Zero-index Metamaterials Research Articles

Low-loss nanoscale zero-index metawaveguides and metadevices

Zero-index metamaterials exhibit a uniform spatial phase distribution, promising numerous applications such as efficient electromagnetic tunneling and high-fidelity optical computing. These applications necessitate an integrated, low-loss zero-index waveguide platform for guiding, routing, and interfering light. Nevertheless, existing zero-index metamaterials grapple with substantial footprints, high losses, and limited flexibility. Here, we present low-loss nanoscale zero-index metawaveguides enabled by quasi-bound states in the continuum, formed through destructive interferences along in-plane and out-of-plane directions. This metawaveguide’s propagation loss is 2 orders of magnitude lower than that of the state-of-the-art zero-index waveguide. Leveraging the low loss and flexibility of this metawaveguide, we realized a zero-index metaring for the first time. This low-loss zero-index metawaveguide offers a versatile platform for integrated photonic circuits with minimal phase errors for classical and quantum information processing.

High-permittivity ceramics enabled highly homogeneous zero-index metamaterials for high-directivity antennas and beyond

Zero-index metamaterials (ZIMs) can support uniform electromagnetic field distributions at any frequency, but their applications are hampered by the ZIM’s homogenization level—only 3 unit cells per free-space wavelength, which is fundamentally limited by the low-permittivity inclusions (εr ≈ 12) and background matrix (εr ≈ 1). Here, by filling high-permittivity SrTiO3 ceramic (εr ≈ 294) pillars in BaTiO3 (εr ≈ 25) background matrix, we demonstrate a highly homogeneous microwave ZIM with an over threefold increase in the homogenization level. Leveraging such a ZIM, we achieve not only an antenna, approaching the fundamental limit in the directivity with outstanding scalability, but also a concave lens with a focal length of as short as 1λ0. Our highly homogeneous ZIM has profound implications in ceramics, ZIM-based waveguides and cavities, free-space wavefront manipulation, and microwave quantum optics, and opens up enormous possibilities in wireless communications, remote sensing, global positioning satellites, etc.

Magnetically tunable zero-index metamaterials

Zero-index metamaterials (ZIMs) feature a uniform electromagnetic mode over a large area in arbitrary shapes, enabling many applications including high-transmission supercouplers with arbitrary shapes, direction-independent phase matching for nonlinear optics, and collective emission of many quantum emitters. However, most ZIMs reported to date are passive; active ZIMs that allow for dynamic modulation of their electromagnetic properties have rarely been reported. Here, we design and fabricate a magnetically tunable ZIM consisting of yttrium iron garnet (YIG) pillars sandwiched between two copper clad laminates in the microwave regime. By harnessing the Cotton–Mouton effect of YIG, the metamaterial was successfully toggled between gapless and bandgap states, leading to a “phase transition” between a zero-index phase and a single negative phase of the metamaterial. Using an S-shaped ZIM supercoupler, we experimentally demonstrated a tunable supercoupling state with a low intrinsic loss of 0.95 dB and a high extinction ratio of up to 30.63 dB at 9 GHz. We have also engineered a transition between the supercoupling state and the topological one-way transmission state at 10.6 GHz. Our work enables dynamic modulation of the electromagnetic characteristics of ZIMs, enabling various applications in tunable linear, nonlinear, quantum, and nonreciprocal electromagnetic devices.

Broadband mid-infrared non-reciprocal absorption using magnetized gradient epsilon-near-zero thin films.

The study of magneto-optical absorption has stimulated diverse energy-technology-related explorations, showing potential in breaking the current theoretical efficiency limits of energy devices compared with reciprocal counterparts. However, experimentally realizing strong infrared non-reciprocal absorption remains an open challenge, and existing proposals of non-reciprocal absorbers are restricted to a narrow working waveband. Here we observe highly asymmetric absorption spectra over a broad mid-infrared band (nearly 10 μm) using doped InAs multilayers with gradient epsilon-near-zero frequencies. We reveal that the magnetized epsilon-near-zero behaviours and material loss play important roles in achieving strongly non-reciprocal absorption under a moderate external magnetic field using a thin epsilon-near-zero film (<λ/40, λ is the wavelength). Our approach enables flexible control over the working frequencies and non-reciprocal bandwidths by designing magnetized InAs films with different doping concentrations. The proposed principles can also be generalized to other III-V semiconductors, magnetized metals, topological Weyl semimetals, magnetized zero-index metamaterials and metasurfaces.

Novel transmission property of Zero-Index Metamaterial waveguide doped with gain and lossy defects

Abstract Taking inspiration from quantum parity-time (PT) symmetries that have gained immense popularity in the emerging fields of non- Hermitian optics and photonics, the interest of exploring more generalized gain-loss interactions is never seen down. In this paper we theoretically present new fantastic properties through a zero-index metamaterial (ZIM) waveguide loaded gain and loss defects. For the case of epsilon-and-mu-near-zero (EMNZ) based ZIM medium, electromagnetic (EM) waves are cumulative and the system behaves as an amplifier when the loss cavity coefficient is greater than the gain cavity coefficient. Conversely, when loss is less than gain, EM waves are dissipated and the system behaves as an attenuator. Moreover, our investigation is extended to non-Hermitian scenarios characterized by tailored distributions of gain and loss in the epsilon-near-zero (ENZ) host medium. The transport effect in ZIM waveguide is amplified in one mode, while it is dissipative in the other mode, which breaks the common sense and its physic is analyzed by magnetic flux. That is which cavity has the smaller loss/gain coefficient, the larger its magnetic flux, which cavity dominates. This paper is of significant importance in the manipulation of electromagnetic waves and light amplification as well as the enhancement of matter interactions.

Asymmetric angular selected transmission in phase gradient metagratings and zero index metamaterials

Phase gradient metagrating (PGM) refers to introduction of a local abrupt phase change covering 2π at an interface, which generates a phase gradient to control the direction and propagation of electromagnetic waves. PGM has provided unprecedented opportunities for wavefront manipulation. In this work, we combine PGMs and zero-index metamaterials to achieve high-efficiency asymmetric angular selected transmission. Our research shows that the wave can pass through the system only at a specific incident angle. Furthermore, the incident angle corresponding to the angular selected transmission can be adjusted by modifying the period length of the PGM. This design philosophy is applicable to both electromagnetic wave and acoustic wave systems. Our results open innovative avenues to extend the potential applications of PGM.

Optical parity-time induced perfect resonance transmission in zero index metamaterials.

Non-Hermitian photonic systems with balanced gain and loss have become significantly more popular due to their potential applications in communications and lasing. In this study, we introduce the concept of optical parity-time (PT) symmetry to zero-index metamaterials (ZIMs) to investigate the transport of electromagnetic (EM) waves through a PT-ZIM junction in a waveguide system. The PT-ZIM junction is formed by doping two dielectric defects of the same geometry in the ZIM, with one being the gain and the other being the loss. It is found that the balanced gain and loss can induce a perfect transmission resonance in a perfect reflection background, and the resonant linewidth is controllable and determined by the gain/loss. The smaller the gain/loss, the narrower the linewidth and the larger the quality (Q) factor of the resonance. This finding originates from the fact that the introduced PT symmetry breaks the spatial symmetry of the structure, leading to the excitation of quasi-bound states in the continuum (quasi-BIC). Additionally, we also show that the lateral displacements of the two cylinders play a crucial role in the electromagnetic transport properties in ZIMs with PT symmetry, which breaks the common sense that the transport effect in ZIMs is location-independent. Our results provide a new approach to manipulate the interaction of EM waves with defects in ZIMs using gain and loss to achieve anomalous transmission, and a pathway to investigate non-Hermitian photonics in ZIMs with potential applications in sensing, lasing, and nonlinear optics.

An ultra-narrow-band optical filter based on zero refractive index metamaterial

Owing to the photonic band gap effect and defect state effect, photonic metamaterials have received much attention in the design of narrow bandpass filters, which are the key devices of optical communication equipment such as wavelength division multiplexing devices. In this work, based on zero-index metamaterial (ZIM), a compact filter with both high peak transmission coefficient and ultra-narrow bandwidth is proposed. The photonic metamaterial with conical dispersion and Dirac-like point is achieved by optimizing the structure and material component parameters of dielectric rods with square lattice in air. It is demonstrated that a triply degenerate state can be realized at the Dirac-like point, which relates this metamaterial to a zero-index medium with effective permittivity and permeability equal to zero simultaneously. Electromagnetic (EM) wave can propagate without any phase delay at this frequency, and strong dispersion occurs in the adjacent frequency cone, leading to dramatic changes in optical properties. We introduce a ZIM into photonic metamaterial defect filter to compress the bandwidth to the realization of ultra-narrow bandpass filter. The ZIM is embedded into the resonant cavity of line defect filter, which is also composed of dielectric rods with square lattice in air. In order to increase the sensitivity of the phase change with frequency, the Dirac-like frequency is adjusted to match the resonant frequency of the filter. We study the transmission spectrum of the structure through the COMSOL Multiphysics simulation software, and find that the peak width at half-maximum of the filter decreases as the thickness of ZIM increases, and the peak transmittance is still high when bandwidth is greatly compressed. The zero phase delay inside the slab can be observed. Through field distribution analysis, the zero-phase delay and strong coupling characteristics of electromagnetic field are observed at peak frequency. The comparison with conventional photonic metamaterials filter is discussed. We believe that this work is helpful in investigating the realization of ultra-narrow bandpass filters.

Zero-index metamaterials for classical and quantum light

Zero-index metamaterials for classical and quantum light

Landstorfer Printed Log-Periodic Dipole Array Antenna With Enhanced Stable High Gain for 5G Communication

A novel wideband stable high-gain Landstorfer printed log-periodic dipole array (PLPDA) antenna loaded with zero-index metamaterials (ZIMs) is proposed for wireless communication system. Based on the Landstorfer-shaped method, the dipoles and ground of the PLPDA antenna are Landstorfer-shaped and strung together by feeding lines to form a novel Landstorfer PLPDA antenna, which obtains a gain enhancement compared with the PLPDA antenna. To further enhance both the antenna gain and gain flatness, a series of novel ZIM are added to the shorter end of the Landstorfer PLPDA antenna. As an example, four-element Landstorfer PLPDA antennas with/without ZIM are designed, implemented, and measured in Ka-band for 5G application. Compared with the original PLPDA antenna, the proposed Landstorfer PLPDA antenna exhibits a maximum gain enhancement of 2.2 dB within the operating band of 26.5–40 GHz. Loaded with ZIM, the antenna gain can be further improved by 0.5–1.9 dB with a measured gain flatness of 10.5 ± 0.5 dBi over the band of 25.7–40 GHz while the size of antenna keeping unchanged. The proposed antenna presents a stable high-gain performance in terms of the parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta G$ </tex-math></inline-formula> /BW ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta G$ </tex-math></inline-formula> is the gain variation, BW is the bandwidth) less than 0.07 within the whole Ka-band.

Producing near-zero-index/directivity-tunable metamaterials using transformation optics

In some literature [Prog. Electromagn. Res. 106, 107 (2010)PELREX1043-626X10.2528/PIER10060103], zero-index metamaterials are regarded as non-transformation optics (TO) materials. In this paper, for the first time, to the best of our knowledge, sets of transformation mapping functions are introduced to produce near-zero-index metamaterials using TO. In addition, other than producing near-zero materials, it is shown that the proposed structures can be used in applications like radiators with highly tunable directivity when the parameters of the transformation functions are adjusted. In near-zero-index metamaterials, the refractive index is near zero when either permittivity or permeability, or both, are near zero. The introduced mapping functions are applied to a desired space. Then, using Maxwell’s equations, the wave equation and consequently the wavenumber of the transformed space is obtained. From the wave equation the obtained wavenumber is near zero. Therefore, it is concluded that the transformed space is a near-zero-index material. The mapping is provided for open and enclosed spaces. At the end, a parametric numerical analysis is provided for various sets of obtained parameters for the introduced near-zero-index materials. From the analysis it is shown that the proposed structures can also be used as radiators with tunable directivity.

Geometry symmetry-free and higher-order optical bound states in the continuum

Geometrical symmetry plays a significant role in implementing robust, symmetry-protected, bound states in the continuum (BICs). However, this benefit is only theoretical in many cases since fabricated samples’ unavoidable imperfections may easily break the stringent geometrical requirements. Here we propose an approach by introducing the concept of geometrical-symmetry-free but symmetry-protected BICs, realized using the static-like environment induced by a zero-index metamaterial (ZIM). We find that robust BICs exist and are protected from the disordered distribution of multiple objects inside the ZIM host by its physical symmetries rather than geometrical ones. The geometric-symmetry-free BICs are robust, regardless of the objects’ external shapes and material parameters in the ZIM host. We further show theoretically and numerically that the existence of those higher-order BICs depends only on the number of objects. By practically designing a structural ZIM waveguide, the existence of BICs is numerically confirmed, as well as their independence on the presence of geometrical symmetry. Our findings provide a way of realizing higher-order BICs and link their properties to the disorder of photonic systems.

High‐gain omnidirectional patch antenna for conformal application based on near‐zero‐index metamaterials

Achieving a high-gain omnidirectional patch antenna with low profile is a challenging task. On the basis of improving antennas by metamaterials, a broadband near-zero-index metamaterial (NZIM) is designed to enhance the horizontal gain of the omnidirectional patch antenna. The zero-index metamaterial is loaded on the substrate of three antennas with different working bands (4.99–5.50 GHz). The results show that the horizontal gain of antennas is enhanced by more than 0.6 dB at various frequencies. The roundness of antennas is also optimised. Therefore, using NZIM can increase the horizontal gain with a low profile (λ/30) structure size. The low profile, high-gain omnidirectional patch antenna has great potential in the conformal field.

Tunable asymmetric acoustic transmission via binary metasurface and zero-index metamaterials

The pursuit of tunable asymmetric sound transmission has been a long-term topic since it could contribute to providing more flexibilities in many areas of acoustic engineering. The interference effect can be a feasible approach in which two waves with the same frequency superposed to form the resultant wave with manipulated amplitude according to the relative phase difference between them. However, strictly speaking, restricted by the spatial variance of phase, the manipulated domain created by the specific phase difference is always limited to a spot with dimensions much smaller than the wavelength. Here, we proposed a design to break this barrier that can realize the tunable asymmetric transmission via the combination of zero-index metamaterials and the binary metasurface. The zero-index metamaterial can provide the effective extremely large speed to shrink the infinite domain into a spot acoustically and the binary metasurface can be used to tune the specific phase difference. Numerical simulations and experimental measurements have good agreement and show that the acoustic waves impinged from the side of metasurface will be manipulated to have controllable transmission, while the acoustic waves impinged from the side of zero-index metamaterials will keep a high transmission. We think the proposed design is full of physical significance, which may find potential applications in many fields, like noise cancelation, acoustic imaging, and ultrasound therapy.

Zero-index metamaterials for Dirac fermion in graphene

We study the response of ballistic electron waves in graphene to a square array composed of gate-defined quantum dots when the incident energy is at the neutral point of the Dirac conical dispersion. The effective medium theory shows that the array will behave as a zero-refractive-index medium. Our simulations based on the rigorous multiple scattering theory corroborate that it can indeed exhibit various typical applications of zero-refractive-index media, such as the focusing effect of electron waves, the control over the propagation direction, and directional emission. The array is usually about one-wavelength thick so that the incident wave can penetrate it and the wave front can be tailored by engineering the geometrical shape of the emergent surface. The wave-manipulating behaviors based on zero-index metamaterials could open unprecedented opportunities to steer the flow of graphene electrons in novel manners and shape the wave front of electron beams at will.

Designing Electromagnetic Field Rotators with Homogenous and Isotropic Materials

This paper reports the design features of an electromagnetic device with an improved mapping transformation function allowing perfect electromagnetic rotation. Based on transformation optics theory, theoretical analysis has been used to show the constitutive parameters of both unimproved and improved perfect case which has ultimately validated via full-wave simulations. For implementation of this device, an alternating structure of zero-index metamaterials and perfect electric conductors has been used. The functionality of the proposed structure was verified by numerical simulations.

Wideband near-zero-index metamaterials created by using a supercell microstructure in the terahertz range

Zero-index metamaterials are widely used in electromagnetic devices due to their favorable optical properties such as infinite large-phase velocity and quasi-infinite wavelength. However, insufficient bandwidth and high loss seriously limit the wide application of zero refractive index metamaterials (ZIMs), especially in the terahertz band. Here, we propose a kind of supercell microstructure metamaterial to realize a broadband near-zero refractive index. This new superlattice structure contains four subunit structures of different sizes in a single unit. Dual broadband characteristics of the near-zero refractive index can be demonstrated in our design. The bandwidth of the near-zero index can reach about 30 GHz with low loss. The resonance principle of the supercell microstructure and the generation mechanism of the near-zero refractive index are discussed in detail. The influence of the thickness of the medium, of the thickness of the metal sheet, and of the period of the central area and the metal line width on the near-zero refractive index is analyzed. Dual broadband zero-refractive metamaterials bring wider applications to various optoelectronic devices for arbitrary wavefront conversion, directional radiation, and obstacle-free light guiding.

Dopant-modulated sound transmission with zero index acoustic metamaterials.

Zero index metamaterials have shown the ability to achieve total transmission or reflection by embedding particular defects with various effective parameters. Here, we present that tunable sound transmission can be realized by configuring a subwavelength-sized dopant inside zero index acoustic metamaterials. Despite its small spatial signature, the dopant is able to strongly interact with the acoustic waves inside the whole zero index metamaterials. It is due to the essence of the zero effective index that can homogenize the pressure field within the metamaterials. Sound transmission can thus be fully switched on and off by adjusting the dopant's surface impedance. A simple rectangular cavity with varied lengths is proposed to provide the required impedance boundary. Our model of correlating the dopant design with sound transmission performance is validated theoretically and numerically. We further demonstrate the utilization of the proposed design to effectively modulate the sound focusing effect. Such a dopant-modulated sound transmission scheme, with its simplicity and capability, has potential applications in fields like noise control and ultrasonography.

A Dual-Band Zero-Index Metamaterial Superstrate for Concurrent Antenna Gain Enhancement at 2.4 and 3.5 GHz

ABSTRACT The classical printed monopole antenna exhibits wideband operation with low gain and an omnidirectional radiation pattern. In order to boost the gain of the antenna at two frequency bands, a novel dual-band zero-index metamaterial (ZIM) based on both electric and magnetic resonance is developed. The printed monopole is also backed by a metallic ground plane for unidirectional radiation pattern towards boreside. The presented unit cell is comprised of two Schurig split-ring resonators nested together in such an arrangement that it leads to a zero refractive index at two distinct frequencies. Using the proposed design methodology, a ZIM is designed operating at 2.4 and 3.5 GHz. To substantiate the improvement in gain, the proposed design is fabricated and used with a cavity-backed printed monopole antenna. The metamaterial superstrate increases the measured realized gain of the printed monopole antenna by 3.2 dB at 3.5 GHz and by 2.5 dB at 2.4 GHz, in agreement with simulations.

Anti-reflection coatings of zero-index metamaterial for solar cells

Anti-reflection coatings of zero index metamaterials (ZIMs) are proposed for maximum absorption of light in solar cells. A thin layer of a ZIM is shown to help trap light inside a solar cell. The outer surface of a ZIM layer is planar, and the inner surface has periodic corrugations in order for the incident light to pass through but block the re-transmission of the light back into free space. Using rigorous calculations for light absorption efficiency integrated over the AM1.5 solar spectrum, the basic design of the anti-reflection coating using a ZIM is studied by comparing the results with a common anti-reflection coating and a ZIM layer planar on both sides.