Huygens Source Research Articles

Isotropic Huygens sources made of clusters of nanoparticles for metasurfaces applications

We present spherical clusters, composed by spherical dielectric or metallic inclusions, as a new kind of efficient and isotropic Huygens sources. We demonstrate that this design can allow a large and multimode overlapping of the electric and magnetic resonances excited at visible frequencies. They may also serve as building blocks for metasurface applications. We investigate their possible uses in high transmittance devices requiring a local phase control, and in thin absorber metalattices. They are particularly suited to bottom-up fabrication and self-assembly, offering an alternative to the classical lithography compatible Huygens meta-atom.

Source Current Polarization Impact on the Cross-Polarization Definition of Practical Antenna Elements: Theory and Applications

With the growing interest in polarization diversity in communications and radar systems, the use of Ludwig's second and third definitions has become controversial among scientists and antenna engineers. Therefore, this paper is an attempt to clarify some of the ambiguity and confusion caused by these definitions. A detailed comparison of Ludwig's second and thirrd definitions of cross polarization, as applied to linearly polarized antennas, was performed. The results show that, in the diagonal plane, Ludwig's second definition leads to a lower crosspolarization level than the third definition for x- or y-polarized current sources. For a Huygens source, by Ludwig's third definition, the radiation pattern has a lower cross-polarization level than that obtained by Ludwig's second definition. For some applications, the antenna is usually placed in the yz plane. Therefore, new polarization bases are proposed according to which the source is used as a reference, and also on how this source is oriented in the yz plane. To complement the theoretical framework demonstrated in this contribution and to provide readers with a better and simpler understanding of the crosspolarization definition, the analysis of several practical antennas for diverse applications was presented. Numerical and measured radiation patterns of wire and printed dipoles, rectangular patch, pyramidal horn, and open-ended rectangular waveguide (OEWG) antennas were investigated according to the polarization formulations presented in this paper. In addition, a dual-polarized element and a dual-polarized active phased array at broadside were utilized to generalize the application.

Design of highly transmissive all-dielectric metasurface based on silicon nanodisks

Using the developed discrete dipole approximation method, we investigate the behavior of silicon nanodisks acting as Huygens sources. First, we study the correlation between the radius of a silicon nanodisk and its electric resonance and also between the height of a nanodisk and its magnetic resonance and find a significant correlation between them. Then, we investigate the transmission of an array of silicon nanodisks. Using the overlap between the electric and magnetic resonances, we obtain a nearly perfect impedance match between the environment and silicon nanodisk array. We achieve a transmission above 99% over the entire phase range of 0–2π radians at the wavelength of 1190 nm. Our results can be extended to microwave, terahertz and optical frequencies and be used to design metasurfaces for different applications such as beam shaping lenses, beam deflectors and holograms.

Janus and Huygens Dipoles: Near-Field Directionality Beyond Spin-Momentum Locking.

Unidirectional scattering from circularly polarized dipoles has been demonstrated in near-field optics, where the quantum spin-Hall effect of light translates into spin-momentum locking. By considering the whole electromagnetic field, instead of its spin component alone, near-field directionality can be achieved beyond spin-momentum locking. This unveils the existence of the Janus dipole, with side-dependent topologically protected coupling to waveguides, and reveals the near-field directionality of Huygens dipoles, generalizing Kerker's condition. Circular dipoles, together with Huygens and Janus sources, form the complete set of all possible directional dipolar sources in the far- and near-field. This allows the designing of directional emission, scattering, and waveguiding, fundamental for quantum optical technology, integrated nanophotonics, and new metasurface designs.

A Study of 28 GHz, Planar, Multilayered, Electrically Small, Broadside Radiating, Huygens Source Antennas

Two 28 GHz, planar, electrically small Huygens source antennas are presented that are broadside radiating and are based on multilayer PCB technology. The designs seamlessly integrate electric Egyptian axe dipole and magnetic capacitively loaded loop near-field resonant parasitic elements with a coax-fed dipole radiator. Both linearly polarized (LP) and circularly polarized (CP) systems are demonstrated. The simulations of the LP system indicate that it is electrically small: ka = 0.961; has peak realized gains and front-to-back ratios (FTBRs) in the range from, respectively, 3.77 to 4.54 dBi and 7.16 to 33.92 dB; and radiation efficiencies higher than 81.14% over its entire 2.14%, −10-dB fractional impedance bandwidth (FBW−10dB). A prototype was fabricated and tested; the measured and simulated results are in good agreement. The CP system exhibits similar properties: ka = 0.942; 1.41% FBW−10dB with a 0.47% 3-dB axial ratio fractional bandwidth; and peak realized gain, FTBR, and radiation efficiency values equal to 2.03 dBi, 26.72 dB, and 73.4%, respectively. To confirm their efficacy for on-body applications, the specific absorption rate values of both the LP and CP Huygens source antennas were evaluated and found to be very low.

Isotropic Huygens dipoles and multipoles with colloidal particles

Huygens sources are elements that scatter light in the forward direction as used in the Huygens-Fresnel principle. They have remained fictitious until recently where experimental systems have been fabricated. In this letter, we propose isotropic meta-atoms that act as Huygens sources. Using clusters of plasmonic or dielectric colloidal particles, Huygens dipoles that resonate at visible frequencies can be achieved with scattering cross-sections as high as 5 times the geometric cross-section of the particle surpassing anything achievable with a hypothetical simple spherical particle. Examples are given that predict extremely broadband scattering in the forward direction over a 1000 nm wavelength range at optical frequencies. These systems are important to the fields of nanoantennas, metamaterials and wave physics in general as well as any application that requires local control over the radiation properties of a system as in solar cells or bio-sensing.

Degrees of Freedom and Maximum Directivity of Antennas: A bound on maximum directivity of nonsuperreactive antennas

We derive a general fundamental directivity limitation formula that applies to nonsuperreactive antennas of any size that fit within a minimum sphere of any given radius rmin. The derivation is done by using a new concept: the degrees of freedom (DoF) of the field radiated by arbitrary sources within the minimum sphere must be twice the maximum directivity. The formula converges to the known bound of the directivity for large rmin. For small spheres, it becomes equal to three, i.e., 4.8 dBi, which is the directivity of the Huygens source. The transition region between these two limiting cases is determined by counting the most significant spherical modes at the surface of the minimum sphere. This is not trivial, because spherical modes have a gradual cutoff when their order approaches krmin. Therefore, we use a weighted summation where the weighting factor is inversely proportional to the radiation-Q of the modes. This extends the DoF from a discrete to continuous function of the minimum sphere radius. The final maximum directivity is similar to a previously published heuristic limit. The formulas are compared to results for measured antennas with large directivity and superdirectivity.

Low-Profile, Electrically Small, Huygens Source Antenna With Pattern-Reconfigurability That Covers the Entire Azimuthal Plane

A pattern-reconfigurable, low-profile, efficient, electrically small, near-field resonant parasitic (NFRP), Huygens source antenna is presented. The design incorporates both electric and magnetic NFRP elements. The electric ones are made reconfigurable by the inclusion of a set of p-i-n diodes. By arranging these electric and magnetic NFRP elements properly, a set of three Huygens sources are attained, each covering a 120° sector. Pattern reconfigurability is obtained by switching the diodes on or off; it encompasses the entire 360° azimuth range. A prototype was fabricated and tested. The numerical and experimental studies are in good agreement. The experimental results indicate that in each of its instantaneous states at f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 1.564 GHz, the antenna provides uniform peak realized gains, front-toback ratios, and radiation efficiencies, respectively, as high as 3.55 dBi, 17.5 dB, and 84.9%, even though it is electrically small: ka = 0.92, and low profile: 0.05 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> .

Vectorial control of nonlinear emission via chiral butterfly nanoantennas: generation of pure high order nonlinear vortex beams.

We report on a chiral gap-nanostructure, which we term a "butterfly nanoantenna," that offers full vectorial control over nonlinear emission. The field enhancement in its gap occurs for only one circular polarization but for every incident linear polarization. As the polarization, phase and amplitude of the linear field in the gap are highly controlled, the linear field can drive nonlinear emitters within the gap, which behave as an idealized Huygens source. A general framework is thereby proposed wherein the butterfly nanoantennas can be arranged in a metasurface, and the nonlinear Huygens sources exploited to produce a highly structured far-field optical beam. Nonlinearity allows us to shape the light at shorter wavelengths, not accessible by linear plasmonics, and resulting in high purity beams. The chirality of the butterfly allows us to create orbital angular momentum states using a linearly polarized excitation. A third harmonic Laguerre-Gauss beam carrying an optical orbital angular momentum of 41 is demonstrated as an example, through large-scale simulations on a high-performance computing platform of the full plasmonic metasurface with an area large enough to contain up to 3600 nanoantennas.

Electrically Small, Broadside Radiating Huygens Source Antenna Augmented With Internal Non-Foster Elements to Increase Its Bandwidth

A broadside radiating, linearly polarized, electrically small Huygens source antenna system that has a large impedance bandwidth is reported. The bandwidth performance is facilitated by embedding non-Foster components into the near-field resonant parasitic elements of this metamaterial-inspired antenna. High-quality and stable radiation performance characteristics are achieved over the entire operational bandwidth. When the ideal non-Foster components are introduced, the simulated impedance bandwidth witnesses approximately a 17-fold enhancement over the passive case. Within this –10-dB bandwidth, its maximum realized gain, radiation efficiency, and front-to-back ratio (FTBR) are, respectively, 4.00 dB, 88%, and 26.95 dB. When the anticipated actual negative impedance convertor circuits are incorporated, the impedance bandwidth still sustains more than a 10-fold enhancement. The peak realized gain, radiation efficiency, and FTBR values are, respectively, 3.74 dB, 80%, and 28.01 dB, which are very comparable to the ideal values.

Design and Testing of Simple, Electrically Small, Low-Profile, Huygens Source Antennas With Broadside Radiation Performance

The efficacy of a simple, electrically small, low-profile, Huygens source antenna that radiates in its broadside direction is demonstrated numerically and experimentally. First, two types of electrically small, near-field resonant parasitic (NFRP) antennas are introduced and their individual radiation performance characteristics are discussed. The electric one is based on a modified Egyptian axe dipole NFRP element; the magnetic one is based on a capacitively loaded loop NFRP element. In both cases, the driven element is a simple coax-fed dipole antenna, and there is no ground plane. By organically combining these two elements, Huygens source antennas are obtained. A forward propagating demonstrator version was fabricated and tested. The experimental results are in good agreement with their analytical and simulated values. This low profile, $\sim 0.05\lambda _{0}$ , and electrically small, $ka = 0.645$ , prototype yielded a peak realized gain of 2.03 dBi in the broadside direction with a front-to-back ratio of 16.92 dB. A backward radiating version is also obtained; its simulated current distribution behavior is compared with that of the forward version to illustrate the design principles.

Single-order transmission diffraction gratings based on dispersion engineered all-dielectric metasurfaces.

A single-order transmission diffraction grating based on dispersion engineered all-dielectric metasurfaces is proposed, and its wavelength discriminating properties have been theoretically described and confirmed using numerical simulations. The metasurface is designed using a 2D array of all-dielectric resonators, which emulates a Huygens' source configuration to achieve a perfect match to free space in broad bandwidth. Using a holey dielectric nanodisk structure as the unit cell, the resonant wavelength is tapered across the metasurface to engineer the wavelength-dependent spatial phase gradient, to emulate a dispersive prism. Consequently, different wavelengths are steered toward different directions and thus are discriminated on the output image plane. Due to the subwavelength periodicities involved, wavelength discrimination is achieved directly in the zeroth diffraction order of the device, unlike conventional diffraction gratings, thereby providing a high-efficiency wavelength discriminating device.

Helical-Ring Antenna for Hemispherical Radiation in Circular Polarization

A compact helical-ring antenna is proposed to realize a Huygens source in circular polarization. The radiation pattern of this antenna is mainly directed toward an half-space without the use of any reflector. The principle relies on the combination of two sets of collocated electric and magnetic short dipoles. The helical ring is excited by means of in-quadrature capacitive probes. Solutions are proposed to adjust the resonant frequency and balance the contributions of the magnetic and electric dipoles. The direction of the radiation pattern can be electrically switched frontward or backward by modifying the in-quadrature probes. Besides, the size of the antenna can be reduced by increasing the number of turns, therefore, without high-permittivity materials. The properties of this antenna are analyzed by means of simulations and confirmed via measurements on a prototype.

Low-reflection beam refractions by ultrathin Huygens metasurface

We propose a Huygens source unit cell to develop an ultrathin low-reflection metasurface, which could provide extreme controls of phases of the transmitted waves. Both electric and magnetic currents are supported by the proposed unit cell, thus leading to highly efficient and full controls of phases. The coupling between electric and magnetic responses is negligible, which will significantly reduce the difficulty of design. Since the unit cell of metasurface is printed on two bonded boards, the fabrication process is simplified and the thickness of metasurface is reduced. Based on the proposed unit cell, a beam-refracting metasurface with low-reflection is designed and manufactured. Both near-field and far-field characteristics of the beam-refracting metasurface are investigated by simulations and measurements, which indicate that the proposed Huygens metasurface performs well in controlling electromagnetic waves.

Polarization Considerations for Scalar Huygens Metasurfaces and Characterization for 2-D Refraction

Transmitarrays and transmissive metasurfaces must efficiently couple incident power to the transmitted beam. Inefficiency is manifested in sidelobe levels, reflections, and insertion loss. The Huygens metasurface embodies the Huygens and equivalence principles, suppressing these sidelobe levels and reflections. This is accomplished with a single thin layer of Huygens sources, which contains both an electric and a magnetic response. In this paper, 2-D interfacial refraction is implemented with a scalar Huygens metasurface. The measured total efficiency is on average 71.06% for a 70° range, the maximum being 80.87% at θ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> =0°. Moreover, it is on average 68.64% over a fractional bandwidth of 8%, the maximum being 80.87% at 10.0 GHz. This demonstrates that the insertion loss, reflection, and sidelobe powers are low in our design. Furthermore, the questions of polarization purity, and the appropriate polarization definition for a scalar Huygens metasurface, are addressed. Our design contains only printed elements, and consists of two bonded boards, instead of many stacked interspaced layers. This simplifies fabrication, and makes it scalable to millimeter-wave frequencies and beyond. The design is also λ/9.3 thick, in contrast to traditional transmitarrays, which require 3-4 λ/4 spaced layers to obtain the same degree of phase control and matching.

A Low-Profile, Low-Backlobe and Wideband Complementary Antenna for Wireless Application

A new complementary antenna (or Huygens source) with a very low profile is presented. This antenna, composed of a pair of planar monopoles, a pair of bowtie shaped slots and a microstrip antenna, provides a broadband operation while its height is only $0.035 \lambda_{{{0}}}$ , where $\lambda _{{{0}}}$ is the wavelength referring to the center frequency of the operating band. More importantly, the proposed antenna radiates unidirectionally without the need of a metallic reflector. And therefore the total antenna volume is reduced to $0.011 \lambda _{{{0}}}^{{{3}}}$ . A prototype was fabricated and measured. It exhibits an impedance bandwidth $({\rm SWR} \leq 2)$ of 20.7% from 1.387 GHz to 1.696 GHz, a stable boresight gain of 6.11 dBi and a unidirectional radiation pattern with low back radiation ( ${ dB), low cross-polarization ( ${ dB).

Low Profile, Broadside Radiating, Electrically Small Huygens Source Antennas

It is demonstrated numerically that a metamaterial-inspired, low profile (height approximately λ/80), electrically small (ka = 0.45) Huygens source antenna can be designed to radiate at 300 MHz in its broadside direction with a high radiation efficiency and a large front-to-back ratio. Two electrically small, near-field resonant parasitic (NFRP) antennas are first designed. Both are based on a coax-fed dipole antenna. An electric dipole response is obtained by combining it with a tunable Egyptian axe dipole (EAD) NFRP element. A magnetic dipole response is obtained by spatially loading the driven dipole with tunable, extruded capacitively loaded loop (CLL) NFRP elements. The driven dipole and the EAD and CLL NFRP elements are combined together and retuned to achieve a broadside radiating Huygens source antenna. Two different designs, one with two CLL elements and one with four, are obtained, and their performance characteristics are compared.

Wideband Compact 4-Port Dual Polarized Self-Grounded Bowtie Antenna

We present a dual-polarized self-grounded bowtie antenna. The antenna is mechanically simple and wideband. One of the characteristics is its flexibility: it can be used either as a 4-port antenna in multiple-input-multiple-output (MIMO) systems or as a directional 2-port dual-polarized antenna in radar or sensor systems. The latter application requires a balun. The 4-petal geometry of the antenna has been optimized for the best matching to 50 Ω coax over 1.5-3 GHz. Both simulated and measured results are presented to verify the design. The antenna radiates over parts of the bandwidth similar to four Huygens source, one from each petal, each having directivity close to 4.8 dBi.

Nonlinear active Huygens metasurfaces for reflectionless phase conjugation of electromagnetic waves in electrically thin layers

A possibility of almost perfect phase conjugation in electrically thin nonlinear sheets is shown for the normal incidence of the signal wave, using either symmetrical or asymmetrical particles and nondegenerate interaction with slightly different frequencies of the incident and phase-conjugated waves. Under these conditions, the use of linear and nonlinear Huygens sources for both linear (signal) and nonlinear (phase-conjugated) waves ensures zero or very small reflection for both of them. In perspective, this can lead to realization of reflectionless superlens based on three-wave interactions, as well to other applications based on reflection-free and electrically thin nonlinear layers.

Discontinuous electromagnetic fields using orthogonal electric and magnetic currents for wavefront manipulation

We introduce the idea of discontinuous electric and magnetic fields at a boundary to design and shape wavefronts in an arbitrary manner. To create this discontinuity in the field we use orthogonal electric and magnetic currents which act like Huygens source to radiate the desired wavefront. These currents can be synthesized either by an array of electric and magnetic dipoles or by a combined impedance and admittance surface. A dipole array is an active implementation to impose discontinuous fields while the impedance/admittance surface acts as a passive one. We then expand on our previous work showing how electric and magnetic dipole arrays can be used to cloak an object demonstrating novel cloaking and anti-cloaking schemes. We also show how to arbitrarily refract a beam using a set of impedance and admittance surfaces. Refraction using the idea of discontinuous fields is shown to be a more general case of refraction than using simple phase discontinuities.