High Refractive Index Metamaterials Research Articles

Vertically‐Aligned Ag Nanowire Array‐Based Metamaterial with High Refractive Index and Low‐Dispersion in the Infrared Band

AbstractTo open new vistas for designing nanophotonic systems, exploiting high refractive index (RI) availability is now drawing attention worldwide, especially in the visible‐infrared range. Based upon the nanoscale close‐packed arrangement of metallic nano‐units, few optical‐frequency metamaterials with high RIs have been demonstrated. However, there are still significant challenges in their RI promotion, tuneability, and integrated preparation for practical applications. Herein, a novel Ag nanowire‐based metamaterial scheme is put forward, in which vertically aligned Ag nanowire (NW) array embedded‐Si or ‐Ge composites are made. Thanks to flexible control on microstructure parameters of Ag NWs (meta‐unit) and their dielectric environment, the synergistic effect of enhanced capacitive coupling and weakened diamagnetic response is created, leading to a recorded near‐infrared high‐RI of ≈7.2, low‐dispersion from 2 to 10 µm, and customizable RIs. Further, an air nanohole array‐Si composite layer, acting as an impedance matching layer, is utilized to realize the transmittance increase of the metamaterial film by ≈50% at specific wavelengths, along with the bulk plasmon‐polariton (BPP) modes resonances in the interwire nanocavities. This approach to increasing the infrared refractive index can circumvent physical effect conflict in high RI metamaterials and pave a basis for future scalable and on‐demand photonic applications.

Deflected beam pattern through reconfigurable metamaterial structure at 3.5 GHz for 5G applications

In this paper, the reconfigurable metamaterial is proposed to deflect the radiated beam of the bow-tie antenna at a 5G band of 3.5 GHz. The split square resonator (SSR) is designed at a sub-6 GHz band and reconfigured to produce different refractive indices. For obtaining the desired deflection angles, 3 × 7 SSR unit cells with ON and OFF configurations are printed into the antenna substrate. This arrangement creates two metamaterial configurations with different refractive indices in the closeness of the radiating elements, thereby deflecting the main beam to the direction of high refractive index metamaterial. In the passive beam deflection antenna, the reconfigurable SSRs deflect the radiation beam of the antenna by angles of ±39° in E-plane at 3.5 GHz. Furthermore, the embedding of SSRs helps improve the gain by 2.4 and 2.3 dB for the positive and negative deflection angles, respectively. On the other hand, in the active beam deflection antenna, the reconfigurable SSRs based on real PIN diodes can deflect the antenna radiated beam by angles of ±36° in the E-plane. These reconfigurable antennas have the potential to be used in 5G beam deflection applications. Both passive and active metamaterial antennas are fabricated, and their performances are measured.

High Refractive Index Metamaterial Superstrate for Microstrip Patch Antenna Performance Improvement

We propose a high refractive index metamaterial （HRIM）that is formed by symmetrically etching square metal patches on both sides of a dielectric plate. By reducing the gap of adjacent patches and the thickness of dielectric plate, the proposed metamaterial can suppress the diamagnetic response and enhance the capacitance coupling, so that the refractive index of the metamaterial is higher than 10 in wideband from 4 GHz to 8 GHz. Attributing to the property of high refractive index, the metamaterial can effectively improve the performance of a microstrip antenna when it is loaded as a superstrate. The measured results reveal that the impedance bandwidth of S11< -10 dB is 690 MHz (12.4%) and the maximum gain reaches to 13 dB at 5.6 GHz. The 1 dB and 3 dB gain bandwidth are 380 MHz (6.7%) and 590 MHz (10.3%), respectively. The antenna has a subwavelength profile of 0.19λ ( λ _0 is the free space wavelength of the antenna central frequency). Furthermore, the profile can be further reduced to 0.15λ when the radiation patch of the antenna is surrounded by metasurface.

Design of circularly polarized patch antennas using anisotropic high refractive index metamaterial loading

ABSTRACTIn this paper, for the first time, the development of the circularly polarized square-shaped patch antenna is performed with the assistance of anisotropic high refractive index (HRI) metamaterial. The HRI metamaterial is realized with the help of an investigation conducted on the single ring split ring resonator (SR-SRR). With the benefit of the concept of duality, the complementary SR-SRR is considered to load on to ground plane (instead of SR-SRR) to function as an effective HRI medium. The concept of the High refractive index in slowing down the waves is explained quantitatively. The anisotropy of the effective HRI metamaterial medium is used to create the phase difference between the orthogonal components. Anisotropy of the unit cell was used to realize one RHCP and one LHCP antenna from the same fundamental design by just rotating the unit cell. For both antennas, the simulated and measured results are agreed on the same.

A technique for reduction of mutual coupling by steering surface wave propagation

AbstractIn this article, we present an efficient approach to reduce mutual coupling in a planar slot antenna array by using high refractive index metamaterial. The metamaterial structure is employed to steer the propagation of the surface wave. For this purpose, array of complementary slot coupled split‐ring resonator structures are designed on the two sides of the slot array. The proposed configuration is fabricated and both simulation and measurement results confirm that an average of 10 dB improvements in isolation level is achieved throughout the wide operating bandwidth. Moreover, due to the implementation of the complementary metamaterial structure, the proposed configuration becomes planar and easy to fabricate.

Miniaturization of monopole antenna using high refractive index metamaterial loading

In this article, miniaturization of the wire monopole antenna with the help of high refractive index (HRI) metamaterial is presented. For the first time, HRI medium is realized by using the array of single ring split ring resonator. By surrounding the wire monopole with the array of SR-SRRs, the effective wavelength is squeezed in the vicinity of the near field dominance. By the loading of the monopole with such an HRI medium, the size of the antenna is reduced from 29.5 to 16 mm, without any deformation in the current distribution and radiation pattern corresponding to the fundamental resonance. The simulated and measured results are agreed to the same.

Advancements in high refractive index media: from quantum coherence in atomic systems to deep sub-wavelength coupling in metamaterials [Invited

Refractive index enhancement is crucial in the field of lithography, imaging, optical communications, solar devices and many more. We present a review of advancements in the process of designing high refractive index metamaterials, starting from quantum coupling and photonic bandgap materials to metamaterials utilizing deep subwavelength coupling to achieve ever-high values of refractive index. A crisp and critical impression of each scheme are presented. The understanding of evolution of material design from intrinsic electronic states manipulation to meta-atoms design is not only fascinating but prerequisite to developing successful devices and applications.

Realization of ultrahigh refractive index in terahertz region by multiple layers coupled metal ring metamaterials

ABSTRACTWe report a polarization-independent unnaturally high refractive index metamaterial. It is found that the effective permittivity of the designed metamaterial can be drastically increased by the capacitive coupling effect. Also, the effective permeability of metamaterial can be increased by reducing the diamagnetic effect by designing a thin structure. In our work, the single-layer multi-band high refractive index metamaterials with a peak achieving to 26.8 at 0.9 THz is demonstrated. Furthermore, the three-dimensional metamaterial possessing high refractive index more than 85 around 0.91 THz can be constructed by a double-layer tricyclic structure. Ultrabroadband and ultrahigh refractive index of designed metamaterials can be obtained by multilayer coupling effect. In addition, the polarization-independent property and wide-angle performance of high refractive index metamaterial are exhibited.

High Refractive Index Metamaterials by Using Higher Order Modes Resonances of Hollow Cylindrical Nanostructure in Visible Region

In order to design optical metamaterials to surpass the limitation of unnaturally high refractive index in visible range, we propose the nanocylinders nanostructure with hexagonal close-packed arrangement to precisely control the electric and magnetic resonances, resulting in the manipulation of the effective index of metamaterials. By drastically increasing the strong capacitive coupling between nanocylinders and decreasing the diamagnetic effect with core-shell nanostructure, the unnaturally high refractive index of designed metamaterials can be achieved to be 10.5 in visible region. The diamagnetic effect of solid nanocylinders can be significantly suppressed by using the design strategy of hollow nanocylinders with higher order mode resonances, leading to great enhancement of the effective refractive index in broadband visible region. Expanding the refractive index into an unnaturally high positive region in visible range can complete the spectrum of attainable refractive index, and the high refractive index metamaterials can provide more design flexibility for nanophotonics.

High-performance lens antenna using high refractive index metamaterials**Project supported by the National Natural Science Foundation of China (Grant Nos. 61761010 and 61461016), in part by the Natural Science Foundation of Guangxi Zhuang Autonomous Region, China (Grant No. 2015jjBB7002), in part by the Guangxi Key Laboratory of Wireless Wideband Communication and Signal Processing, and in part by the Innovation Project of

In this paper, a high refractive index metamaterial (HRM), whose element is composed of bilayer square patch (BSP) spaced by a dielectric plate, is proposed. By reducing the thickness of the dielectric plate and the gap between adjacent patches, the BSP can effectively enhance capacitive coupling and simultaneously suppress diamagnetic response, which significantly increases the refractive index of the proposed metamaterial. Furthermore, the high refractive index region is far away from the resonant region of the metamaterial, resulting in broadband. Based on these characteristics of BSP, a gradient refractive index (GRIN) lens with thin thickness (0.34λ0, where λ0 is the wavelength at 5.75 GHz) is designed. By using this lens, we then design a circularly polarized horn antenna with high performance. The measurement results show that the 3-dB axial ratio bandwidth is 34.8% (4.75 GHz∼ 6.75 GHz) and the antenna gain in this frequency range is increased by an average value of 3.4 dB. The proposed method opens up a new avenue to design high-performance antenna.

Terahertz multiband ultrahigh index metamaterials by bilayer metallic grating structure

One-dimensional metallic grating structure was proposed to realize high refractive index metamaterial in the terahertz region. By drastically increasing the effective permittivity by means of intense capacitive coupling and reducing the diamagnetic effect using a thin metallic thickness, a peak refractive index of 15.81 at the resonant frequency in embedded metallic grating can be obtained. Multiband high refractive index metamaterial can be realized by double symmetric metallic grating and asymmetric grating structure. For asymmetric grating metamaterial structure, two separate transmission peaks appear and result in two separate high refractive index. Interestingly, a near zero refractive index metamaterial can be obtained by the introduction of double asymmetric design. It was found that our designed ultrahigh refractive index metamaterials depend on the electric field coupling effect and the magnetic field diamagnetic response.

Design of ultrahigh refractive index metamaterials in the terahertz regime

A high refractive index metamaterial was theoretically designed in the terahertz region, and the proposed structure is to easy implementation. By decreasing the diamagnetic effect with a thin “I”-shaped metallic patch structure and drastically increasing the effective permittivity through strong capacitive coupling in the unit cell, the higher refractive index of designed metamaterial can be obtained while maintaining low losses in the resonant frequency. The physical mechanism of metamaterial possessing an enhanced refractive index is revealed by the surface current, the electric field and magnetic field distributions. Besides, the polarization characteristics and broad incident angles robustness for the high refractive index are demonstrated.

High refractive index metamaterials using corrugated metallic slots.

We report on a method for realizing high refractive index metamaterials using corrugated metallic slot structures at terahertz frequencies. The effective refractive index and peak index frequency can be controlled by varying the width of the air gap in the corrugated slot arrays. The phenomenon occurs because of the secondary resonance effect due to the fundamental inductive-capacitive resonance, which generates a red-shift of the fundamental resonance determined by twice the length of the corrugated metallic slots. In addition, multiple gaps in the corrugated slots act as plasmonic hotspots which have the properties of three-dimensional subwavelength confinement due to extremely strong enhancement of the terahertz waves. The versatile characteristics of the structures may have many potential applications in designing compact optical devices incorporating various functionalities and in developing highly sensitive spectroscopic/imaging systems.

Improvement of Gain and Elevation Tilt Angle Using Metamaterial Loading for Millimeter-Wave Applications

Elevation-plane beam tilting is demonstrated for a printed dipole antenna operating over 57–64 GHz. This is achieved using a 3x4 array of high refractive-index metamaterial (HRIM) unit cells. The unit cell comprises a modified H-shaped structure with stub loading to control the refractive index of the unit cell over a finite frequency range. Integration of the 3x4 array in the H-plane of a dipole antenna is shown to deflect the main beam by +30 degrees with respect to the endfire direction over 57–64 GHz. In addition, the proposed technique provides 5 dB gain enhancement.

Transforming acoustic sensing with high refractive index metamaterials

Acoustic sensors play an important role in many areas, such as safety (e.g., sonar arrays), public health (e.g., ultrasonic imaging), surveillance (e.g., underwater communication and navigation), and industry (e.g., non-destructive damage detection). However, conventional acoustic sensors inevitably suffer from the fundamental pressure detection limit, which hinders the performance of current acoustic sensing technologies. Here, we design high refractive index acoustic metamaterials that have strong wave compression effect to obtain direct amplification of pressure fields in metamaterials. This enables a novel metamaterial enhanced acoustic sensing system that can help overcome the detection limit of conventional systems. Through analytical, numerical, and experimental studies, we demonstrate that this novel acoustic sensing system can help achieve over an order of magnitude enhancement in acoustic pressure detection limit. This will allow the detection of weak acoustic signals below the noise floor of the conventional acoustic sensing systems. This work is expected to impact many fronts that require high performance acoustic sensing.

High-resolution electrohydrodynamic jet printing for the direct fabrication of 3D multilayer terahertz metamaterial of high refractive index

The fabrication of 3D metamaterials, such as multilayer structures, is of great interest in practical applications of the metamaterial. Here we present an electrohydrodynamic jet printing technique as a direct fabrication method of 3D multilayer metamaterial. By alignment of the nozzle movement, we could fabricate multiple layers of the metamaterial. Controlling an electrical pulse to make droplets on-demand, we fabricated a high refractive index metamaterial and compared the optical performances of a single layer and multiple layers, with 10 µm width and 5 µm gap of I-shaped meta-atoms on the polyimide substrate. The peak refractive index was 25.7 at 0.46 THz for a four-layer metamaterial.

Tapered labyrinthine acoustic metamaterials for coherent controlling of acoustic wave

Acoustic metamaterials with their exotic material properties enable unprecedented control over acoustic wave propagation and reflection. Besides utilizing locally resonating structures or non-resonant composite effective media, non-locally resonating spatial coiling structures have recently been adopted to design negative or high positive refractive index metamaterials. We have in the past experimentally demonstrated the unit cell characteristics of one kind of labyrinthine metamterial (Xie et al. PRL 2013) and its impedance matching improved versions (Xie et al. APL 2013). In this work, we present several coherent modulation devices based on our recently proposed tapered labyrinthine metamaterials. With thickness of only one or two metamaterial cell layers, we can create an acoustic blazed diffraction grating, a phase conjugation lens, or a flat lens that can perform plane wave-cylindrical wave conversion. The design process and experimental demonstrations will be presented. The coherent controlling devices feature precise phase modulation, high-energy throughput, broad operating bandwidth, and sub-wavelength thickness. Our work demonstrates that labyrinthine metamaterials can be the unit cells of choice for functional coherent acoustic modulation devices.

Analytical description of subwavelength plasmonic MIM resonators and of their combination

We show that a periodic array of metal-insulator-metal resonators can be described as a high refractive index metamaterial. This approach permits to obtain analytically the optical properties of the structure and thus to establish conception rules on the quality factor or on total absorption. Furthermore, we extend this formalism to the combination of two independent resonators.

Broadband and low loss high refractive index metamaterials in the microwave regime

A broadband, low-loss, isotropic, high refractive index metamaterial comprising an array of close-packed cubic elements of cross-linked square metal plates is demonstrated in the microwave regime. The structuring of the cubic elements reduces the diamagnetic response exhibited by arrays of solid metal cubes, whilst maintaining the strongly enhanced permittivity arising from capacitative coupling between adjacent metal elements. The corresponding increase in refractive index can be tailored through appropriate structuring of the faces and/or spacing of the cubes.

High negative refractive index in chiral metamaterials

Conjugated bi-layer C8 metamaterials are proposed. Chirality and refractive index can be controlled effectively by the structure geometry. Extreme chirality results in a high negative refractive index for both right- and left-circular polarized excitations. The conjugated bi-layer C8 structures are fabricated and tested in microwave regimes. The measurement results are in good agreement with the simulation results. These proposed structures are additional promising candidates for high negative refractive index metamaterials which will facilitate different applications.