Refractive Index Metamaterials Research Articles

New Horizons in Near-Zero Refractive Index Photonics and Hyperbolic Metamaterials.

The engineering of the spatial and temporal properties of both the electric permittivity and the refractive index of materials is at the core of photonics. When vanishing to zero, those two variables provide efficient knobs to control light-matter interactions. This Perspective aims at providing an overview of the state of the art and the challenges in emerging research areas where the use of near-zero refractive index and hyperbolic metamaterials is pivotal, in particular, light and thermal emission, nonlinear optics, sensing applications, and time-varying photonics.

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.

Automatic design of pixelated near-zero refractive index metamaterials based on elite-preserving genetic algorithm optimization

Near-zero refractive index metamaterials (NZIM) have drawn a lot of attention recently for the development of high directivity antennas. However, their application has been constrained by the tedious and time-consuming manual design process. In this work, we offer an automated optimization design technique for NZIM that combines a modeling process based on pixelated metamaterials with an optimization process based on heuristic algorithms. The interactive CST-Python simulation is utilized for accomplishing the above automated design. In detail, the elite-preserving genetic algorithm (EGA) is specifically employed because of its improved capacity in locating the superior unit, and the fitness function proposed guarantees the performance of bandwidth and optimization process based on the principles of proportionate integration and penalty-like functions, respectively. Utilizing the suggested technique, we designed a NZIM unit that operates between 8.0 and 8.52 GHz. In order to validate the performance of the NZIM, a rectangular microstrip patch antenna (MPA) resonating at 8.2 GHz was employed as the radiation source for studies. According to the simulation results, the developed NZIM has a clearly beam focusing effect which causes the MPA's spherical waves to converge into quasi-plane waves. Furthermore, discussing the NZIM’s application in high directivity antennas designing, the prototypes of the NZIM and MPA were fabricated and tested. The results revealed that the MPA’s gain enhancement achieves 5.3 dB on average and 6.05 dB on maximum. Due to the automated optimization and design, this work is promised to advance NZIM's further applicability in high directivity antennas.

Far-Field Acoustic Imaging Enhancement Using Compact Graded Refractive Index Metamaterials

Far-Field Acoustic Imaging Enhancement Using Compact Graded Refractive Index Metamaterials

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.

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.

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

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.

A Compact Wideband CSRR Near Zero Refractive Index and Epsilon Negative Metamaterial for Wearable Microwave Applications

A complementary split-ring resonator (CSRR) decagonal shaped textile-based single-negative metamaterial (MTM), considering a frequency range from 1 to 15 GHz, is presented in this paper. Seven different unit cell arrays (i.e., 1×1, 1×2, 1×3, 2×1, 2×2, 1×3 and 3×3) are analysed to evaluate the effects of the unit-cell arrays on the resonance frequencies of the MTM. The designed unit cell arrays exhibit average negative permittivity bandwidth of 12.87 GHz (from 1 to 12.87 GHz) and an average near-zero-refractive-index (NZRI) bandwidth of 11.98 GHz (from 1.015 to 12.995 GHz). Simultaneous negative permittivity and NZRI results at L, S, C, X and Ku frequency bands indicate the proposed MTM is suitable for various wearable applications in these frequency regimes.

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.

An Efficient Semianalytical Modal Analysis of Rectangular Waveguides Containing Metamaterials with Graded Inhomogeneity

Rectangular waveguides containing inhomogeneous metamaterials with graded refractive-index profiles have potential applications in bending waveguides and radiation-enhanced antennas, and accurate eigenvalue solutions are prerequisite. Commonly used commercial electromagnetic solvers such as HFSS, COMSOL, and CST could not efficiently calculate the eigenvalues of waveguides containing graded refractive-index dielectrics. In this paper, an accurate and efficient semianalytical method based on the modal expansion has been proposed to solve these waveguides. The proposed method has been employed to calculate the eigenvalues, including the cutoff wavenumbers and dispersion relations, for metamaterials with various graded refractive-index profiles. Calculated results are then validated by comparison, using commercial solver HFSS, which indicates the superiority of the proposed method in accuracy and efficiency. Below-cutoff backward wave propagation is observed in waveguides filled with graded refractive-index metamaterials, which provides a new approach for waveguide miniaturization.

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.

"Short circuit" of power flux of light in near-zero refractive index metamaterials.

An interesting phenomenon of "short circuit" of the power flux of light arising from the electromagnetic decoupling has been discovered in a composite structure composed of epsilon-mu-near-zero medium (EMNZ) and epsilon-near-zero (ENZ). In the composite structure, the power flux of light goes around the ENZ components with any shapes and sizes and perfectly passes through the composite structure via the EMNZ until the connection of the EMNZ is broken. The ENZ components can freely manipulate the power flux of light flowing along arbitrary paths inside the EMNZ without reflection. In addition, we discover that the embedded ENZ components enhance the stability of transmission property of impure EMNZ effectively. The arbitrarily-shaped non-ENZ dopants of EMNZ are able to be well hidden by coating an ENZ cladding with moderate thickness, which is demonstrated through an actual structure composed of photonic crystals. In the area of transmission and processing of optical signal in two-dimensional integrated optical systems, the EMNZ-ENZ composite structure we construct employs potential application prospects.

Topologically protected zero refraction of elastic waves in pseudospin-Hall phononic crystals

Zero-angle refraction of elastic waves in metamaterials has attracted attention for its extraordinary wave collimation properties. However, earlier implementations relied on the specific flat equifrequency curve of the phononic crystals suffer from a narrow range of incident angles or operating bandwidths, which severely hinders the exploration and design of functional devices. Here, we propose an elastic near-zero refractive index metamaterial of a triangular lattice to realize topological zero refraction with arbitrary angles of incidence and wide working frequency range. Topological robustness of the zero-angle refraction of pseudospin-Hall edge state against defects is experimentally demonstrated. Furthermore, tunable wave mode conversion associated with the zero-angle refraction is revealed and discussed. These results provide a paradigm for the simultaneous control of the refraction properties of longitudinal and transverse waves that can be employed for designing the topological elastic antennas and elastic wave collimator.

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.

Space wave channeling enabled by conformal transformation optics

Transformation optics (TO) methodology provides an unconventional approach to mold the path of electromagnetic (EM) waves. Most of the previous works reported today are limited to control EM waves in either reflection or transmission mode. However, by merging TO and geometrical optics, we present a concept of space wave channeling metamaterial (SWCM) to manipulate backward and forward scattered waves simultaneously. As proof-of-principle, backward anomalous scattering, forward anomalous scattering, and multibeam arbitrary scattering patterns are realized through only one inhomogeneous isotropic dielectric medium with proper arrangement of embedded perfect electric conductor (PEC) pieces. Numerical full-wave simulations are performed to verify the potential realization of SWCM with all-dielectric gradient refractive index metamaterials. The proposed designing approach paves the way to the development of EM wave manipulations with feasible constitutive materials.