Metamaterial Waveguide Structure Research Articles

Ultra-compact and polarization-insensitive silicon waveguide 3 × 3 star-crossing based on composite subwavelength grating metamaterials.

We present what we believe is the first report on a polarization-insensitive 3 × 3 silicon star-crossing utilizing a composite subwavelength metamaterial waveguide structure. Two different types of subwavelength grating metamaterials (nanohole grating and fan-shaped bent subwavelength grating) are respectively used to address diffraction issues in the crossing region and mode interference issues caused by a compact non-adiabatic design. This approach results in a device with an ultra-compact footprint of 12.68 × 10.98 µm2 on a standard 220 nm silicon-on-insulator (SOI) platform. Simulation results show low insertion loss (IL) values of <0.2 dB/0.3 dB and suppressed cross talk (CT) levels of <-27.2 dB/-23.6 dB for TE/TM polarizations across a wavelength range of 100 nm (1500-1600 nm). Experimental measurements of the fabricated devices confirm outstanding performance, with IL values of <0.35 dB/0.4 dB and CT levels of <-31.5 dB/-28.6 dB for TE/TM polarization in the C-band.

Analytical and Numerical Analyses of Multilayer Photonic Metamaterial Slab Optical Waveguide Structures with Kerr-Type Nonlinear Cladding and Substrate

In this paper, we propose the analytical and numerical analyses of multilayer photonic metamaterial slab optical waveguide structures with Kerr-type nonlinear cladding and substrate. The multiple-quantum-well (MQW) photonic metamaterial optical waveguide structure with Kerr-type nonlinear cladding and substrate was also analyzed. We can use the proposed method to study the multilayer optical metamaterial slab optical waveguide structure with the linear cladding and substrate.

High-&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$Q$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Multiple Fano Resonances Sensor in Single Dark Mode Metamaterial Waveguide Structure

We demonstrate multiple Fano resonances in an integrated single dark mode hybrid metamaterial waveguide structure, which consists of three gold cut wires placed on a dielectric board waveguide. With a symmetrical pattern, the design shows double sharp Fano peaks with high $Q$ -factors of 950 and 216, which are attributed to the interaction between plasmon mode and quasi-guided modes. By breaking the structural symmetry, a quadrupole mode on middle vertical cut wire is excited and further results in multiple Fano resonances through both the diffracted waves and near-field coupling with bright–dark mode. Based on the above-mentioned properties, we show a high $Q$ -factor refractive index sensor with figure of merit values of 330 and 281. Interestingly, these $Q$ -factors of Fano peaks can be increased to 1357 and 361 to improve the sensing performance, by capping a silicon layer on the metal surface. This letter provides a way to obtain multiple high $Q$ -factor Fano resonances, which can widen channels for fabricating device in biochemical sensing.

Numerical analysis of high-Q multiple Fano resonances

We design and numerically analyze high-quality (Q) multiple Fano resonances based on a hybrid metamaterial waveguide structure, which consists of T-shaped gold cut wires placed on a dielectric board waveguide. There are three sharp Fano peaks arising from the interference between plasmon dipole mode and different guided modes. The physical origin of obvious Fano behaviors is explained by the three-level plasmonic system and slab waveguide theory. The largest Q-factor reaches 547, and modulation depth of the peak C can get to nearly 100%, making it possible to perfectly realize the Fano switch function. Combining the cramped spectral lines with large near-field confinement, we demonstrate an optical refractive index sensor with a sensitivity of 4920 nm/RIU and a figure of merit of 188. This work provides a way to obtain multiple high Q-factor Fano resonances, which can widen channels for fabricating devices in biochemical sensing and optical switching.

Loosely-bound low-loss surface plasmons in hyperbolic metamaterial

Surface plasmons (SPs) carry electromagnetic energy in the form of collective oscillation of electrons at metal surface and commonly demonstrate two important features: strong lateral confinement and short propagation lengths. In this work we have investigated the trade-off relationship existing between propagation length and lateral confinement of SP fields in a hyperbolic metamaterial system, and explored loosening of lateral confinement as a means of increasing propagation length. By performing finite-difference time-domain analysis of Ag/SiO2 thin-film stacked structure we demonstrate long range (~ 100 mm) propagation of SPs at 1.3 µm wavelength. In designing low-loss loosely-bound SPs, our approach is to maximally deplete electric fields (both tangential and normal components to the interface) inside metal layers and to support SP fields primarily in the dielectric layers part of metamaterial. Such highly-localized field distributions are attained in a hyperbolic metamaterial structure, whose dielectric tensor is designed to be highly anisotropic, that is, low-loss dielectric (Re(ε) > 0; Im(ε) ~ 0) along the transverse direction (i.e., normal to the interface) and metallic (large negative Re(ε)) along the longitudinal direction, and by closely matching external dielectric to the normal component of metamaterial’s dielectric tensor. Suppressing the tangential component of electric field is shown to naturally result in weakly-confined SPs with penetration depths in the range of 3–10 µm. An effective-medium approximation method is used in designing the metamaterial waveguide structure, and we have tested its validity in applying to a minimally structured core-layer case (i.e., composed of one or two metal layers). Low-loss loosely-bound SPs may find alternative applications in far-field evanescent-wave sensing and optics.

Piezoelectric T-matrix approach and multiple scattering of electroacoustic waves in thin plates

Metamaterial-enhanced harvesting (MEH) of wave energy in thin plates and other structures has appeared recently for powering small sensors and devices. To support continued MEH concept development, this paper proposes a fully coupled T-matrix formulation for analyzing scattering of incident wave energy from a piezoelectric patch attached to a thin plate. More generally, the T-matrix represents an input–output relationship between incident and reflected waves from inclusions in a host layer, and is introduced herein for a piezoelectric patch connected to an external circuit. The utility of a T-matrix formalism is most apparent in scenarios employing multiple piezoelectric harvesters, where it can be re-used with other T-matrices (such as those previously formulated for rigid, void, and elastic inclusions) in a multiple scattering context to compute the total wavefield and other response quantities, such as harvested power. Following development of the requisite T-matrix, harvesting in an example funnel-shaped metamaterial waveguide structure is predicted using the multiple scattering approach. Enhanced wave energy harvesting predictions are verified through comparisons to experimental results of a funnel-shaped waveguide formed by placing rigid aluminum inclusions in, and multiple piezoelectric harvesters on, a Lexan plate. Good agreement with predicted response quantities is noted.

Analyzing the multilayer metamaterial waveguide structure with the Kerr-type nonlinear cladding

A general method for analyzing the multilayer metamaterial optical waveguides with the Kerr-type nonlinear cladding was proposed. To prove the accuracy of the proposed general method, a degenerated example was introduced. The analytical and numerical results show excellent agreement. The similar process can also be used to analyze TM waves propagating in the multilayer metamaterial waveguide structure with the Kerr-type nonlinear cladding.

Enhanced absorption in silicon metamaterials waveguide structure

Metamaterial waveguide structures for silicon solar cells are a novel approach to antireflection coating structures that can be used for the achievement of high absorption in silicon solar cells. This paper investigates numerically the possibility of improving the performance of a planar waveguide silicon solar cell by incorporating a pair of silicon nitride/metamaterial layer between a semi-infinite glass cover layer and a semi-infinite silicon substrate layer. The optimized layer thicknesses of the pair are determined under the solar spectrum AM1.5 by the effective average reflectance method. The transmission and reflection coefficients are derived by the transfer matrix method for values of metamaterial’s refractive index in visible and near-infrared radiation. In addition, the absorption coefficient is examined for several angles of incidence of the transverse electric polarized (TE), transverse magnetic polarized (TM) and the total (TE&TM) guided waves. Numerical results provide an extremely high absorption. The absorptivity of the structure achieves greater than 98 %.

Analysis of evanescent field of TE and TM mode in the grounded slab metamaterial waveguide structure

The propagation constant in terms of β/k with function of normalized frequency (V) for grounded slab metamaterial waveguides of TE and TM mode is calculated and its graphical results is shown with the help of MATLAB. These results are compared with the three layer metamaterial waveguide structure. The evanescent fields are investigated for various modes in a perfectly grounded metamaterial planer slab waveguide for both cases. The power distribution inside the cover and film region is investigated and distribution of energy flux along the transverse profile of waveguide for different normalized frequency (V) is shown properly. Metamaterials show the unusual properties over normal material and due to the grounded slab structure, the evanescent fields are propagated more inside the cover region. This analysis can be useful to enhance the sensitivity of metamaterial optical waveguide sensor.

Ultra-broadband absorption in mid-infrared spectrum with graded permittivity metamaterial waveguide structure

In this work, we demonstrate an ultra-broadband metamaterial absorber working in mid-infrared spectrum with a graded permittivity waveguide structure. The proposed absorber can achieve near-perfect absorption at wavelengths in the range from 3.4 to 5.8 µm. The absorption bandwidth is maintained quite well even at large angles of incidence. The gradually changing permittivity of the dielectric films in the multilayered absorber allows light capture through slow light mode and energy dissipation through localized electromagnetic resonance. The design approach presented here is effective for the construction of broadband absorbers and is also helpful as a theoretical support for the fabrication of other photonic devices.

Antireflection Coating at Metamaterial Waveguide Structure by Using Superlattices (LANS)

The characteristics of electromagnetic wave reflection and transmission by multilayered structures consisting of a pair of left-handed material (LHM) and superlattices (LANS) slabs inserted between two semi-infinite dielectric media are investigated for photovoltaic and solar energy applications. Maxwell’s equations are used to determine the electric and magnetic fields of the perpendicular polarized wave incident at each layer. Snell’s law is applied and the boundary conditions are imposed at each layer interface to calculate the reflected and transmitted coefficients of the structure. The reflected, transmitted powers are determined using these coefficients by a recursive method. The reflected and transmitted powers are computed in both visible and microwave spectral band with the appropriate LHM for each band and appropriate location of LANS in the structure. They are illustrated as a function of the incident wavelength, angle of incidence, magnetic fraction of LANS and thickness of the slabs with the emphasis on the appropriate refractive indices. I found that, zero reflectance and maximum transmittance of the incident powers are achieved for visible spectral band at a single frequency if LHM and LANS have the same refractive index of opposite signs with the same width and more magnetic material of LANS while the reflected power is zero for less magnetic material of LANS in the microwave spectral band which realizes antireflection coating in this structure.

Antireflection Coating at Metamaterial Waveguide Structures for Solar Energy Applications

The optical reflection properties of a periodic metamaterial-dielectric multilayered structure are investigated theoretically. The structure is an antireflection coating if it is placed between two semi-infinite media of the same kind and the two slabs constituted each period have the same width and opposite refractive indices. Maxwell's equations are used to determine the electric and magnetic fields of the incident waves at each layer. Snell's law is applied and the boundary conditions are imposed at each layer interface to calculate the reflected and transmitted coefficients of the structure. The reflected and transmitted powers are determined using these coefficients by a recursive method. In the numerical results, the reflected powers are computed and illustrated as a function of wavelength of the incident waves, angle of incidence and number of periods with the emphasis on the appropriate refractive indices.

Efficient time-domain analysis of highly dispersive linear and non-linear metamaterial waveguide and antenna structures operated in the impulse-regime

A numerical method is proposed for the analysis of highly dispersive linear and non-linear metamaterial structures. Firstly, pulse propagation along non-linear composite right/left-handed (CRLH) transmission line (TL) is studied. The dispersive CRLH line is loaded with hyper-abrupt diodes to achieve non-linearity. The characteristic impedance and propagation constant of each unit-cell of the CRLH TL is varied at each time-step, and the non-uniform line is then analysed. A novel interpolation scheme, based on the smooth behaviour of the propagation constant with small variations of the shunt capacitor is proposed. Numerical results show that high accuracy and up to 80% reduction of the computational cost is achieved. A general example which combines dispersion and non-linearity is presented, validated and discussed. Secondly, the impulse-regime radiation of a leaky-wave antenna is analysed and used to describe a real-time spectrogram analyser (RTSA). A new calibration method for the system, which takes into account the directivity variation of the antenna with frequency, is proposed. This method provides an extremely fast and accurate tool to characterise the behaviour of leaky-wave-based RTSA systems.

Effect of backward radiation of electromagnetic waves by a metamaterial waveguide structure

The anomalous properties of electromagnetic radiation from an antenna in the form of a rectangular section waveguide tube made of a metamaterial with the negative real parts of permittivity and permeability have been demonstrated. The metamaterial is an isotropic two-dimensional array of left and right-twisted coils placed on a thin polyurethane substrate. The coils of the sample are directed in equal numbers along the x, y, and z axes. The possibility of the preferential backward radiation of the structure has been shown on the basis of numerical calculations with the use of the method of moments and the measurements of the radiation pattern of the antenna in an anechoic chamber at frequencies close to the 3-GHz resonance frequency of the metamaterial. The causes and existence conditions of the effect have been revealed.

Can light be stopped in realistic metamaterials?

Arising from: K. L. Tsakmakidis, A. D. Boardman & O. Hess , 397–401 (2007)10.1038/nature06285 ; Tsakmakidis et al. reply Tsakmakidis et al.1 extend earlier work (for example, refs 2, 3) in proposing a novel metamaterial waveguide structure that can stop broadband light, producing so-called “trapped rainbows”. The authors make the bold assumption that metamaterial loss can be ignored: but material loss, with dispersion, is an inherent feature of negative-index metamaterials (for example, refs 4, 5); any realistic model must include loss and dispersion to satisfy the fundamental principle of causality6. Here we revisit the authors’ predictions1 and show that even when an arbitrarily small metamaterial loss is introduced, it is impossible to stop the light; moreover, we find that all slow-light modes in such structures give impractically large propagation losses.

Guided modes of elliptical metamaterial waveguides

The propagation of guided electromagnetic waves in open elliptical metamaterial waveguide structures is investigated. The waveguide contains a negative-index media core, where the permittivity $\ensuremath{\epsilon}$ and permeability $\ensuremath{\mu}$ are negative over a given bandwidth. The allowed mode spectrum for these structures is numerically calculated by solving a dispersion relation that is expressed in terms of Mathieu functions. By probing certain regions of parameter space, we find the possibility exists to have extremely localized waves that transmit along the surface of the waveguide.