Metamaterial Slab Research Articles

Determination of Effective Constitutive Parameters of Inhomogeneous Metamaterials With Bianisotropy

A recursive algorithm is proposed to accurately extract the electromagnetic parameters of isotropic/bianisotropic metamaterial (MM) slabs of inhomogeneous structures. The algorithm is based on the recursive calculations of measured/ simulated forward and backward scattering (S-) parameters from previously extracted wave impedances and refractive indices of each homogeneous layer (except the analyzed intermediate layer) by a suitable retrieval method. There are two main advantages of our proposed method over existing extraction algorithms. First, it could retrieve the electromagnetic parameters of both isotropic and bianisotropic MM slabs. Second, it is applicable for electromagnetic parameter extraction of inhomogeneous structures. For validation of our method and assessing its accuracy over existing extraction methods, we used the simulation results of various inhomogeneous structures constructed by different isotropic/bianisotropic MM slabs (split-ring-resonator (SRR), SRR-wire, omega, and omega-wire). For completeness, the effects of noise present in S-parameters and iterative error (by increasing the value of iteration numbers) are examined to test the robustness of the proposed algorithm.

Thin Hybrid Metamaterial Slab With Negative and Zero Permeability for High Efficiency and Low Electromagnetic Field in Wireless Power Transfer Systems

Current wireless power transfer (WPT) systems have limited charging distance and high induced electromagnetic field (EMF) leakage. Thus, we first proposed a thin printed circuit board (PCB) type hybrid metamaterial slab (HMS) combining two kinds of metamaterial cell structures. The metamaterial cells in the center area of the HMS have zero relative permeability and straighten the magnetic field direction. The metamaterial cells located at the edges of the HMS have negative relative permeability and change the outgoing magnetic fields to opposite direction by magnetic boundary condition. Therefore, the magnetic field can be more confined between transmitter and receiver coils, enhancing the power transfer efficiency, while decreasing the EMF leakage in a WPT system. In this paper, we demonstrated that increased power transfer efficiency from 34.5% to 41.7% and reduced EMF leakage from −19.21 to −26.03 dBm in 6.78-MHz WPT system. Furthermore, we proposed new analysis method for relative permeability measurement of the metamaterial using a novel cubic structure with perfect electrical conductor and perfect magnetic conductor boundary.

Switching Purcell effect with nonlinear epsilon-near-zero media

An optical topological transition is defined as the change in the photonic iso-frequency surface around epsilon-near-zero (ENZ) frequencies which can considerably change the spontaneous emission of a quantum emitter placed near a metamaterial slab. Here, we show that due to the strong Kerr nonlinearity at ENZ frequencies, a high-power pulse can induce a sudden transition in the topology of the iso-frequency dispersion curve, leading to a significant change in the transmission of propagating as well as evanescent waves through the metamaterial slab. This evanescent wave switch effect allows for the control of spontaneous emission through modulation of the Purcell effect. We develop a theory of the enhanced nonlinear response of ENZ media to s and p polarized inputs and show that this nonlinear effect is stronger for p polarization and is almost independent of the incident angle. We perform finite-difference time-domain simulations to demonstrate the transient response of the metamaterial slab to an ultrafast pulse and fast switching of the Purcell effect at the sub-picosecond scale. The Purcell factor changes at ENZ by almost a factor of three which is an order of magnitude stronger than that away from ENZ. We also show that due to the inhomogeneous spatial field distribution inside the multilayer metal-dielectric super-lattice, a unique spatial topological transition metamaterial can be achieved by the control pulse induced nonlinearity. Our work can lead to ultra-fast control of quantum phenomena in ENZ metamaterials.

Propagation Constant Measurement of Microwave Networks With Symmetric/Asymmetric Reflections

We proposed a Line-Line (LL) method for complex propagation constant $\gamma $ determination of uniform transmission lines or uniform lines loaded by samples having symmetric or asymmetric reflections using calibration-free scattering (S-) parameter measurements. To achieve our goal, we first derived pseudo wave-cascading-matrix (WCM) presentation of a network modeling a uniform line with asymmetric reflections ( $S_{11} \ne S_{22}$ ). Then, we proved that such a network can be represented by the product of three matrices: two matrices for impedance transitions and one matrix for phase and amplitude changes. While the first impedance transition matrix is a $2\times 2$ square matrix with unity values at diagonal entries and two different reflection coefficient values at off-diagonal entries, the second one is the inverse of the first one. Besides, the phase-amplitude change matrix is a diagonal matrix with diagonal entries corresponding to phase and amplitude changes. Next, using such representation of the pseudo WCM, we implemented the LL method to determine $\gamma $ of a network with asymmetric (or symmetric) reflections by a simple trace operation (summation of the diagonal terms in a square matrix) without resorting to ascertaining the reflection coefficient corresponding to an impedance transition. The method was validated by $\gamma $ measurements of an empty waveguide section with length 7.70 mm, the same section loaded by a polyethylene sample, and another waveguide section with length 10.16 mm loaded by bianisotropic metamaterial slabs in C-shape form.

μ-near-zero metamaterial slabs for a new concept of plasmonic sensing platforms

We demonstrate that the excitation of magnetic bulk plasmon-like resonances in μ-near-zero double-negative metamaterial slabs is suitable for the design of new sensing platforms, where light-to-plasmon coupling is reached without requiring a prism or grating coupler. This allows for excitation with light coming directly from the air and for dielectric substrates with any refractive index. In the microwave region this architecture is able to detect changes as small as 10−2 in the refractive index of the superstrate. If the metamaterial slab is backed by a metallic substrate, on the other hand, the system can be used as a light-absorber for light harvesting applications.

Local field in finite-size metamaterials: Application to composites of dielectrics and metal nanoparticles

The theory of the optical response of a metamaterial slab which is represented by metal nanoparticles embedded in a dielectric matrix is developed. It is demonstrated that the account of the reflections from the slab boundaries essentially modifies the local field in the slab and leads to the anisotropy and spatial dispersion of its dielectric function as well as to the emergence of modes which do not exist in an infinite metamaterial. It is shown that these features introduce the existence of self-excited normal waves (polaritons) and mechanical excitons (polarization waves). These findings reveal that the metamaterial slab can be regarded as an active device (``plasmonic oscillator'') which generates sustained polaritons in the presence of dissipation. A relation of this effect with the phenomenon of a plasmonic blackbody or perfect absorber, observed in such structures, is discussed and a possible mechanism of this phenomenon is proposed.

Agile Beamwidth Control and Directivity Enhancement for Aperture Radiation With Low-Profile Metasurfaces

Uniaxial anisotropic near-zero metamaterials have been utilized to design 2-D apertures with beamwidth control and directivity enhancement. Theoretical study shows that the two principal plane beamwidth of an aperture can be significantly reduced, both independently and simultaneously. With two proposed simplified loading models as a transitional structure, the 3-D bulk metamaterials are eventually reduced to 2-D metasurfaces with an array of metallic inclusions. In the design procedure, a critical equivalence between 2-D metasurfaces and 3-D metamaterial slabs with a specific thickness is discussed and validated. Measured results of the fabricated prototypes have demonstrated that a final aperture design loaded with low-profile electric LC and split-ring resonator metasurfaces has a substantial beamwidth reduction on both principal planes and a 4.32 dBi gain enhancement around the operating frequency.

Tunable optical response at the plasmon-polariton frequency in dielectric-graphene-metamaterial systems

By using the scattering matrix formalism, it is studied the optical properties of one dimensional photonic crystals made of multiple layers of dielectric and uniaxial anisotropic single negative electric metamaterial with Drude type responses, with inclusions of graphene in between the dielectric-dielectric interfaces (DGMPC). The transmission spectra for transverse electric (TE) and magnetic (TM) polarization are presented as a function of the incidence angle, the graphene chemical potential, and the metamaterial plasma frequencies. It is found for the TM polarization the tunability of the DGMPC optical response with the graphene chemical potential, which can be observed by means of transmission or reflexion bands around the metamaterial plasmon-polariton frequency, with bandwidths depending on both the incidence angle and the metamaterial plasma frequency. Also, the transmission band is observed when losses in the metamaterial slabs are considered for finite systems. The conditions for the appearance of these bands are shown analytically. We consider this work contributes to open new possibilities to the design of photonic devices with DGMPCs.

Cylindrical to rectangular coordinate transformation for planar phase front synthesis

Coordinate transformation technique is employed to engineer a metamaterial slab that enables planar wave emission from a planar or line localised source. First a rigorous cylindrical to rectangular coordinate transformation is applied resulting in with a sheet current generating a plane wave with impedance matching at the boundary between the device and the surrounding medium. Then, based on physical reasoning, the material constitutive parameters are further reduced to having only one entry of the spatially varying permeability tensor instead of two entries as discussed in the references. This further reduction is equivalent to modifying the near field in the transformed domain. Finally, due to a specific mu-near-zero condition near the origin, the sheet source is transformed to a localised one in the transformed domain and via full-wave simulations, the authors show that the field is still propagating as a plane wave. Moreover, the transmission line (TL) metamaterial implementation with discretised spatially varying permeability is designed. The capacitor- and inductor-loaded two-dimensional TL grid is used to synthesise the required spatially varying relative permeability in the wavefront converter metamaterial slab. The TL metamaterial design is proven to successfully synthesise a planar wave front using a microwave circuit simulator.

Ultra-Broadband Perfect Absorption with Stacked Asymmetric Hyperbolic Metamaterial Slabs

ABSTRACTWe propose a method to realize ultra-broadband perfect absorption by using multiple slabs of asymmetric hyperbolic metamaterial (AHM) made of doped silicon nanowire arrays. Our numerical results show that the absorptance of the structure is greater than 0.99 in the wavelength range from to for an incident transverse magnetic (TM) plane wave at an angle of incidence equal to . Moreover, the broadband absorptance can still be above 0.9 when the angle of incidence is in the range from to . The underlying mechanism is elucidated as due to the combination of matching of impedance at the interfaces and enhanced absorption in the AHM slabs of the structure. This work may provide in the design of metamaterial absorbers with some inspiring guidelines for obtaining highly enhanced absorption over an ultra-broadband and in a wide range of angle of incidence.

Launching and controlling Gaussian beams from point sources via planar transformation media

Based on operations prescribed under the paradigm of Complex Transformation Optics (CTO) [1-5], it was recently shown in [5] that a complex source point (CSP) can be mimicked by a parity-time ($\mathcal{PT}$) transformation media. Such coordinate transformation has a mirror symmetry for the imaginary part, and results in a balanced loss/gain metamaterial slab. A CSP produces a Gaussian beam and, consequently, a point source placed at the center of such metamaterial slab produces a Gaussian beam propagating away from the slab. Here, we extend the CTO analysis to non-symmetric complex coordinate transformations as put forth in [6] and verify that, by using simply a (homogeneous) doubly anisotropic gain-media metamaterial slab, one can still mimic a CSP and produce Gaussian beam. In addition, we show that a Gaussian-like beams can be produced by point sources placed {\it outside} the slab as well [6]. By making use of the extra degrees of freedom (real and imaginary part of the coordinate transformation) provided by CTO, the near-zero requirement on the real part of the resulting constitutive parameters can be relaxed to facilitate potential realization of Gaussian-like beams. We illustrate how beam properties such as peak amplitude and waist location can be controlled by a proper choice of (complex-valued) CTO Jacobian elements. In particular, the beam waist location may be moved bidirectionally by allowing for negative entries in the Jacobian (equivalent to inducing negative refraction effects). These results are then interpreted in light of the ensuing CSP location.

Intrinsic Properties of Metamaterial Slab in Wireless Power Transfer for Efficiency Improvement of Wireless Communication Systems

Metamaterial (MTM) slabs, realized using Split Ring Resonators (SRR’s) to enhance efficiency of Wireless Power Transfer (WPT) systems is discussed here. A detailed study of intrinsic properties obtained by simulation of the actual MTM slab is presented. An improvement in the wavelength to unit cell size of 813, at a low operating frequency of 5680 kHz is shown. Forward transmission coefficients (S₂₁) for with and without MTM slab cases are compared. In this low frequency regime, with simulated S-parameters an efficiency of ~ 68% is achieved by placing the MTM slab closer to receiver coil than the transmitter coil.

Second‐Harmonic Generation from Hyperbolic Plasmonic Nanorod Metamaterial Slab

AbstractHyperbolic plasmonic metamaterials provide numerous opportunities for designing unusual linear and nonlinear optical properties. In this work, second‐harmonic generation in a hyperbolic metamaterial due to a free‐electron nonlinear response of a plasmonic component of the metamaterial is studied. It is shown that owing to a rich modal structure of an anisotropic plasmonic metamaterial slab, the overlap of fundamental and second‐harmonic modes results in the broadband enhancement of radiated second‐harmonic intensity by up to 2 orders of magnitude for TM‐ and TE‐polarized fundamental light, compared to a smooth Au film under TM‐polarised illumination. Compared to the radiated second‐harmonic intensity from a bulk LiNbO3 nonlinear crystal of the same thickness, the SHG intensity from a metamaterial slab may be up to 2 orders of magnitude higher at the certain metamaterial resonances. The results open up possibilities to design tuneable frequency‐doubling integratable metamaterial with the goal to overcome limitations associated with classical phase matching conditions in thick nonlinear crystals.

Efficient design of electrically small antenna using metamaterials for wireless applications

In this paper, a simple and efficient approach is presented for the design of a compact CPW fed slot antenna loaded with complementary split ring resonators (CSRRs) and metamaterial (MTM) slab. It has been shown that the fundamental resonant frequency of the conventional slot antenna can be lowered considerably by loading CSRRs and an MTM slab. The MTM slab consists of an array of 1 × 3 single-sided periodic structure of split ring resonator unit cells. The CSRRs are incorporated on both sides of the rectangular slot and the MTM slab is placed below the dielectric substrate of the antenna to achieve miniaturization. About 56.7% miniaturization is attained for the proposed CSRRs and MTM slab loaded antenna in comparison to the conventional antenna. It is also observed that the minimal effect on the co-polarization and cross-polarization is obtained in the case of proposed antenna. The overall antenna size is 0.23 λ0 × 0.23 λ0 × 0.009 λ0, where λ0 is the free space wavelength. In this proposed antenna a high radiation efficiency of about 84.2% is obtained. Owing to the high efficiency and compact size, the proposed antenna will find broad applications in the field of wireless communication.

Tunable spin splitting of Laguerre–Gaussian beams in graphene metamaterials

Optical spin splitting has attracted significant attention owing to its potential applications in quantum information and precision metrology. However, it is typically small and cannot be controlled efficiently. Here, we enhance the spin splitting by transmitting higher-order Laguerre–Gaussian (LG) beams through graphene metamaterial slabs. The interaction between LG beams and metamaterial results in an orbital-angular-momentum- (OAM) dependent spin splitting. The upper bound of the OAM-dependent spin splitting is found, which varies with the incident OAM and beam waist. Moreover, the spin splitting can be flexibly tuned by modulating the Fermi energy of the graphene sheets. This tunable spin splitting has potential applications in the development of spin-based applications and the manipulation of mid-infrared waves.

Coherent perfect absorber and laser modes in purely imaginary metamaterials

Conjugate metamaterials, in which the permittivity and the permeability are complex conjugates of each other, possess the elements of loss and gain simultaneously. By employing a conjugate metamaterial with a purely imaginary form, we propose a mechanism for realizing both coherent perfect absorber (CPA) and laser modes, which have been widely investigated in parity-time symmetric systems. Moreover, the general conditions for obtaining CPA and laser modes, including obtaining them simultaneously, are revealed by analyzing the wave scattering properties of a slab made of purely imaginary conjugate metamaterials. Specifically, in a purely imaginary conjugate metamaterial slab with a sub-unity effective refractive index, perfect absorption can be realized for the incident wave from air.

Spontaneous Emission inside a Hyperbolic Metamaterial Waveguide

The ability to control the rate of spontaneous emission via the design of nanostructured materials with appropriate electromagnetic properties is important in the development of novel fast sources of incoherent illumination, single photon emitters for quantum optical applications, laser physics and de-excitation of electronic states leading to photodegradation in organic materials. Here, for an emitter placed inside a hyperbolic metamaterial slab of finite thickness comprised of an array of gold nanorods, we experimentally demonstrate an enhancement of the fluorescence coupled to the waveguided plasmon-polariton modes of the metamaterial. We show that fluorescence properties in such finite-size metamaterial design behave differently from commonly studied infinite metamaterials or when the emitters are placed near the metamaterial interface. The emitters inside the metamaterial waveguide exhibit an almost 50-fold reduction of their lifetime, whereas a much smaller reduction (a factor of 2-3) is observed fo...

Calibration-free extraction of constitutive parameters of magnetically coupled anisotropic metamaterials using waveguide measurements.

A calibration-free method for extraction of electromagnetic properties of magnetically coupled anisotropic biaxial metamaterial (MM) slabs from waveguide measurements is proposed. It relies on three measurement steps (thru, empty line, and the same line arbitrarily loaded by the MM slab) to extract electromagnetic properties. It is evaluated against another calibration-dependent method for retrieval of electromagnetic properties of a MM slab constructed by C-shaped resonators. From evaluation analysis, we note that our method, as compared to the calibration-dependent method, not only accurately extracts electromagnetic properties without requiring the use of expensive calibration standards but also is reference-invariant.

Radiative Pulling Forces, Exerted by Evanescent Fields Along a Hyperbolic Metamaterial Slab

We show and investigate in detail the optical pulling forces acting on a particle in the vicinity of a waveguiding structure made of a hyperbolic metamaterial. We have shown analytically that if the waveguide dispersion is negative, the longitudinal radiative force outside of hyperbolic metamaterial is always a pulling force, that is directed toward the source of electromagnetic energy flow in contrast to the common situation when a strong propulsion force emerges near the surface of an open waveguide.

Investigation of various cavity configurations for metamaterial-enhanced field-localizing wireless power transfer

Controlling power to an unintended area is an important issue for enabling wireless power transfer (WPT) systems. The control allows us to enhance efficiency as well as suppress unnecessary flux leakage. The flux leakage from WPT can be reduced effectively via selective field localization. To realize field localization, we propose the use of cavities formed on a single metamaterial slab that acts as a defected metasurface. The cavity is formed by strong field confinement using a hybridization bandgap (HBG), which is created by wave interaction with a two-dimensional array of local resonators on the metasurface. This approach using an HBG demonstrates strong field localization around the cavity regions. Motivated by this result, we further investigate various cavity configurations for different sizes of the transmitter (Tx) and receiver (Rx) resonators. Experiments show that the area of field localization increases with the number of cavities, confirming the successful control of different cavity configurations on the metasurface. Transmission measurements of different cavities show that the number of cavities is an important parameter for efficiency, and excess cavities do not enhance the efficiency but increase unnecessary power leakage. Thus, there exists an optimum number of cavities for a given size ratio between the Tx and Rx resonators. For a 6:1 size ratio, this approach achieves efficiency improvements of 3.69× and 1.59× compared to free space and a uniform metasurface, respectively. For 10:1 and 10:2 size ratios, the efficiency improvements are 3.26× and 1.98× compared to free space and a uniform metasurface, respectively.