Near-field Plate Research Articles

Nonradiative subdiffraction near-field patterns using metagratings

We present a synthesis scheme to mold periodic nonradiative field patterns in transmission using the recent concept of metagratings (MGs). To this end, we utilize our previously developed analytical model to analyze the interaction of an incoming plane wave with these sparse periodic arrangements of polarizable particles (meta-atoms). As the model reliably predicts coupling to all scattered Floquet–Bloch modes, both propagating and evanescent, desired reactive near-field profiles with deep subwavelength features can be generated. This approach forms an appealing alternative to previously proposed near-field plates based on metasurfaces, where abstract homogenization introduces uncertainties regarding utilization of highly evanescent spectrum, and meta-atom realization incurs full-wave optimization. In contrast, the outlined MG-based methodology, verified via full-wave simulations, directly yields fabrication-ready printed-circuit-board configurations, enabling versatile control of reactive near fields with no interfering radiative components, with potential uses in sensing, selective microwave heating, and wireless power transfer.

Near-field plates and the near zone of metasurfaces

AbstractA brief, tutorial account is given of the differences between the near and far regions of the electromagnetic field emphasizing the source-dependent behavior of the former and the universal properties of the latter. Field patterns of near-field plates, that is, metasurfaces used for sub-wavelength applications, are discussed in some detail. Examples are given of fields that decay away from the plates in an exponential manner, a ubiquitous feature of many interface problems, and metasurfaces for which the decay is not exponential, but algebraic. It is also shown that a properly designed system of two parallel near-field plates can produce fields that exhibit pseudo minima, which are potentially useful for near-field tweezer-like applications.

Electron Beam Aberration Correction Using Optical Near Fields.

The interaction between free electrons and optical near fields is attracting increasing attention as a way to manipulate the electron wave function in space, time, and energy. Relying on currently attainable experimental capabilities, we design optical near-field plates to imprint a lateral phase on the electron wave function that can largely correct spherical aberration without the involvement of electric or magnetic lenses in the electron optics, and further generate on-demand lateral focal spot profiles. Our work introduces a disruptive and powerful approach toward aberration correction based on light-electron interactions that could lead to compact and versatile time-resolved free-electron microscopy and spectroscopy.

Electromagnetic-wave beam-scanning antenna using near-field rotatable graded-dielectric plates

The paper proposes an electromagnetic-wave beam-scanning antenna system using a simple rotation of a pair of near-field graded-dielectric plates (GDPs). The antenna system requires an electromagnetic (EM) illuminator, with nearly symmetric aperture field distribution, as the base antenna and two types of GDPs: one radially graded dielectric (RGD) and two linearly graded dielectric (LGD) plates. The RGD first focuses the beam of the base antenna in the broadside direction, and the LGD then tilts the focused beam at an offset angle. For some types of base antennas, having fairly uniform aperture phase distributions, an RGD may not be required and only a pair of LGDs are sufficient. Irrespective of the antenna configuration, using a simple rotation of the two LGDs, the beam can be scanned to any position within a large conical region. The antenna system presented as a proof of concept has a resonant-cavity antenna as the base antenna and three graded-dielectric plates. The aperture length of the antenna system is 6λ0 and its height is 2.2λ0. The results predicted by numerical simulations indicate that the antenna system has highest peak directivity of 21 dBi. The beam can be scanned to any direction within a cone having an apex angle of 20°.

A Multiband Antenna Associating Wireless Monitoring and Nonleaky Wireless Power Transfer System for Biomedical Implants

This paper presents a multiband conformal antenna for implantable as well as ingestible devices. The proposed antenna has the following three bands: medical implanted communication service (MICS: 402–405 MHz), the midfield band (1.45–1.6 GHz), and the industrial, scientific, and medical band (ISM: 2.4–2.45 GHz) for telemetry or wireless monitoring, wireless power transfer (WPT), and power conservation, respectively. A T-shaped ground slot is used to tune the antenna, and this antenna is wrapped inside a printed 3-D capsule prototype to demonstrate its applicability in different biomedical devices. Initially, the performance of the proposed antenna was measured in an American Society for Testing and Materials phantom containing a porcine heart in the MICS band for an implantable case. Furthermore, to stretch the scope of the suggested antenna to ingestible devices, the antenna performance was simulated and measured using a minced pork muscle in the ISM band. A modified version of the midfield power transfer method was incorporated to replicate the idea of WPT within the implantable 3-D printed capsule. Moreover, a near-field plate (NFP) was employed to control the leakage of power from the WPT transmitter. From the simulation and measurements, we found that use of a ground slot in the implantable antenna can improve antenna performance and can also reduce the specific absorption rate. Furthermore, by including the NFP with the midfield WPT transmitter system, unidirectional wireless power can be obtained and WPT efficiency can be increased.

Two dimensional subdiffraction focusing beyond the near-field diffraction limit via metasurface

Near-field plates with the capabilities of modulating the near-field pattern and forcing the incident wave to a subwavelength spot have been experimentally investigated at microwave wavelengths. Their superlensing prop-erties result from the radiationless electromagnetic interference. However, the material’s loss and limitations of state-of-the-art nanofabricating technology pose great challenges to scale down the microwave near-field plates to the infrared or optical region. In this paper, a related but alternative approach based on metasurface is introduced which breaks the near-field diffraction limit at mid-infrared region (10.6 μm). The metasurface consists of periodic arrangement of chromium dipolar antennas with the same geometry but spatially varying orientations, which plays the dual roles in achieving the prescribed amplitude modulation and the abrupt π phase change between the subwavelength neighboring elements. As a result, a two dimensional subdiffraction focus as small as 0.037λ2 at ~0.15λ above the metasurface is presented. In addition, the broadband response and ease fabrication bridge the gap between the theoretical investigation and valuable applications, such as near-field data storage, subdiffraction imaging and nanolithography.

Unidirectional wireless power transfer using near-field plates

One of the obstacles preventing wireless power transfer from becoming ubiquitous is their leakage of power: high-amplitude electromagnetic fields that can interfere with other electronic devices, increase health concerns, or hinder power metering. In this paper, we present near-field plates (NFPs) as a novel method to tailor the electromagnetic fields generated by a wireless power transfer system while maintaining high efficiency. NFPs are modulated arrays or surfaces designed to form prescribed near-field patterns. The NFP proposed in this paper consists of an array of loaded loops that are designed to confine the electromagnetic fields of a resonant transmitting loop to the desired direction (receiving loop) while suppressing fields in other directions. The step-by-step design procedure for this device is outlined. Two NFPs are designed and examined in full-wave simulation. Their performance is shown to be in close agreement with the design predictions, thereby verifying the proposed design and operation. A NFP is also fabricated and experimentally shown to form a unidirectional wireless power transfer link with high efficiency.

Programmable Screen for Patterning Magnetic Fields

We present the design and experimental demonstration of a magnetic screen, which can pattern magnetic field into 256 programmable spots of small sub-wavelength dimension. A design methodology is outlined for designing a near-field plate to achieve a desired focus, and its implementation is discussed. Simulation using a full-wave electromagnetic solver clearly demonstrates focused magnetic field spots and that the presented structure has a stronger magnetic field compared with a conventional coil at the same focusing distance. A sample of thickness 2.1 mm with 16 layers, in which there are 32 unclosed loops for focusing on each of the eight layers and optimized guiding strips on the other eight layers, is fabricated and measured. The measurement results agree well with the simulation results. Finally, the response of the magnetic screen in the time domain is measured and computed showing the screen can be operated under pulse excitation. Such a device, capable of producing a programmable focused magnetic field, will find applications in biomedical devices, near-field imaging systems, data storage, and electromechanical actuators.

A unidirectional subwavelength focusing near-field plate

Near-field plates consist of non-periodically patterned surfaces that can overcome the diffraction limit and confine electromagnetic fields to subwavelength dimensions. Previous near-field plates experimentally demonstrated extreme field tailoring capabilities. However, their performance suffered from radiation/reflection in undesired directions, those other than the subwavelength focus. This issue can limit the practical use of near-field plates. In this paper, we address this issue by designing a unidirectional near-field plate that can form a subwavelength focal pattern, while suppressing the field radiated/reflected in other directions. The design and operation of the proposed unidirectional near-field plate are verified through full-wave simulation. The unidirectional near-field plate may find application in high resolution imaging and probing, high density data storage, and wireless power transfer systems. As an example, its utility as a high resolution probe is demonstrated through full-wave electromagnetic simulation.

Axially Steered Spots of Near-Field Fresnel Zone Plate Lens With Sweeping Frequency

The frequency performance of near-field fresnel zone plate (NFFZP) is of fundamental interest in electromagnetic field. An NFFZP lens is designed to focus a spherical wave at a spot. The spots of NFFZP lens are electronically steered forward and backward on the axis according to the operating frequency. Several nonlinear formulas are derived to describe the spots movement with sweeping frequency. The focal shifts of the primary and fractional spots are presented and analyzed. Furthermore, the power drift, the depth of focus, the sidelobe level, and the lens resolution are characterized. All the results are verified by performing full-wave simulations in Ku-band.

Planar Near-Field Plates

This paper presents planar near-field plates that can tailor the electromagnetic near field and produce desired subwavelength focal patterns. These plates consist of concentric annular slots on a circular grounded dielectric slab excited through a coaxial connector. The annular slots are loaded with lumped element impedances which are designed to produce a prescribed subwavelength focal pattern. The procedure to design such plates is outlined. The designed plates are studied through full-wave simulation and the generation of desired subwavelength patterns is demonstrated. Several near-field plates are also fabricated and measured. It is shown that the experimental near-field patterns are in close agreement with simulation. Furthermore, experiments confirming the planar near-field plates ability to tailor the electromagnetic near field are presented. Planar near field plates may find application in near-field imaging and probing systems, biomedical targeting devices, and high density data storage.

Three dimensional subwavelength focus by a near-field plate lens

We implemented the inverse design method to build a thin near-field lens that could produce a desired subwavelength focus by manipulating the near fields of a magnetic dipole source. The flat near-field lens represented by an artificial impedance surface was fabricated by lumped reactive elements (capacitor and inductor) with spatially varying values. In the experiment, a desired annular focusing spot with a characteristic size nearly three times smaller than that allowed by the diffraction limit was obtained. Besides high-resolution imaging, the proposed near-field plate could be extended for other interesting applications, such as wireless power transfer or complex wavefront/beam shaper.

Generating Evanescent Bessel Beams Using Near-Field Plates

We present a near-field plate that can generate an evanescent Bessel beam. The metallic plate consists of nonperiodic concentric corrugations that surround a coaxially fed aperture. The design procedure for such a device is outlined. The designed plate is simulated using a full-wave electromagnetic solver and is shown to produce an evanescent Bessel beam, thereby verifying its design and operation. The performance of the near-field plate is contrasted against a coaxial probe and a near-field plate designed to produce an Airy focal pattern with the same beamwidth. Such a device, capable of producing evanescent Bessel beams, will find applications in near-field probing/imaging systems, data storage, and biomedical devices.

Dual polarized near-field focusing plate for near-field optical focusing in two dimensions

We introduce a dual polarized near-field focusing plate (DP-NFFP) with focusing in two dimensions, designed to operate at the near infrared frequency of 193 THz (λ(0) = 1550 nm). Subwavelength focusing in two dimensions, for both incident polarizations, is achieved at a distance of a quarter wavelength from the DP-NFFP. The design procedure is described in detail and the proposed design could be easily scaled to other working frequencies, from microwave to optics. We show that the use of ideal lossless (i.e., perfect electric conductor) or real lossy (i.e., silver) metals provide with subwavelength focusing at 193 THz, indicating that metal losses do not significantly affect the DP-NFFP performance, and thus confirming the design feasibility at the near-infrared frequency. Results are validated by using two distinct full-wave simulators.

Near-Field Plates: Metamaterial Surfaces/Arrays for Subwavelength Focusing and Probing

In this paper, we present a brief overview of near-field plates, which are nonperiodic grating-like surfaces/arrays that can focus electromagnetic field to subwavelength resolutions. The general properties of near-field plates are described, and the procedure used to design these devices is outlined. The design of two separate near-field plates is discussed in detail. One of the near-field plates produces a subwavelength line (1-D) focus while the other a spot (2-D) focus. Potential applications of near-field plates are also reviewed.

An Experimental Concentric Near-Field Plate

We present a concentrically corrugated near-field plate that can form a subwavelength near-field focal spot. The experimental plate consists of a coaxial aperture surrounded by nonperiodic concentric corrugations. The measured subwavelength patterns are shown to be significantly narrower than those created by a coaxial probe (without corrugations) of similar dimensions. Close agreement between simulated and measurement results is observed. Further, the subwavelength beam emitted by the corrugated near-field plate is shown to be narrower that of the coaxial probe, confirming the superior electromagnetic confinement achieved by the near-field plate over a focal length. Finally, the near-field plate is used to image two sources separated by subwavelength distances. The images obtained using the near-field plate exhibit significantly higher resolution than those obtained using a coaxial probe. The reported near-field plate will find use in near-field probing and microscopy applications.

An analytical investigation of near-field plates

This paper describes an analytical approach to modeling near-field plates, which are non-periodic grating-like structures that can focus electromagnetic waves to subwavelength dimensions. The analysis provides additional insight into the operation and design of such plates that focus cylindrical waves to subwavelength resolutions. Explicit expressions for the current density induced on the plate and its impedance profile are derived. The analytical expressions are validated numerically.

Tailoring near-field patterns with concentrically corrugated plates

We present a device that can form a two-dimensional subwavelength focus along a specified near-field focal plane. The device, referred to as a corrugated near-field plate, consists of a coaxially fed aperture in a concentrically corrugated metallic surface. A procedure for designing such devices is outlined. In addition, simulation results are reported for a design that operates at around 1 GHz and forms a focus with a null-to-null beamwidth of λ/20 at a focal plane λ/15 from its surface. The effect of losses is also studied. Such devices will find use in near-field microscopy and probing systems.

Hypergratings: nanophotonics in planar anisotropic metamaterials

We present a technique capable of producing subwavelength focal spots in planar nonresonant structures not limited to the near-field of the source. The approach combines the diffraction gratings that generate the high-wave-vector-number modes and planar slabs of homogeneous anisotropic metamaterials that propagate these waves and combine them at the subwavelength focal spots. In a sense, the technique combines the benefits of Fresnel lens, near-field zone plates, hyperlens, and superlens and at the same time resolves their fundamental limitations. Several realizations of the proposed technique for visible, near-IR, and mid-IR frequencies are proposed, and their performance is analyzed theoretically and numerically. Generalizations of the developed approach for subdiffractional imaging and on-chip photonics are suggested.

Near-Field Plates: Subdiffraction Focusing with Patterned Surfaces

Using a patterned, grating-like plate to control the electromagnetic near field, we demonstrate focusing well beyond the diffraction limit at approximately 1 gigahertz. The near-field plate consists of only capacitive elements and focuses microwaves emanating from a cylindrical source to a spot of size approximately lambda/20 (half-power beamwidth), where lambda is the free-space wavelength. These plates will find application in antennas, beam-shaping devices, nonradiative wireless power-transfer systems, microscopy, and lithography.