Resonant circuit model for efficient metamaterial absorber

Miniaturized and Actively Tunable Triple-Band Terahertz Metamaterial Absorber Using an Analogy I-Typed Resonator

Triple-band terahertz metamaterial absorber with design of miniaturization and compactness is presented in this work. The unit cell of the terahertz absorber is formed by an analogy I-typed resonator (a rectangular patch with two small notches) deposited on top of dielectric sheet and metallic mirror. The miniaturized structure design exhibits three discrete frequency points with near-perfect absorption at terahertz regime. The three absorption peaks could be ascribed to localized resonances of analogy I-typed resonator, while the response positions of these absorption peaks at the analogy I-typed resonator are different by analyzing the near-field patterns of these resonance peaks. Changes in structure parameters of the analogy I-typed resonator are also investigated. Simulation results revealed that the notch sizes of the rectangular patch are the key factor to form the triple-band near-perfect absorption. Further structure optimization is given to demonstrate triple-band polarization insensitive performance. Moreover, actively tunable absorption properties are realized by inserting or introducing vanadium dioxide with adjustable conductivity into the metamaterial structure. It is revealed that the insulator-metal phase transition of vanadium dioxide is the main reason for the modulation of absorption performance. Compared with previous multiple-band absorbers, the device given here has excellent features of high degrees of simplification, miniaturization, and active modulation, these are important in practical applications.

Broadband RCS Reduction by a Quaternionic Metasurface

A quaternionic metasurface consisting of two pairs of units with destructive phase difference is proposed to extend the bandwidth of radar cross section (RCS) reduction. The two pairs of units are designed to have complementary phase-different bandwidth, which extends the bandwidth of RCS reduction. The overlaps of their bandwidth enhance the RCS reduction, resulting in a metasurface having broadband and strong RCS reduction. This design and the wideband RCS reduction of the quaternionic metasurface were verified by analytical calculation with superposition principle of electric field, numerical simulation with commercial software package CST Microwave Studio and experiment in microwave anechoic chamber. The scattering mechanism and the angular performance of the quaternionic metasurface were also investigated.

Multi-band terahertz superabsorbers based on perforated square-patch metamaterials

This paper presents a multi-band terahertz superabsorber with a surface structure that consists of a square metallic patch with a very small rectangular hole whose area is only 3.94% of the square patch. The introduction of a rectangular hole in the square patch plays an important role in achieving multi-band absorption. Three resonant bands with very high absorption (>95%) were observed in the terahertz range. Different from the near-field distributions of the traditional square patch with no modification, the introduction of a rectangular hole in the square patch can break the near-field distributions of the traditional square patch with no modification or can rearrange them to form some new or extra resonance modes, thereby generating multi-band absorption. Considering the fact that the introduced rectangular hole plays the key role in the rearrangement of the near-field and the introduction of some new resonance modes, the parameter changes of the rectangular hole introduced in the square patch provide considerable freedom in controlling the number of absorption peaks, and the resonant bands can be tuned to quad- or dual-band absorption. The multi-band superabsorbers given here should have potential applications in numerous areas.

Pyramidal metamaterial absorber for mode damping in microwave resonant structures

In many resonant structures the damping of parasitic or higher order modes is indispensable to guarantee a correct and stable performance. This is particularly true in the microwave region in case of cavities or other resonant systems operating in accelerating structures, where the mitigation of spurious resonance effects is mandatory to achieve high quality particle beams. We present the results on the mode suppression in a real pillbox cavity by inserting a properly designed pyramidal metamaterial that acts as light, small volume damper for specific resonances in the range 3-4 GHz, only slightly perturbing other intrinsic modes. Measurements of the cavity response without and with the metamaterial absorber are presented and compared with full wave simulations. Field distribution for the pillbox intrinsic modes under scrutiny is also presented, showing that damping induced by the metamaterial critically depends on its relative position inside the cavity.

Generalized circuit model for all-dielectric narrowband metasurface absorbers

All-dielectric metasurface absorbers have great potential in many scientific and technical applications. The emerging metasurfaces show strong and versatile capabilities in controlling absorptance, reflectance, and transmittance of electromagnetic waves. In this work, we propose and investigate all-dielectric metasurface absorbers with an equivalent circuit model. In the proposed circuit model, we satisfy the first Kerker condition. To verify the accuracy of the proposed model, the obtained results for an all-dielectric cubic metasurface absorber are compared with the existing experimental data. Moreover, using the proposed circuit model, we propose a hemisphere structure and compare the results of the proposed model with those of full-wave simulations. With this novel structure, we achieve higher absorptance and quality factor in comparison to a cubic one. Additionally, our proposed model reduces the calculation time and needs less memory compared to full-wave simulations. The results of the circuit model have an acceptable agreement with the experimental data and those of full-wave simulations. The proposed circuit model is simple yet general. It provides physical insight into the design and operation of various sub-wavelength structures in the broad frequency range, including THz and visible regions.

A broadband terahertz metamaterial absorber enabled by the simple design of a rectangular-shaped resonator with an elongated slot

Broadband metamaterial absorbers are of critical importance in practical applications, but their obtainment approaches are quite complex at present. We demonstrate here that a fairly simple structure design formed by a rectangular-shaped resonator having an elongated slot can be utilized to achieve a broadband absorption response at terahertz frequencies. More than 50% absorption in a continuous frequency range of 1.62 THz (with a central frequency of 2.05 THz) can be gained, and its relative absorption bandwidth is 79.02%, which is superior to that of previous broadband absorption devices. The basic principle of the broadband absorption originates from the superposition of four different but narrowly separated resonance peaks that resulted from different response positions of the suggested resonator. Results further reveal that the broadband terahertz absorption performance (or its four resonance peaks) can be controlled by the resonator dimensions. The suggested method can provide a new type of design strategy to realize broadband integrated terahertz absorption devices.

Analytical method for metal-insulator-metal surface plasmon polaritons waveguide networks

Metal-insulator-metal (MIM) surface plasmon polaritons (SPPs) waveguides with side-coupled resonators have been widely studied through various approaches. However, few methods are both physically transparent and complete. Here, an analytical approach, which is based on the Green's function method, is developed in order to investigate electromagnetic wave transmission across SPPs MIM waveguide networks. The proposed method is applied in order to model different MIM-waveguide geometries with weakly-coupled side stubs, comparing to the geometries with strongly-coupled stubs. The weak coupling between the backbone and stubs is taken into account by the electromagnetic field leakage at metal-insulator interface. Analytical expressions for transmittance in cases of single stub and cavity are obtained straightforwardly. Our method shows excellent computational efficiency in contrast with solving Maxwell equations numerically.

Theoretical excitation of 2-D (1, 1) cavity mode with asymmetric sword-shaped notched square resonators for metamaterial perfect multiband absorbers in infrared range

1-D cavity modes in 1-D resonators, such as strips, have been used to design multiband metamaterial perfect absorbers. As for 2-D resonators, such as squares and crosses, most studies still focus on exciting the 1-D cavity modes. In this paper, a symmetry-breaking idea is proposed for 2-D cavity mode excitation. An asymmetric sword-shaped notched square resonator is proposed to excite a new 2-D (1, 1) cavity mode, in addition to the 1-D (1, 0) and (3, 0) cavity modes. Thus, a triple-band MPA was successfully produced with average absorptivity of 98.8% in the infrared range. The 2-D cavity mode provides better performance as a biosensor than the 1-D cavity modes do. To obtain more absorption bands, a six-band metamaterial perfect absorber with average absorptivity of 97.2% was successfully produced by combining two sword-shaped notched square resonators with different sizes. A nine-band metamaterial perfect absorber was successfully produced with average absorptivity of 94.5% by combining three sword-shaped notched square resonators with different sizes.

A Co-Polarization Broadband Radar Absorber for RCS Reduction

In this article, a single layer co-polarization broadband radar absorber is presented. Under normal incidence, it achieves at least 90% of absorption from 5.6 GHz to 9.1 GHz for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations. Our contribution and the challenge of this work is to achieve broadband absorption using a very thin single layer dielectric and it is achieved by rotating the resonating element by 45 ∘ . An original optimized Underlined U shape has been developed for the resonating element which provides a broadband co-polarization absorption. The structure is 12.7 times thinner than the wavelength at the center frequency. To understand the absorption mechanism, the transmission line model of an absorber and the three near unity absorption peaks at 5.87 GHz, 7.16 GHz and 8.82 GHz have been used to study the electric and magnetic fields. The physical insight of how the three near unity absorption peaks are achieved has also been discussed. After fabricating the structure, the measurements were found to be in good agreement with the simulation results. Furthermore, with the proposed original UUSR resonating element, the operational bandwidth to thickness ratio of 6.43 is obtained making the proposed UUSR very competitive.

Metamaterial perfect absorber with unabated size-independent absorption

Metamaterial absorbers open a new door for the design of optical harvesting devices ranging from the microwave to optical regimes. The top resonator in these structures is critical for the function of metamaterial absorbers. The resonant frequency, bandwidth, and maximum absorption mainly depend on the choice of material, shape, and size of the top resonator. The maximum absorption is generally impaired as the size of the resonator changes, due to the high sensitivity of impedance matching with the medium. In this paper, we experimentally demonstrate a metamaterial perfect absorber with unabated absorption as its resonator's size changes. The perfect absorber is based on an array of metal squares inscribed with a hollow square. The absorption maxima stay above 98% as the size changes from 600 to 1500 nm in the mid-infrared region, agreeing with simulated results yielding an absorption of ~100%. The unabated absorption properties can be interpreted by the equivalent circuit theory. Moreover, the experimental absorption remains above 91% for incident angles change up to 50°, both for TE and TM polarization. Our work offers a method for achieving stable perfect absorption in sensing, filtering, and selective thermal emission.

Design of Quad-Band Terahertz Metamaterial Absorber Using a Perforated Rectangular Resonator for Sensing Applications

Quad-band terahertz absorber with single-sized metamaterial design formed by a perforated rectangular resonator on a gold substrate with a dielectric gap in between is investigated. The designed metamaterial structure enables four absorption peaks, of which the first three peaks have large absorption coefficient while the last peak possesses a high Q (quality factor) value of 98.33. The underlying physical mechanisms of these peaks are explored; it is found that their near-field distributions are different. Moreover, the figure of merit (FOM) of the last absorption peak can reach 101.67, which is much higher than that of the first three absorption modes and even absorption bands of other works operated in the terahertz frequency. The designed device with multiple-band absorption and high FOM could provide numerous potential applications in terahertz technology-related fields.

Tungsten-based Ultrathin Absorber for Visible Regime

Utilizing solar energy requires perfect absorption of light by the photovoltaic cells, particularly solar thermophotovoltaics (STPVs), which can be eventually converted into useful electrical energy. Ultrathin nanostructures, named metasurfaces, provide an intriguing platform to develop the miniaturized solar energy absorbers that can find potential applications in integrated photonics, optical sensing, color imaging, thermal imaging and electromagnetic shielding. Therefore, the quest of novel materials and designs to develop highly efficient absorbers at minuscule scale is an open topic. In this paper, novel absorbers using tungsten-metasurface are developed which give ultrahigh absorbance over a wide frequency spectrum. The proposed designs are two-dimensional, polarization insensitive, broadband and are predicted to give better response under high temperatures ascribed to high melting point of tungsten i.e. 3422 °C. Amongst these designs, cross alignment is found optimum for tungsten, because it is impedance matched with the free space for visible spectrum. This cross arrangement is further tweaked by changing width, height and length resulting in 7 different optimized solutions giving an average absorbance greater than 98%. One, amongst these solutions, gave a maximum average absorbance of 99.3%.

Six-band terahertz metamaterial absorber based on the combination of multiple-order responses of metallic patches in a dual-layer stacked resonance structure

This paper reports on a numerical study of the six-band metamaterial absorber composed of two alternating stack of metallic-dielectric layers on top of a continuous metallic plane. Six obvious resonance peaks with high absorption performance (average larger than 99.37%) are realized. The first, third, fifth, and the second, fourth, sixth resonance absorption bands are attributed to the multiple-order responses (i.e., the 1-, 3- and 5-order responses) of the bottom- and top-layer of the structure, respectively, and thus the absorption mechanism of six-band absorber is due to the combination of two sets of the multiple-order resonances of these two layers. Besides, the size changes of the metallic layers have the ability to tune the frequencies of the six-band absorber. Employing the results, we also present a six-band polarization tunable absorber through varying the sizes of the structure in two orthogonal polarization directions. Moreover, nine-band terahertz absorber can be achieved by using a three-layer stacked structure. Simulation results indicate that the absorber possesses nine distinct resonance bands, and average absorptivities of them are larger than 94.03%. The six-band or nine-band absorbers obtained here have potential applications in many optoelectronic and engineering technology areas.

Quad-band terahertz absorption enabled using a rectangle-shaped resonator cut with an air gap

Quad-band terahertz absorption responses were theoretically investigated in a simple design of a metamaterial absorber, which consisted of only one rectangle-shaped metallic resonator with an air gap.

Broadening the absorption bandwidth of metamaterial absorbers by transverse magnetic harmonics of 210 mode

By investigating a square-shaped metamaterial structure we discover that wave diffraction at diagonal corners of such a structure excites transverse magnetic harmonics of 210 mode (TM₂₁₀ harmonics). Multi-layer overlapping and deliberately regulating period length between adjacent unit cells can significantly enhance TM₂₁₀ harmonics, leading to a strong absorption waveband. On such a basis, a design strategy is proposed to achieve broadband, thin-thickness multi-layered metamaterial absorbers (MMAs). In this strategy big pyramidal arrays placed in the “white blanks” of a chessboard exhibit two isolated absorption bands due to their fundamental and TM₂₁₀ harmonics, which are further connected by another absorption band from small pyramidal arrays in the “black blanks” of the chessboard. The as-designed MMA at a total thickness (h) of 4.36 mm shows an absorption of above 0.9 in the whole frequency range of 7–18 GHz, which is 38% broader with respect to previous design methods at the same h. This strategy provides an effective route to extend the absorption bandwidth of MMAs without increasing h.

Ultra-flexible polarization-insensitive multiband terahertz metamaterial absorber

A thin-flexible and polarization-insensitive multiband terahertz metamaterial absorber (MMA) has been investigated. Each unit cell of the MMA consists of two metallic structures, which include the top metal resonator ring and the bottom metal ground plane, separated by a thin-flexible dielectric spacer. Finite element simulation indicates that this MMA has three high absorption peaks in the terahertz region, with absorptivities of 89% at 0.72 THz, 98% at 1.4 THz, and 85% at 2.3 THz. However, because of its rotationally symmetric structure, this MMA is polarization-insensitive and can perform very well at a wide range of incident angles, namely, 30° for transverse electric waves and 40° for transverse magnetic waves. The thin-flexible device structure and good performance shows that this MMA is very promising to disguise objects and make them less detectable to radar in the terahertz region.

Nanostructure arrays in free-space: optical properties and applications

Dielectric and metallic gratings have been studied for more than a century. Nevertheless, novel optical phenomena and fabrication techniques have emerged recently and have opened new perspectives for applications in the visible and infrared domains. Here, we review the design rules and the resonant mechanisms that can lead to very efficient light-matter interactions in sub-wavelength nanostructure arrays. We emphasize the role of symmetries and free-space coupling of resonant structures. We present the different scenarios for perfect optical absorption, transmission or reflection of plane waves in resonant nanostructures. We discuss the fabrication issues, experimental achievements and emerging applications of resonant nanostructure arrays.