Orthogonal Magnetic Dipoles Research Articles

Bipartite dielectric Huygens’ metasurface for anomalous refraction

Huygens’ metasurfaces—fundamentally based on Schelkunoff's equivalence principle, Huygens’ metasurfaces consist of a two-dimensional array of Huygens’ sources formed by co-located orthogonal electric and magnetic dipoles. Such metasurfaces provide electric and magnetic responses to an incoming electromagnetic (EM) wave, leading to unidirectional scattering and 2π phase coverage. We herein report a near-reflectionless coarsely discretized dielectric Huygens’ metasurface that performs anomalous refraction, offering a low-loss platform for wave manipulation at high frequencies as compared to their lossy metallic analogue. The coarse discretization dramatically simplifies the design, resulting in a metasurface that is highly efficient, cost-effective and robust. In this paper, the proposed metasurface comprises two meta-atoms per period, and is hence named the bipartite dielectric Huygens’ metasurface. Through full-wave simulations at 28 GHz, we show that the proposed metasurface can reroute an incident EM wave from θ i = 15° to θ t = − 44.5° with a very high efficiency: 87% of the scattered power is anomalously transmitted to θ t . Based on our observations, a coarsely discretized dielectric Huygens’ metasurface platform can be efficacious to design meta-devices with multifaceted functionalities in different frequency regimes.

Circularly Polarized Scattering Radiation From a Silicon Nanosphere

AbstractA dielectric nanosphere with orthogonal electric dipole (ED) and magnetic dipole (MD) Mie resonances can be a nanoantenna radiating circularly polarized light in specific directions if the amplitudes and the phase relations are properly designed. First, theoretical calculations show that a silicon nanosphere illuminated with a linearly polarized plane wave radiates circularly polarized light at the wavelength in between the ED and MD resonances if the refractive index of a surrounding medium (nm) is ≈1.3; the ellipticity of the scattered light can be >0.99 when nm is in a 1.19–1.35 range. Size‐purified silicon nanospheres suspended in water (nm = 1.33) are then prepared, and the angle‐ and circular‐polarization‐resolved scattering spectra are studied. It is experimentally demonstrated that circularly polarized light is radiated in specific directions under linearly polarized plane wave illumination. The results also show that the wavelength of the radiation of circularly polarized light can be controlled in the whole visible range by controlling the silicon nanosphere diameter in 100–200 nm range.

Wearable Ungrounded Tag Antenna for UHF RFID Applications

A single-layer ungrounded tag antenna is designed for on-body radio frequency identification (RFID) applications in the ultrahigh frequency (UHF) band. The ungrounded tag antenna is designed based on a broadside radiating Huygens source. The electric dipole is generated by the center part of an original dipole, while two orthogonal magnetic dipoles are created by stacking another two metallic strips at the edge of the original dipole. The tag antenna shows good robustness of input matching and radiation efficiency with respect to the human-body effect. Furthermore, the designed tag antenna is also able to work with metallic objects and its resonant frequency is stable around 920 MHz. Finally, three tag prototypes, including the proposed tag antenna, are fabricated and tested. The measured reading range confirms the numerical simulations and demonstrates the robustness of the proposed design with respect to different distances between human-body and tag antenna. The proposed tag provides a reading range of 5.0 m (near-body), 8.0 m (free space), and 16.5 m ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$200\times200$ </tex-math></inline-formula> mm metallic plate), respectively.

A Rotating-Permanent-Magnet Array for ULF Through-the-Sea Magnetic Communications

To realize magnetic induction (MI) communication through seawater, a rotating-permanent-magnet array (RPMA) operating in the ultralow frequency (ULF) band is proposed. The array consists of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5\times $ </tex-math></inline-formula> 5 cylindrical Nd–Fe–B magnets driven by servo motors. To maximize the transmitting magnetic moment, the motors are controlled to rotate synchronously through network communication. The design of the magnet and array configuration is implemented by analyzing the torque and magnetic moment. Magnetic forces between the array magnets are analyzed by using the equivalent static magnetic dipole model. Permalloy sleeves are used to reduce the influence of magnetic force on the synchronization of magnets. The total transmitting magnetic moment of the RPMA is 360 A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot ~\text{m}^{2}$ </tex-math></inline-formula> , and its power efficiency is 14.4 times higher than the conventional transmitting system using coils. Binary frequency shift keying (BFSK) modulation is realized by switching the rotating speed of the magnets and verified by an indoor test. The equivalent orthogonal horizontal magnetic dipole (HMD) model of the array is used, and the magnetic field distribution in layered media is simulated using FEKO. The calculated MI is 100 fT in the position with a distance of 1 km and a depth of 5 m in the sea. The results of the through-the-sea experiment show that the magnetic field of the RPMA can be detected in a range of 500 m by magnetic inductive sensors immersed at a depth of 5 m in seawater and agree well with the simulated results. Transmission bit rates of 10 and 5 b/s can be achieved in the positions of 60 and 110 m.

Efficient Spin‐Direction Coupling between Circular Polarized Electric and Magnetic Dipoles and Photonic Modes

AbstractIntegrated single photon sources are essential elements in quantum computing, simulation, communication, and photonic neural networks. The directional radiation and scattering of single‐photon sources play a crucial role in light manipulation and rely on electric and magnetic dipole moments. Although clever physical insights and designer intuition strategies have been successfully applied in the development of integrated sources, inverse strategies could enhance and maximize its performance. Recently, topology‐optimized couplers for on‐chip single‐photon sources are designed to efficiently couple a guided mode and an electric dipole. However, the superposition of orthogonal electric and magnetic dipoles can also be harnessed due to the additional degrees of freedom via their interference. Here, the authors have extended the strategy to couplers that enhance spin‐direction coupling of circularly polarized, Huygens, and Janus dipoles. The authors demonstrate that optimization not only increases the coupling to a desire mode but also enhances the electric and magnetic local density of states (LDOS) while maintaining the amplitude and phase relation between the orthogonal dipoles for unidirectional coupling. Currently, coupling efficiency and enhanced LDOS of up to 88%, 94%, and 93% and up to 9.4, 18.6, and 14.9 are obtained for these dipoles. The topology optimization improves the performances of 3D integrated photonic devices.

A Planar Bidirectional Circularly Polarized Antenna Using Orthogonal Magnetic Dipoles Without Extra Phase Shift Line

A planar bidirectional circularly polarized (CP) antenna using orthogonal magnetic dipoles (OMDs) without an extra phase shift line is proposed in this work. It consists of a rectangular microstrip magnetic dipole antenna (MMDA) and a loop radiator with two arms directly connected to the open edge of MMDA. The working principle is revealed by the equivalent sources model to provide a clear physical insight at first. The theoretical investigations imply that by controlling the width and the feed locations of the loop antenna on the aperture, the phase and magnitude for CP radiation can be realized in both endfire and backfire directions without introducing an additional phase shift line structure. For demonstration, one antenna prototype is designed and fabricated. The experimental results indicate that an overlapping bandwidth of 5.91% from 2.30 to 2.44 GHz is attained. Compared with the counterpart using OMDs, the proposed antenna exhibits a low design complexity and improved gains of 3.56/1.83 dBic in the endfire/backfire directions.

Polarization-independent near-infrared superabsorption in transition metal dichalcogenide Huygens metasurfaces by degenerate critical coupling

Strong near-infrared absorption of light in semiconducting transition metal dichalcogenides (TMDs) is essential for improving the photocarrier extraction efficiency in optoelectronic devices. Here, we numerically demonstrate that an original TMD Huygens metasurface is specifically designed to overcome the 50% absorptance limit of a subwavelength thin film. The unique metasurface comprising a TMD nanodisk array shows evidence of characteristic Mie resonances including electric and magnetic dipoles. By carefully optimizing the aspect ratio of nanodisks, the extraordinary spectral overlapping of orthogonal electric and magnetic dipole resonances is successfully realized and, thus, enables the super-high absorptance up to 87% in a subwavelength-thin ($\ensuremath{\approx}0.19{\ensuremath{\lambda}}_{0}$) semiconductor structure by degenerate critical coupling at the wavelength of ${\ensuremath{\lambda}}_{0}=903\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. All numerical results are confirmed by the temporal coupled-mode theory and again via the multipole decomposition method. Importantly, the near-unity absorptance in this TMD Huygens metasurface by degenerate critical coupling not only is tailored throughout the entire near-infrared region but also is both polarization independent and angle insensitive. The absorptance maintaining &gt;80% is observed with the incident angle up to \ifmmode\pm\else\textpm\fi{}10 \ifmmode^\circ\else\textdegree\fi{} at least. Our findings are not only of fundamental interest but could also offer a platform for TMD-based high-speed photodetecting, energy harvesting, and thermal emission.

An Analytic Algorithm for Dipole Electromagnetic Field in Fully Anisotropic Planar-Stratified Media

This article presents an efficient analytic algorithm to calculate the electromagnetic (EM) fields radiated from orthogonal magnetic and electric dipoles in the fully anisotropic planar-stratified media. The EM fields are transformed into the frequency–wavenumber domain by the double Fourier transform, which yields a set of ordinary differential equations, consisting of a system matrix, a field vector, and a source vector. The eigenvalues of the system matrix are uniquely determined, but the corresponding eigenvectors can be multiplied by an arbitrary constant or be normalized by energy. With these eigenvalues and eigenvectors, the field vectors are expressed as mode fields consisting of the up-going and down-going fields. The propagation of the mode fields in each layer is described by the generalized reflection/transmission method. To accelerate the evaluation of the inverse Fourier transform of EM field from spectral domain to spatial domain, an approximated primary field was subtracted from total field. Numerical results from a finite element method (FEM) validate the proposed method. Then, we apply this new algorithm to a variety of real applications in geophysical prospecting.

Tunable circularly polarized graphene antenna for THz applications

A technique is implemented for the generation of tunable circularly polarized THz wave using patterned graphene patch. The aspect ratio of graphene patch is selected to operate with the harmonics at frequency 1.7, 2.23, 3.26 and 3.41 THz covering three bands 1.67 - 1.73, 2.18 - 2.27 and 3.17 - 3.49 THz. The graphene patch is perturbed by inserting a patterned slot to generate the set of orthogonal magnetic dipoles and hence the CP wave. Insertion of slot shifts the resonant frequency of the generated harmonics at 2.95, 3.59, 4.89 and 5.068 THz covering the bands 2.86 - 2.99, 3.54 - 3.66 and 4.83 - 5.12 THz. Also, the dual-band circular polarization is obtained with 3-dB axial ratio bandwidth of 1.68 % (2.94 - 2.99 THz) and 0.4 % (4.99 - 5.01 THz) in the first and third operating band, respectively. Furthermore, the utilization of graphene in the radiator provides the frequency tunability of generated CP wave by varying the chemical potential. The antenna can be utilized in the biomedical applications at THz frequency band.

An Analytic Algorithm for Electromagnetic Field in Planar-Stratified Biaxial Anisotropic Formation

An analytic algorithm is developed to solve the electromagnetic (EM) field excited by orthogonal magnetic dipoles in the multilayered biaxial anisotropic medium. It allows forward modeling and inversion of EM well logging in the planar-stratified formation without borehole and invasion zones. First, the transverse components of the EM fields are divided into up/down-going waves in the frequency wavenumber domain. A novel concept of reflection/transmission is introduced to describe the propagation of the EM waves through the boundaries. Different from previous work, those reflection/transmission coefficients are concerning the propagation of up/down-going waves directly rather than the amplitude of the total EM field in each layer, which makes the derivation process clearer and easier to understand. Then, we adopt the cubic spline interpolation to calculate the 2-D Fourier integral that transforms the field from the spectral domain to the spatial domain, and an efficient sampling rule is also provided to achieve high accuracy and stability in numerical calculation. For avoiding the singularity when the field locations are close to the source along the vertical direction, the primary field is subtracted in the spectral domain to make the integrand decay fast. Finally, we validate our new formulas compared with another numerical method and analyze the responses of the triaxial induction tool in biaxial anisotropic formations.

Dual-Mode Compression of Dipole Antenna by Loading Electrically Small Loop Resonator

This communication presents a dual-mode dipole antenna for impedance bandwidth improvement. In our approach, an electrically small loop (ESL) resonator is properly loaded to compress the operating frequencies of first- and third-order modes. With the existence of ESL, the third-order mode shifts to its first-order mode to obtain a wide bandwidth with stable polarization and radiation patterns, due to the fact that ESL is equivalent to an orthogonal magnetic dipole at the third-order mode. To validate the design strategy, a prototype was fabricated and tested. Omnidirectional radiation pattern in H-plane is achieved over the entire operating band from 2.94 to 3.89 GHz while keeping a smaller size of $0.45\lambda _{0}\times 0.17\lambda _{0} (\lambda _{0}$ is the wavelength in free space at the center frequency). Good agreement has been obtained between measurement and simulation.

An Additively Manufactured 3-D Antenna-in-Package With Quasi-Isotropic Radiation for Marine Animals Monitoring System

A low-cost and additively manufactured three-dimensional (3-D) antenna-in-package with quasi-isotropic radiation is proposed for a marine animals monitoring system. The antenna is based on a meandered dipole folded as a split-ring resonator structure, which can generate simultaneously a pair of orthogonal electric and magnetic dipoles, thus providing a quasi-isotropic radiation pattern. The antenna (integrated with a balun) has been inkjet-printed on a 3-D-printed buoyant cone structure, which acts also as the system package to house the electronics and the battery. The antenna designed at 2.4 GHz is electrically small, with a $ka\; = 0.49$ , and has a bandwidth of 70 MHz (2.9%). The measured gain deviation of the antenna (maximum to minimum) is near 3 dB in bandwidth, thus qualifying it as a quasi-isotropic antenna. Field tests of the antenna in the active state (integrated with the electronics) confirm a reliable communication range of 240 m in any direction in the azimuthal plane.

A new approach to vector scattering: the 3s boundary source method.

This paper describes a novel Boundary Source Method (BSM) applied to the vector calculation of electromagnetic fields from a surface defined by the interface between homogenous, isotropic media. In this way, the reflected and transmitted fields are represented as an expansion of the electric fields generated by a basis of orthogonal electric and magnetic dipole sources that are tangential to, and evenly distributed over the surface of interest. The dipole moments required to generate these fields are then calculated according to the extinction theorem of Ewald and Oseen applied at control points situated at either side of the boundary. It is shown that the sources are essentially vector-equivalent Huygens' wavelets applied at discrete points at the boundary and special attention is given to their placement and the corresponding placement of control points according to the Nyquist sampling criteria. The central result of this paper is that the extinction theorem should be applied at control points situated at a distance d = 3s (where s is the separation of the sources) and consequently we refer to the method as 3sBSM. The method is applied to reflection at a plane dielectric surface and a spherical dielectric sphere and good agreement is demonstrated in comparison with the Fresnel equations and Mie series expansion respectively (even at resonance). We conclude that 3sBSM provides an accurate solution to electromagnetic scattering from a bandlimited surface and efficiently avoids the singular surface integrals and special basis functions proposed by others.

Performance analysis of spinning magnet as mechanical antenna

Long wavelength results in the low radiation efficiency of a portable conventional antenna operating at very low frequency (VLF) and below. This has motivated one to develop an innovative approach to design an electrically small antenna in a frequency band lower than VLF. The time-varying electromagnetic fields can be generated by spinning a permanent magnet. In this way, the mechanical energy is converted to the electromagnetic energy, and the impedance matching networks with nonnegligible insertion loss are not required. Therefore, this mechanical antenna with spinning magnet can improve radiation efficiency in a low frequency band. In this paper, we give the detailed analysis procedure for the spinning magnet, which is seldom discussed in other published reports. In order to analyze the electromagnetic characteristics of the spinning magnet, in this paper we use the ampere return circuit theorem to investigate the equivalent relation between a spinning magnet and the orthogonal magnetic dipole. We introduce an initial spinning angle of the magnet into the dyadic green’s function. With this modification, we provide the rigorous analytic formula for field computation of the orthogonal magnetic dipole. Thus the electromagnetic characteristics of the spinning magnet and spinning magnet array can also be analyzed. For a spinning NdFeB magnet with a magnetization of &lt;i&gt;B&lt;/i&gt;&lt;sub&gt;r&lt;/sub&gt; = 0.8 T and a volume of &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;r&lt;/sub&gt; = 270 cm&lt;sup&gt;3&lt;/sup&gt; as well as 9600 revolutions per minute, the simulation results reveal that the magnetic field of 15 fT at 1 km in air space can be obtained. But the magnetic field of the spinning magnet decreases quickly to 1 fT at 250 m in sea water. Considering the potential demand for increasing the field strength in the near field region, we recommend to use a magnet array with small-sized elements. The magnet array can be used to control the near field pattern. We take two magnets as an example for studying the performance. It can be found from the simulation results that the magnetic near field is increased by 3 dB with the linear magnet array consisting of two elements. With the initial spinning angle of the magnet element adjusted, the near field pattern of the magnet array can be controlled. This is analogous to beam steering of traditional phased array for high band operation. It can be concluded from our study that the spinning magnet is a possible alternative solution for low frequency small transmitter antenna.

A Wideband Dual-Polarized Dielectric Magnetoelectric Dipole Antenna

A new wideband ±45° dual-polarized metal loop dielectric resonator magnetoelectric antenna is proposed in this communication. The antenna consists of two orthogonal dielectric bars that support two orthogonal electric dipoles and two cross interlaced metal semi-loops that are equivalent to two orthogonal magnetic dipoles above the ground. With a legitimate combination of the electric and magnetic dipoles, a unidirectional radiation with low backward radiation and equal E-plane and H-plane radiation patterns can be achieved. The antenna can be made of a monolithic dielectric block with simple installation. To validate the new antenna configuration, a prototype is designed, manufactured, and measured. The prototype antenna using ceramic with the electrical size of $0.33\lambda _{o} \times 0.33\lambda _{o} \times 0.21\lambda _{o}$ demonstrates that an impedance bandwidth of 11.4% ( $\vert S_{11}\vert dB) in the 3.5 GHz frequency band can be achieved with the measured forward/backward ratio of better than 22 dB. The proposed antenna is suitable as antenna element for massive-MIMO array antennas, where a large number of antenna elements need to be installed in a compact space.

Unidirectional excitation of plasmonic waves via a multilayered metal-dielectric-metal Huygens' nanoantenna.

Huygens' nanoantennas maintain orthogonal electric and magnetic dipole resonances satisfying the Kerker condition and can generate directional radiation in both the near-field and far-field regimes. Here we study a multilayered metal-dielectric-metal (MDM) Huygens' type nanoantenna which is capable of launching surface plasmon polaritons (SPPs) unidirectionally when excited by a dipole source. We show that the radiative decay rates of the dipole source are strongly enhanced by the antenna, and the generated SPP waves propagate in opposite directions at two different wavelengths. The directionality of the excited SPPs can be switched by changing the geometry and the material composition. We further demonstrated that the beam width of the SPP waves can be narrowed by arranging the MDM antennas in a chain.

Huygens’ metasurfaces from microwaves to optics: a review

AbstractIn this article, the basic principles and the main applications of Huygens’ metasurfaces (HMSs) are reviewed from microwaves to optics. In general, HMSs comprise a thin layer of orthogonal electric and magnetic dipoles, which form an array of Huygens’ sources. In a refraction setting, these sources radiate mostly in the forward direction and can be used to manipulate an incident electromagnetic wave at will. In the case of passive HMSs, the Huygens’ sources are induced by an incident electromagnetic field. Examples of passive manipulations include reflectionless refraction, perfect anomalous reflection, and arbitrary antenna beam forming. In the case of active HMSs, the Huygens’ sources are impressed active sources. Active HMS manipulations include cloaking and subwavelength spot formation in a cavity environment.

Quality factor assessment of finite-size all-dielectric metasurfaces at the magnetic dipole resonance

Recently there has been a large interest in achieving metasurface resonances with large quality factors. In this article, we examine metasurfaces that comprised a finite number of magnetic dipoles oriented parallel or orthogonal to the plane of the metasurface and determine analytic formulas for their resonances’ quality factors. These conditions are experimentally achievable in finite-size metasurfaces made of dielectric cubic resonators at the magnetic dipole resonance. Our results show that finite metasurfaces made of parallel (to the plane) magnetic dipoles exhibit low quality factor resonances with a quality factor that is independent of the number of resonators. More importantly, finite metasurfaces made of orthogonal (to the plane) magnetic dipoles lead to resonances with large quality factors, which ultimately depend on the number of resonators comprising the metasurface. In particular, by properly modulating the array of dipole moments by having a distribution of resonator polarizabilities, one can potentially increase the quality factor of metasurface resonances even further. These results provide design guidelines to achieve a sought quality factor applicable to any resonator geometry for the development of new devices such as photodetectors, modulators, and sensors.

Radiation Null Compensation and Isolation Enhancement Between Two Orthogonal Magnetic Dipole Antennas

New approaches to tackle the radiation null and enhance the isolation between two magnetic dipole antennas are presented in this letter. By employing metal-via wall, the magnetic dipole and half-mode substrate-integrated waveguide (HMSIW) cavity are formed. Based on the orthogonal configuration of the magnetic dipoles, null compensation contributes to the nonblind point on the radiation pattern of the dipole. Furthermore, a defected surface structure is adopted to be inserted into the HMSIW cavity. The isolation enhancement with 42.6 dB between the antennas is achieved. With high isolation, the orthogonal integration of the magnetic dipoles is realized. The antenna is fabricated and measured to verify the proposed design.

Splitting of magnetic dipole modes in anisotropic TiO2 micro‐spheres (Laser Photonics Rev. 10(4)/2016)

The sphere in the centre of the cover picture is an SEM (scanning electron microscopy) image of one of the titanium dioxide spheres studied by Khromova et al. The incoming broadband terahertz (THz) pulse (white pulse and white beam) excites two non-degenerate orthogonal magnetic dipole modes in the sphere (due to material anisotropy). These modes are represented as ripples of the excited propagating fields (blue and red) and as corresponding signature pulses (red and blue) as detected in the authors' experiment using time-domain spectroscopy. The blue and red colours as well as the pulse shapes represent the difference in the resonance frequency of the two excited modes. (Picture: Irina Khromova et al., 10.1002/lpor.201600084 pp. 681–687, in this issue)