In this article, we review the topic of Huygens' metasurfaces with an emphasis on existing and emerging applications at microwave frequencies. Huygens' metasurfaces have demonstrated unprecedented capabilities of controlling electromagnetic wavefronts by means of electric and magnetic dipole moments arranged in a thin sheet. We present the fundamental principles of Huygens' metasurfaces based on the boundary conditions governing their operation. Then, we discuss the aspect of practical realization of Huygens' metasurfaces and the different types of constituent subwavelength scatterers (unit cells). Moreover, we summarize recent developments in several areas related to metasurfaces, such as perfect anomalous refraction, polarization control, antenna beamforming and reconfigurable metasurfaces. Lastly, we provide a brief outlook on emerging metasurface-based microwave technologies that are expected to further grow in the future.

Huygens' metasurfaces, typically designed using sub-wavelength unit cells, allow the arbitrary control of electromagnetic waves. Two well-known unit-cell topologies are the wire-loop and stacked-layer designs. The wire-loop unit cell utilizes a conductive wire and loop to control its electric and magnetic responses, while the stacked-layer unit cell utilizes cascaded impedance sheets. Due to their versatility, Huygens' metasurfaces have been used for numerous applications. One interesting application has been perfect or reflectionless wide-angle refraction. In recent years, it was shown that for perfect refraction, omega-bianisotropy was required in Huygens' metasurfaces. To realize omega-bianisotropic designs, asymmetric unit cells were proposed and demonstrated utilizing stacked-layer unit cells. However, an omega-bianisotropic wire-loop unit cell for modulated Huygens' metasurfaces has yet to be demonstrated. This article demonstrates the design, simulation results, and measurements of an omega-bianisotropic wire-loop Huygens' metasurface for reflectionless wide-angle refraction of 71.8 degrees at 20 GHz. The design and simulation results of both TE and TM transverse-electric (TE) and transverse-magnetic (TM) designs are presented. Additionally, an optimized TM metasurface is experimentally verified through a combination of quasi-optical and far-field measurements. The presented results demonstrate negligible reflections, high scattered refraction efficiency, and a 0.7 GHz bandwidth, which validates the wire-loop unit-cell topology for realizing omega-bianisotropic Huygens' metasurfaces.

Metasurfaces are thin two-dimensional metamaterial layers that allow or inhibit the propagation of electromagnetic waves in desired directions. For example, metasurfaces have been demonstrated to produce unusual scattering properties of incident plane waves or to guide and modulate surface waves to obtain desired radiation properties. These properties have been employed, for example, to create innovative wireless receivers and transmitters. In addition, metasurfaces have recently been proposed to confine electromagnetic waves, thereby avoiding undesired leakage of energy and increasing the overall efficiency of electromagnetic instruments and devices. The main advantages of metasurfaces with respect to the existing conventional technology include their low cost, low level of absorption in comparison with bulky metamaterials, and easy integration due to their thin profile. Due to these advantages, they are promising candidates for real-world solutions to overcome the challenges posed by the next generation of transmitters and receivers of future high-rate communication systems that require highly precise and efficient antennas, sensors, active components, filters, and integrated technologies. This Roadmap is aimed at binding together the experiences of prominent researchers in the field of metasurfaces, from which explanations for the physics behind the extraordinary properties of these structures shall be provided from viewpoints of diverse theoretical backgrounds. Other goals of this endeavour are to underline the advantages and limitations of metasurfaces, as well as to lay out guidelines for their use in present and future electromagnetic devices. This Roadmap is divided into five sections: 1. Metasurface based antennas. In the last few years, metasurfaces have shown possibilities for advanced manipulations of electromagnetic waves, opening new frontiers in the design of antennas. In this section, the authors explain how metasurfaces can be employed to tailor the radiation properties of antennas, their remarkable advantages in comparison with conventional antennas, and the future challenges to be solved. 2. Optical metasurfaces. Although many of the present demonstrators operate in the microwave regime, due either to the reduced cost of manufacturing and testing or to satisfy the interest of the communications or aerospace industries, part of the potential use of metasurfaces is found in the optical regime. In this section, the authors summarize the classical applications and explain new possibilities for optical metasurfaces, such as the generation of superoscillatory fields and energy harvesters. 3. Reconfigurable and active metasurfaces. Dynamic metasurfaces are promising new platforms for 5G communications, remote sensing and radar applications. By the insertion of active elements, metasurfaces can break the fundamental limitations of passive and static systems. In this section, we have contributions that describe the challenges and potential uses of active components in metasurfaces, including new studies on non-Foster, parity-time symmetric, and non-reciprocal metasurfaces. 4. Metasurfaces with higher symmetries. Recent studies have demonstrated that the properties of metasurfaces are influenced by the symmetries of their constituent elements. Therefore, by controlling the properties of these constitutive elements and their arrangement, one can control the way in which the waves interact with the metasurface. In this section, the authors analyze the possibilities of combining more than one layer of metasurface, creating a higher symmetry, increasing the operational bandwidth of flat lenses, or producing cost-effective electromagnetic bandgaps. 5. Numerical and analytical modelling of metasurfaces. In most occasions, metasurfaces are electrically large objects, which cannot be simulated with conventional software. Modelling tools that allow the engineering of the metasurface properties to get the desired response are essential in the design of practical electromagnetic devices. This section includes the recent advances and future challenges in three groups of techniques that are broadly used to analyze and synthesize metasurfaces: circuit models, analytical solutions and computational methods.

In this paper, a novel concept of a leaky-wave antenna is proposed, based on the use of Huygens’ metasurfaces. It consists of a parallel-plate waveguide in which the top plate is replaced by a bianisotropic metasurface of the omega type. It is shown that there is an exact solution to transform the guided mode into a leaky mode with arbitrary control of the constant leakage factor and the pointing direction. Although the solution turns out to be periodic, only one Floquet mode is excited and radiates, even for electrically long periods. Thanks to the intrinsic spurious Floquet mode suppression, broadside radiation can be achieved without any degradation. Simulations with idealized reactance sheets verify the concept. Moreover, physical structures compatible with PCB fabrication have been proposed and designed, considering aspects such as the effect of losses. Finally, experimental results of two prototypes are presented and discussed.

Huygens' metasurfaces allow the arbitrary control of electromagnetic waves by utilizing both electric and magnetic currents. These electrically thin structures interact with incident electromagnetic waves on a sub-wavelength scale to produce desired wave effects. To realize Huygens' metasurfaces, different unit cell topologies such as the wire-loop or the stacked-layer configurations can be utilized. In recent years, Omega-bianisotropic Huygens' metasurfaces have been demonstrated utilizing asymmetric stacked-layer unit cells, allowing the realization of reflectionless transformations. However, an Omega-bianisotropic version of the wire-loop unit cell has yet to be demonstrated for modulated Huygens' metasurfaces. In this paper, the design and fullwave simulation results of Omega-bianisotropic Huygens' wire-loop metasurfaces for reflectionless wide-angle refraction from normal incidence to 71.8° for both transverse-electric and transverse-magnetic polarizations centered at 20GHz are presented.

In this communication, the experimental verification of a leaky-wave antenna that uses a Huygens' metasurface to have a control on the radiation parameters is addressed. The antenna consists of a parallel-plate waveguide in which the top plate has been replaced by an omega-type bianisotropic Huygens' metasurface, in order to transform the guided mode into the desired leaky-mode with arbitrary choice of the leakage rates, pointing direction and waveguide height. The physical implementation of the metasurface is explained. Two design examples, radiating at broadside with different leakage factors, are implemented, manufactured and measured. Simulated and experimental results of the achieved directivity, scanning performance and radiation patterns are provided, experimentally corroborating the concept.

Huygens' metasurfaces have demonstrated their versatility in numerous applications, such as wide-angle refraction and antenna beamforming. These electrically thin structures allow the enhancement of antenna systems with passive designs. Such enhancements include improving the gain and scanning capability of feed antennas. In this paper, the design, simulation results, and measurements of a metasurface lens for gain enhancement of frequency-scanning slotted-waveguide antennas are shown. Both transverse-electric (TE) and transverse-magnetic (TM) metasurface designs are verified with full-wave simulations, which demonstrate upward of 13 dB of directivity enhancement. Additionally, a fabricated 40 lambda long by 15 lambda wide TM metasurface lens centered at 34.3 GHz was experimentally verified in conjunction with a TM frequency-scanning slotted-waveguide antenna. The waveguide antenna radiates through a 1-D slot array operating between 33.5 and 35.3 GHz and produces a 16 dB fan beam which scans in the H-plane between -26 degrees and broadside within its operational bandwidth. The realized metasurface lens, placed at 3.05 lambda at 34.3 GHz away from the waveguide antenna, was able to increase its realized gain by upward of 10 dB while maintaining its scanning capabilities. With the design verified in both simulation and measurements, the proposed metasurface serves as an easy-to-fabricate, efficient, low-profile, and lightweight alternative to standard microwave lenses.

In 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.

Bianisotropy in metasurface designs offers a solution to produce arbitrary wave transformations even when the incident and transmitted waves differ largely in their transverse wave impedances. By using asymmetry in unit cell designs, bianisotropy can be generated. Such asymmetric bianisotropic unit cells have been discussed and investigated utilizing the stacked-layer topology. However, an asymmetric version of the more natural dipole-loop unit cell has yet to be proposed. In this paper, we present an asymmetric dipole-loop unit cell which demonstrates matching capabilities that may be used for bianisotropic reflectionless wide-angle refraction. This concept is verified using fullwave results for a normally incident plane wave intended for refraction at 71.8° at 20GHz.

Lenses at the microwave/millimeter-wave frequencies can be used to improve the performance of antennas. However traditional dielectric lenses suffer from high reflections and large physical thickness. To combat these issues, we propose a Huygens' metasurface lens design which can perform the desired lensing effects while minimizing reflections and keeping a low profile. The proposed metasurface utilizes a wide variety of unit cells to accurately model the theoretical boundary conditions necessary for a focusing wave transformation. An example of a metasurface cylindrical lens is designed to improve the gain of a 40λ long frequency-scanned slotted waveguide antenna operating in the frequency range of 33.4GHz to 35.2GHz. The proposed metasurface is designed to increase the gain of the feed antenna while maintaining its scanning capabilities.

There is an established equivalence between Huygens’ metasurfaces and lattice networks. This paper proposes an extension to this equivalence for bianisotropic Huygens’ metasurfaces, where bianisotropy refers to the magnetoelectric coupling between the effective electric and magnetic responses. A modified version of the lattice network is proposed that exhibits a one-to-one mapping to the bianisotropic sheet transition conditions by incorporating ideal transformers. A possible realization of bianisotropic Huygens’ unit cells is also proposed that relies on offsetting the wire with respect to the loop in the wire/loop topology. This is followed by deriving analytical expressions for the image impedances and the corresponding transfer function of bianisotropic Huygens’ metasurfaces. Inverse analytical expressions are also derived that govern the required effective electric and magnetic responses and the magnetoelectric coupling for achieving wideband electrical transparency (symmetric case) and wideband impedance transformation (bianisotropic case) with these Huygens’ metasurface unit cells. Moreover, it is shown that these expressions can be realized with transmission-line stubs. These expressions are then used to design subwavelength thin Huygens’ metasurfaces immersed in an air gap between two dielectric media. It is shown, through full-wave simulations, that wideband electrical transparency and impedance transformation are realized for a normally impinging plane wave.

Huygens' metasurfaces are electrically thin devices which allow arbitrary field transformations. Beam refraction is among the first demonstrations of realized metasurfaces. As previously shown for extreme-angle refraction, control over only the electric impedance and magnetic admittance of the Huygens' metasurface proved insufficient to produce the desired reflectionless field transformation. To maintain zero reflections for wide refraction angles, magnetoelectric coupling between the electric and magnetic response of the metasurface, leading to bianisotropy, can be introduced. In this paper, we report the theory, design, and experimental characterization of a reflectionless bianisotropic metasurface for extreme-angle refraction of a normally incident plane wave towards 71.8$^\circ$ at 20 GHz. The theory and design of three-layer asymmetric bianisotropic unit cells are discussed. The realized printed circuit board structure was tested via full-wave simulations as well as experimental characterization. To experimentally verify the prototype, two setups were used. A quasi-optical experiment was conducted to assess the specular reflections of the metasurface, while a far-field antenna measurement characterized its refraction nature. The measurements verify that the fabricated metasurface has negligible reflections and the majority of the scattered power is refracted to the desired Floquet mode. This provides an experimental demonstration of a reflectionless wide-angle refracting metasurface using a bianisotropic Huygens' metasurface at microwave frequencies.

In this paper recent advances in Huygens' metasurfaces are described. These include novel broadband and bi-anisotropic unit cells, as well as beam expansion for antenna directivity enhancement using phase- changing metasurfaces.

We report the design, fabrication, and characterization of bianisotropic Huygens' metasurfaces (BHMSs) for refraction of normally incident beams towards 71.8 degrees. As previously shown, all three BHMS degrees of freedom, namely, electric polarizability, magnetic polarizability and omega-type magnetoelectric coupling, are required to ensure no reflections occur for such wide-angle impedance mismatch. The unit cells are composed of three metallic layers, yielding a printed-circuit-board (PCB) structure. The fabricated BHMS is characterized in a quasi-optical setup, used to accurately assess specular reflections. Subsequently, the horn-illuminated BHMS' radiation pattern is measured in a far-field chamber, to evaluate the device's refraction characteristics. The measured results verify that the BHMS has negligible reflections, and the majority of the scattered power is coupled to the desirable Floquet-Bloch mode. To the best of our knowledge, this is the first experimental demonstration of such a reflectionless wide-angle refracting metasurface.

We introduce a new method of integrating holography into microwave antennas. Interference patterns of a reference wave with a desired wave can be represented using tilted radiating elements. By either feeding the elements directly or with an incident wave excitation, the Angle Holography array can then reproduce the desired output wave. The proposed design aims to provide features current holographic systems do not posses such as frequency scalability, simplicity in feeding, and ease of design and fabrication. Two 1D array designs are discussed, one for beam steering and the other for Fresnel zone focusing. Through full wave simulations, preliminary results validate the proposed idea and designs.