Negative Refractive Index Metamaterials Research Articles

A solid-solution approach for high frequency 3-dimensional isotropic negative-refractive-index metamaterials based on microwave ceramics

Negative refraction has yielded valuable insights and solutions in electromagnetics. However, progress in this field has been hindered primarily by implementation challenges, including the limited availability of raw materials and assembly difficulties, particularly at high frequencies. In this paper, a novel flexible self-assembly method was proposed, through which a controllable solid solution of close-packed structures can be attained. By integrating the flexible self-assembly method with an all-dielectric left-handed material structure, a negative-refractive-index metamaterial (NRIM) model was conceived. Magnetism is not a prerequisite under this scheme; therefore, a demonstration based on a microwave dielectric ceramic was proposed, wherein two subdivisions of close-packed structures were studied. Analysis of the field distribution and electromagnetic eigenmodes using finite element analysis revealed a negative refraction band at around 20 GHz, with a relative bandwidth of 4.2%. The analysis of dispersion behavior verified spatial isotropism in the NRIM. In summary, the designed passive NRIM only requires moderate dielectrics and is isotropic in three dimensions, thus bearing a high resemblance to natural mediums.

Microwave imaging sensor system using metamaterial lens for subwavelength resolution

Lenses made of negative refractive index metamaterials can overcome the diffraction-limited resolution of imaging at working distances in the far-field. This paper describes a novel negative index metamaterial (NIM) lens imaging sensor system for subwavelength microwave nondestructive evaluation (NDE) using homodyne detection measurements. The split-ring resonator (SRR) - wire based NIM lens is optimized to work at 6.3 GHz (wavelength λ = 48 mm). Coherent homodyne detection scheme provides a simple, low-cost, and highly sensitive NIM lens imaging sensor system that can be used in the field under practical conditions. Imaging of an omnidirectional source using negative refraction is shown using the proposed system. A focal spot of size 0.62λ was obtained in the image plane of the metamaterial lens with a source to image distance of 1.67λ. The focal spot position was further shifted in the azimuthal plane by simple movement of the source. The study demonstrates unique focus-scanning capability of NIM lenses for target localization without the need for complex tuning mechanisms. The feasibility of the system to image subwavelength defects inside dielectric materials in reflection mode is also investigated for its potential application as an NDE sensor. Defects comprising subwavelength hole of diameter 0.25λ and a groove of dimensions 0.31λ x 0.1λ placed at the focal plane of the lens was imaged using the proposed sensor system.

An analytical study of wireless power transmission system with metamaterials

Abstract Computation of transmission system efficiency of electromagnetic radiation becomes inaccurate when Friis formula is used in Fresnel zone (near-field zone) of wireless power transmission (WPT). A novel methodology has been proposed in this paper for estimation of the power density, received power and transmission system efficiency at a particular operating frequency in the near-field zone. The power transfer and transmission system efficiency need to be maximized for minimal dimensions of transmitter and receiver. Utilization of negative refractive index metamaterials (MM) between transmitter and receiver is proposed to achieve maximum power transfer. In the present paper, variation of received power and system efficiency with respect to transmitting distance is computed and compared for WPT system with and without metamaterials. The variation of system efficiency is studied for varied location of metamaterial from transmitter by assuming metamaterial diameter at a given location is greater than or equal to the beam diameter at that location. Different configurations with varied input power and transmitting distances from shorter to longer range are considered in the study to analyse parameters that affect system efficiency. The approach for identifying optimal number of metamaterials and their placement between transmitter and receiver for maximum power transfer has also been discussed.

Design Method of Acoustic Metamaterials for Negative Refractive Index Acoustic Lenses Based on the Transmission-Line Theory

The design theory for electromagnetic metamaterials with negative refractive indices by using a distributed transmission-line model is introduced to the design of acoustic metamaterials, and a negative refractive index (NRI) acoustic lens is designed theoretically. Adjustments to the negative refractive indices of metamaterials have been carried out by calculations with numerical simulators in conventional design methods. As the results show, many calculations are needed to determine the shape of the unit structures and there are issues in that it is difficult to design those rigorously, meaning that limitations regarding the degree of freedom in the designs are many. On the other hand, the transmission-line model can rigorously design the unit cell structures of both the negative refractive index metamaterials and the background media with the positive refractive indices by calculations with the design formulas and modifying the error from the theory with a small calculation. In this paper, a meander acoustic waveguide unit cell structure is proposed in order to realize a structure with characteristics equivalent to the model, and the waveguide width and length for realizing an NRI acoustic lens are determined from the design formula of the model. The frequency dispersion characteristics of the proposed structure are also computed by eigenvalue analysis and the error in the waveguide length from the theoretical value is modified by a minor adjustment of the waveguide length. In addition, the NRI acoustic lens is constituted by periodically arranging the proposed unit cell structure with the calculated parameters, and the full-wave simulations are carried out to show the validity of the design theory. The results show that the designed lens operates at 2.5 kHz.

Modified fishnet structure with a wide negative refractive index band and a high figure of merit at microwave frequencies

Since realization of negative refractive index (NRI) metamaterials in 2001, which is about 20 years after its introduction as a pure academic concept in 1968, there has been much curiosity and enthusiasm for this field of study. Despite numerous potential applications, different loss mechanisms inherent to such materials have severely restricted their widespread usage. Based on this, NRI metamaterials with low losses are extensively studied in the literature. In this paper, we present a design to realize NRI at microwave frequencies which, to some degree, overcomes the limitations imposed by high intrinsic losses pertinent to such category of materials via bestowing a figure of merit (FOM) as high as 139.3. The layout includes two opposing parts, each containing a square slab in the middle of two crossing wires. The theoretical discussions unveiling the physics behind the NRI behavior, as well as effects of geometrical parameters, temperature, and the use of different metals on the FOM, are extensively discussed.

Modeling of a Negative Refractive Index Metamaterial Unit-Cell and Array for Aircraft Surveillance Applications

In this paper, a unique negative refractive index-metamaterial (NRI-MTM) structure is proposed for traffic alert and collision avoidance system (TCAS) for aircraft application. The NRI-MTM unit-cell structure is a combination of square and circular split ring structures. The proposed square and circular split ring metamaterial (SCM) is designed on the top of a 1.6 mm thick FR-4 substrate, with a single circular split ring on the bottom of the substrate. The total dimension of the SCM unit-cell is 23 × 23mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (0.08λ × 0.08 λ), resonating at 1.06 GHz. This unit-cell is further used to create a 5×4 array structure with a dimension of 130 × 111mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (0.46 λ × 0.4 λ). A High-Frequency Structure Simulator (HFSS) is used to assess the electromagnetic properties of the proposed structures. Furthermore, in order to analyse the double negative (DNG) behaviour of the designed SCM structure, the unit-cell is rotated from 0 to 360 degrees with a step of 60 degrees along the azimuthal plane. On doing so, it is observed that, the proposed SCM structures, including unit-cell and array structures, exhibit DNG and NRI properties over the frequency band of 0.5 to 1.2 GHz, thus making it optimum for aircraft surveillance application. The SCM unit-cell and 5 × 4 SCM array structure were fabricated, and the measured results are in well agreement with the simulated results. Further, the array structure is used as a superstrate on the patch antenna to analyze the impact of the MTM on the antenna performance. In comparison to the antenna without the MTM structure, the 5 × 4 SCM array loaded antenna has a gain improvement of 2.8dB and a bandwidth enhancement of 31.5MHz.

Thz Flat Lenses Based on Wide-Angle, Polarization Independent 3-D Negative Refractive Index Metamaterial for Sub-Wavelength Imaging and Self-Collimation Assisted Beam Transfer Applications

Thz Flat Lenses Based on Wide-Angle, Polarization Independent 3-D Negative Refractive Index Metamaterial for Sub-Wavelength Imaging and Self-Collimation Assisted Beam Transfer Applications

Design of a transmission-line metamaterial with a negative index of refraction at S-Band / Concepção de um metamaterial de linha de transmissão com um índice negativo de refracção na banda S

A metamaterial based on a two-dimensional transmission-line network loaded with inductors and capacitors enabling to achieve negative-refractive-index (NRI) is developed. The dispersion characteristics are calculated by their equivalent circuit model and an operating frequency of 2.95 GHz in which there is impedance matching with free space is chosen in the S-Band. This NRI metamaterial supports transverse electric (TE) waves, thus it can be used in applications such as lensing.

Negative Index Metamaterial Lens for Subwavelength Microwave Detection

Metamaterials are engineered periodic structures designed to have unique properties not encountered in naturally occurring materials. One such unusual property of metamaterials is the ability to exhibit negative refractive index over a prescribed range of frequencies. A lens made of negative refractive index metamaterials can achieve resolution beyond the diffraction limit. This paper presents the design of a metamaterial lens and its use in far-field microwave imaging for subwavelength defect detection in nondestructive evaluation (NDE). Theoretical formulation and numerical studies of the metamaterial lens design are presented followed by experimental demonstration and characterization of metamaterial behavior. Finally, a microwave homodyne receiver-based system is used in conjunction with the metamaterial lens to develop a far-field microwave NDE sensor system. A subwavelength focal spot of size 0.82λ was obtained. The system is shown to be sensitive to a defect of size 0.17λ × 0.06λ in a Teflon sample. Consecutive positions of the defect with a separation of 0.23λ was resolvable using the proposed system.

Active control of the hybridization effect of near-field coupled resonators in metamaterial for a tunable negative refractive index at terahertz frequencies

To achieve broadband negative refraction, which is useful for many practical applications, recent studies have shown that the use of hybridization is one of the most effective methods. Nevertheless, reported studies often propose changing structural parameters rather than external factors. In this article, a method to control actively the bandwidth of negative refractive index (NRI) metamaterials based on second-order hybridization operating in the terahertz regime is proposed numerically by use of a simple disk-grid dimer. By use of the semiconductor InSb as the metallic component, the conductivity is tunable according to temperature, which changes the separation of magnetic resonant frequencies in second-order hybridization scheme and the plasma frequency of metamaterial. Consequently, the fractional bandwidth of the double-NRI region is extended. The mechanism of the phenomenon is explained clearly by both calculations and simulation. Finally, the fractional bandwidth of the NRI region was optimized by variation of the dielectric thickness of the disk-grid monomer and the thickness of the interlayer between the two monomers. The optimal fractional bandwidth is 10.4%, with a high transmitted power of 94% and a high figure of merit of 52.9 at 1.68 THz. Our results pave the way for the implementation of diverse semiconductors in tunable broadband NRI metamaterials at terahertz frequencies.

A Combined WU-shaped NRI Metamaterial for Dual Band Microwave Application

In the field of electromagnetism, artificially constructed metamaterial has opened a new hope for the researchers. Metamaterial is developed artificially and not found available in nature. This composite material shows more irregular properties than the natural materials. Negative refractive index (NRI) is one of the exceptional properties of it. Therefore, many researchers have started investigation with this material. Some metamaterials found in literature have NRI properties but very few of them are found operating simultaneously in C-and X-band. In this study, a new combined WU-shaped NRI metamaterial unit cell is presented. Here, three different substrate materials have been adopted for sensitivity analysis. The used substrate materials are FR-4, Rogers RT6006 and Rogers RT6010 and their permittivity are 4.3, 6.15 and 10.2 respectively. Finite integration technique based numerical tool is adopted for all analyses. The electromagnetic properties are determined by Nicolson-Ross-Weir (NRW) method. Perfect electric-magnetic boundary condition is applied for simulation. The metamaterial unit cell exhibits negative refractive index (NRI) within C-band and X-band for all these substrate materials. C-and X-band are widely used for satellite and long distance communication. Moreover, the proposed material shows better effective medium ratio with increasing relative permittivity which ensures the materials proper operation and sensitivity for any applications.

Plasmonic hybridization in symmetric metamaterial for broadband negative refractive index: simulation, experiment and characterization

This work demonstrates a broad-band negative refractive index (NRI) by using the hybridization effect of dish-net dimer metamaterials (MMs), in the GHz frequency regime. Two symmetric layers of dishnet structure are expected to realize a wideband of negative refractive index with a high transmitted power of 88%, a high figure of merit of 26 at 15.2 GHz and to make the MM insensitive for polarization/wide-incident angle of electromagnetic wave. By adjusting the distance between these layers and their dielectric thickness, the NRI bandwidth are flexibly controlled. Further study indicates that the refractive index strongly depends on tangent loss of the dielectric substrate and number of hybridized modes. Also, we numerically investigate the hybridization characteristic in various operational frequency ranges. The performance of the proposed metamaterial is numerically, theoretically and experimentally demonstrated. This paper strengthens the novelty and computation of NRI metamaterials.

A graphene‐based tunable negative refractive index metamaterial and its application in dynamic beam‐tilting terahertz antenna

AbstractGraphene is emerging as a kind of promising 2D material in designing reconfigurable terahertz (THz) devices owning to its tunability of the surface conductivity, which can be tuned by changing the chemical potential. In this work, we demonstrated a graphene‐based THz tunable negative refractive index metamaterial. The proposed metamaterial unit consists of a metallic resonant structure and embedded graphene, which presents a dynamic negative refractive index in the corresponding bands by modulating the chemical potential of the embedded graphene. In addition, a dynamic beam‐tilting antenna loaded with the metamaterial‐based stacked configuration has been designed. Through changing the refractive index of the metamaterial or the number of layers in the stacked configuration, the steering angle can be adjusted from 0° to −27°. This work demonstrates the great potential of graphene in THz tunable negative refractive index materials, provides a classic case about the practical application of controllable negative refractive index, and also gives an effective way to design dynamic beam‐tilting antenna.

Improved wireless power transfer efficiency with non-perfect lenses

Negative refractive index metamaterials (MMs) have found widespread interest in shaping electromagnetic waves. One attractive area is energy transfer using low frequency magnetic waves or wireless power transfer (WPT). Previous reports have presented an isotropic metamaterial with μr = −1 as a “perfect lens” to focus energy and enhance WPT efficiency. In this work, we show that, while the perfect lens condition does enhance efficiency, anisotropic MMs with μr ≠ −1 can provide a larger enhancement in efficiency. These “non-perfect” lenses offer higher efficiency by enhancing the coupled field while incurring lower losses in the magnetostatic waves excited in or on the materials.

Dual‐band beam tilting antenna with low profile negative refractive index metamaterial

A dual band negative refractive index metamaterial (NRIM) loaded beam tilting antenna has been proposed for 5G applications. The proposed antenna comprises of a dual band quarter wave bow–tie dipole with NRIM array fixed above the ground plane. The dual band NRIM unit cell is derived from the conventional double negative meta-material structure. When NRIM arrays have been integrated with dipole antenna at optimum distance and height, the main beam is deflected by 34.5° and 45° with peak gain of 8.1 and 8.93 dB at 2 and 3.5 GHz, respectively. Unlike the conventional beam tilting antennas, no gain reduction observed for both the bands when the beam is tilted, instead gain has been improved about 0.53 and 1.02 dB at lower and higher bands, respectively. Furthermore, the application of the proposed NRIM dipole in array antennas has been studied and discussed.

Broadband negative-refractive index terahertz metamaterial with optically tunable equivalent-energy level.

An optically tunable terahertz negative-refractive index metamaterial (NIM) is proposed. The NIMs are composed of two aluminum rings and two photosensitive ring-shaped silicon apertures coaxially coated on the both sides of Teflon substrate. The NIMS are also designed to realize wide incident angle, polarization insensitivity, and tunability. Similar to the real atom, the unit cell of NIMs is equivalent to the Teflon nucleus surrounded by top and bottom resonator electrons, which indicates that the equivalent-energy level of NIMs can be dynamically controlled by the resonator electrons, once the scale of substrate nucleus is fixed. Using the LC-circuit model, the dynamic control of the equivalent-energy level of NIMs is studied in detail. Simulation results indicate that the transmission of NIMs is tuned from lowpass to highpass when the conductivity of silicon is increased, and the corresponding phase at lower frequency can be continually tuned. Correspondingly, the negative refractive index of NIMs represents dynamically tunable property, and the tunable negative refraction is simulated by classical wedge prism model. Besides, the phase flow indicates that the direction of phase velocity of NIMs is negative for the single-negative index.

A broadband negative refractive index meta‐atom for quad‐band and sensor applications

AbstractA new modified hexagonal design structure with broadband‐negative refractive index metamaterial was proposed in this manuscript. The proposed structure was composed of two symmetrical metallic layers which had same thickness and an FR4 dielectric layer that was sandwiched between these metallic layers. Frequency domain solver of CST microwave studio, which is commercially available was utilized to observe the results of the design structure. The design structure was analyzed by two orientations with respect to the incident plane wave. For normal wave incidence, the suggested structure displays broadband negative refractive index (3.18 GHz) and (0.9 GHz) with a higher effective medium ratio (17.26) that was indicated small electrical size of the structure, whereas for parallel wave incidence, broadband negative refractive index (2.45 GHz), (1.22 GHz), and (1.93 GHz) bandwidth with a higher effective medium ratio (28.01) was obtained. The negative refractive index was verified by simulation and measurement for quad‐band applications. Furthermore, the effective medium ratio (λ/a) of the proposed broadband negative refractive index metamaterial was considerably improved compared to previous suggested multiband metamaterials. In addition, the design structure can also be utilized as a pressure sensor.

Exceptional electromagnetic interference shielding and microwave absorption properties of room temperature synthesized polythiophene thin films with double negative characteristics (DNG) in the Ku-band region

Negative refractive index (NRI) metamaterials with simultaneously negative permittivity and negative permeability have recently experienced tremendous fundamental and practical interest due to their unique electromagnetic properties. In this work, polythiophene (PTh) thin films were synthesized using an in situ chemical oxidative polymerization route. The double negative characteristics (negative permittivity and permeability) appeared simultaneously in the as-prepared PTh thin films. The structural and microstructural characterizations of as-deposited PTh thin films were examined through different techniques including XRD, FESEM, TEM, XPS and FT-IR spectroscopy. Furthermore, microwave absorption properties such as reflection loss (RL), shielding effectiveness (SE) of the PTh thin films with varying monomer concentration were also investigated. The morphological investigation shows that the unique porous microstructure of the PTh fibers has a dramatic impact in the enhancement kinetics of the microwave absorption performance. The PTh fibers deposited at 0.2 M concentration of thiophene showed minimum reflection loss (RL) of −44.24 dB (99.9% microwave absorption) at 17.59 GHz frequency, with a total shielding effectiveness of −18 dB at 16.3 GHz with only 5.31 microns of matching thickness. The PTh thin films exhibit exceptional microwave absorption ability at a fairly thin thickness and show a greater effective absorption bandwidth (RL < −10 dB) in the Ku-band region; these findings definitely pave the way to explore the study of different conducting polymer materials for utilization in EMI-shielding applications.

A broadband tunable terahertz negative refractive index metamaterial

A strategy to greatly broaden negative refractive index (NRI) band, reduce loss and ease bi-anisotropy of NRI metamaterials (MMs) has been proposed at terahertz frequencies. Due to the symmetric structure of the MM, the transmission and refractive index are independent to polarizations of incident radiations, and a broadband NRI is obtainable for the range of the incident angle from 0° to 26°. In addition, THz MMs’ properties such as transmission, phase and negative refraction exhibit a real-time response by controlling the temperature. The results indicate that the maximum bands of the negative and double-negative refraction are 1.66 THz and 1.37 THz for the temperature of 40 °C and 63 °C, respectively. The figure of merit of the MMs exceeds 10 (that is, low loss) as the frequency increases from 2.44 THz to 2.56 THz in the working temperature range, and the maximum figure of merit is 83.77 at 2.01 THz where the refractive index is −2.81 for a given temperature of 40 °C. Furthermore, the negative refraction of the MMs at the low loss band is verified by the classical method of the wedge, and the symmetric slab waveguide based on the proposed MM has many unique properties.

Focusing ultrasonic waves in a 2-D covered channel above the periodic array of Helmholtz resonators

This paper presents the numerical simulation and experiments on focusing low-frequency ultrasonic waves in a 2-D covered channel above acoustic metamaterials composed of the periodic array of Helmholtz resonators, in which the refractive index can be in a negative value. The 2-D channel above the acoustic metamaterials is covered by a plate to limit the area affected by the negative refractive index metamaterials. Ultrasonic waves propagated in the 2-D covered channel are found to be highly dependent on input frequency and the designed Helmholtz resonator structure, where its negative refractive index causes the focusing phenomenon in this channel. From the numerical simulation and experiments, an amplitude focus spot is observed in the peak-to-peak of the time domain signal and frequency response at 125 kHz by mapping a pressure field in the 2-D covered channel. Different focal points with several input frequencies are also identified. Our research demonstrates the possibility of applying the designed lens based on acoustic metamaterials to improve the focusing effect in ultrasonic testing.