work, a novel raft-like structure that combines noble metal nanoparticles (NPs) with an interconnected layer of hemispherical dielectric shell was fabricated and characterized. It was discovered that this hybrid material can enhance the optoplasmonic interaction between plasmonic and dielectric components, thereby improving the sensing performance in surface-enhanced Raman spectroscopy (SERS). Varied geometric parameters of the fabricated optoplasmonic raft, including the inner diameter and thickness of the dielectric shell, were attempted and analyzed through numerical simulation and experimental SERS measurements. With particular size, thickness, and incident orientation, the silica shell focuses the incident optical flow into the deposited silver NPs, undergoing similar near-field focusing behavior in comparison with other optoplasmonic entities. This optoplasmonic raft floating on the water surface is able to harvest the target molecules effectively through bubble enrichment, which rapidly captures and concentrates analytes from the aqueous phase. With a limited sampling time, the sensing performance of the developed optoplasmonic raft is improved by applying the optimized parameters involved in bubble enrichment. The substrates and corresponding enrichment method were implemented in the detection of methamphetamine (METH), achieving a limit of detection (LOD) down to 0.035 nM. As for practical onsite detection, the developed substrate and bubbling strategy were applied in an assembled set, employing a portable Raman spectrometer and an air pump. This set is able to detect METH dissolved in regular commercial beer, which is quite competent in the investigation of drug abuse.

Miniaturized near-field-focused antenna array

This paper presents a microstrip planar antenna array designed for near-field focusing operating at 10 GHz. It is a challenge to reach the phase and amplitude distribution of near-field-focused antenna arrays with small focal spots. The antenna elements of this array are arranged radially on the aperture of only 5.6λ 0 × 5.6λ 0, which reduces the focal spot diameter. We designed an unequal-amplitude unequal-phase power divider to reach phase distribution and binomial taper distribution. The proposed array suppresses the sidelobes on the focal plane. It increases the electric field intensity by 11.46 dB compared with the horn with the same aperture size and increases 9.42 dB compared with the equal-phase array. The focus spot diameter is only 0.33λ 0. The simulated and measured results are in good agreement, which verifies the high performance of the array. This miniaturized high-focus array has a good prospect in near-field applications like wireless energy transmission and industrial detection.

Au Octahedral Nanosponges: 3D Plasmonic Nanolenses for Near-Field Focusing.

Here, we report the synthesis of three-dimensional plasmonic nanolenses for strong near-field focusing. The nanolens exhibits a distinctive structural arrangement composed of nanoporous sponge-like networks within their interior. We denote these novel nanoparticles as "Au octahedral nanosponges" (Au Oh NSs). Employing a carefully planned multistep synthetic approach with Au octahedra serving as sacrificial templates, we successfully synthesized Au Oh NSs in solution. The porous domains resembling sponges contributed to enhanced scattering and absorption of incident light within metal ligaments. This optical energy was subsequently transferred to the nanospheres at the vertex, where near-field focusing was maximized. We named this observed enhancement a "lightning-sphere effect". Using single particle-by-particle surface-enhanced Raman scattering (SERS), we optimized the morphological dimensions of the spheres and porous domains to achieve the most effective near-field focusing. In the context of bulk SERS measurements targeting weakly adsorbing analytes (2-chloroethyl phenyl sulfide) in the gas phase, we achieved a low detection limit of 10 ppb. For nonadsorbing species (dimethyl methyl phosphonate), utilization of hybrid SERS substrates consisting of Au Oh NSs and metal-organic frameworks as gas-adsorbing intermediate layers was highly effective for successful SERS detection.

Near-field Focusing in Ground Penetrating Radar Based on Phase Compensation Algorithm

Underground energy intensity and electromagnetic wave penetration depth are important parameters to characterize the detection performance of Ground Penetrating Radar (GPR). Near-field focusing (NFF) is an effective means to improve the electromagnetic energy fed into the ground by the antennas. In this paper, the phase compensation algorithm considering underground media is derived. In order to verify the effectiveness of the algorithm, the NFF antenna array is proposed. By changing the phase of the NFF array, the focus distance can be adjusted in the range of 400 mm – 800 mm. The maximum field enhancement coefficient in the focal region can achieve 19.2 dB. This shows that NFF array based on phase compensation algorithm has potential application prospects in increasing GPR detection performance.

Electromagnetic near-field focusing based on metasurfaces

Near-field electromagnetic focusing has become a hot research topic due to its high application value in wireless transmission, wireless sensing, biomedicine, and radio frequency identification. In this paper, anisotropic metasurfaces have been proposed to manipulate the phase of the electromagnetic (EM) wave for obtaining EM near-field focusing. Under orthogonal linearly polarized incidence, dual focus functions can be realized independently for polarization multiplexing on the metasurfaces. Meanwhile, Single focus and multi focus with different energy distributions and focal position is obtained by the proposed metasurfaces. By using the Vivaldi antenna as the feed source positioning at the focal of the metasurfaces, a lens antenna with high gain is obtained with circularly polarized far-field mode and broadband characteristics. Due to the unique advantages of metasurfaces in modulating the phase distribution of EM waves, the proposed method may pave a new way for EM focusing application.

Design and implementation of a near-field focused metasurface for microwave power transmission

In this paper, a near-field focused (NFF) metasurface composed of sub-wavelength structures for microwave power transmission is designed and implemented. Through full-wave simulations, a phase-shifting unit is proposed, which covers a transmission phase range of 360°, maintains a transmission amplitude better than −1.5 dB in most regions. Accurate design of the units at different positions in the array is achieved using phase compensation theory, forming the near-field focusing metasurface. By adjusting the transmission phase of the central unit in the metasurface to 30°, higher transmission efficiency can be achieved. Simulation results indicate that the designed near-field focused metasurface has the properties of high transmission efficiency, polarization insensitivity and distance insensitivity. To verify the transmission performance of the near-field focusing metasurface, a near-field focusing experimental platform is built and the microwave power transmission efficiency is calculated. Experimental results demonstrate that the proposed metasurface enhances the power received at the receiver, increasing the transmission efficiency by 433 %. The proposed near-field focusing metasurface holds potential for applications in high-power wireless power transfer and microwave intracavity focusing.

NEAR FIELD BEAM FORMING IN TERAHERTZ WIDEBAND MASSIVE MIMO SYSTEMS USING CODEBOOK LEARNING

Terahertz (THz) communication technology holds immense potential for enabling ultra-high data rates in wireless communication networks. However, the highly directional nature of THz waves poses significant challenges for achieving precise beam focusing, especially in near-field scenarios. In this study, we propose a novel approach leveraging deep learning techniques for near-field beam focusing in Terahertz Wideband Massive Multiple Input, Multiple Output (MIMO) systems. Our methodology's main novelty is the combination of codebook learning with deep neural networks. We use a convolutional neural network (CNN) architecture to learn complex spatial properties of THz channels and optimise beamforming weights. During the training phase, the codebook, which represents a discrete collection of beamforming vectors, is adaptively modified, allowing for dynamic response to changing channel circumstances. Extensive tests are being carried out to confirm the efficacy of our strategy, employing cutting-edge THz transceivers and MIMO technology. The proposed deep learning model is trained and evaluated using real-world channel data. Comparative comparisons with traditional beamforming approaches demonstrate our methodology's better performance, revealing significant advances in spectral efficiency and signal quality. This paper highlights the transformational influence of codebook learning linked with deep neural networks in boosting the performance of near-field beam focusing in Terahertz Wideband Massive MIMO systems. It not only provides a major achievement in the field of THz communications. Key Words: THz Communication, deep neural networks, MIMO, beamforming, deep learning.

A Near-Field Focused Array Antenna Empowered by Deep Learning for UHF-RFID Smart Gates

A Near-Field Focused Array Antenna Empowered by Deep Learning for UHF-RFID Smart Gates