Metamaterial Filter Research Articles

Mitigating Inaudible Ultrasound Attacks on Voice Assistants With Acoustic Metamaterials

Voice assistants play an important role in facilitating human-machine interactions and have been widely used in audio consumer electronic products. However, it has been shown that they are susceptible to inaudible attacks in which the malicious signals are in the ultrasound regime and cannot be heard by human ears. In this study, we show that a judiciously designed acoustic metamaterial filter can mitigate such attacks by modulating the received signals by the microphones. The metamaterial filter is composed of rigid plates with individual holes which exhibit local resonance phenomena that suppress incoming waves at specific frequencies. The effectiveness of the metamaterial filter is confirmed by experiments which show that a combination of the holes can collectively distort the attack signals and protect the smart speakers. Moreover, normal audible signals are not affected by the proposed metamaterial, which adds to the flexibility of the device. The metamaterial filter has a small footprint and can be easily installed on various audio products. Our proposed strategy expands the capacity of acoustic metamaterials and improves the security of devices that use voice assistants.

An ultra-narrow-band optical filter based on zero refractive index metamaterial

Owing to the photonic band gap effect and defect state effect, photonic metamaterials have received much attention in the design of narrow bandpass filters, which are the key devices of optical communication equipment such as wavelength division multiplexing devices. In this work, based on zero-index metamaterial (ZIM), a compact filter with both high peak transmission coefficient and ultra-narrow bandwidth is proposed. The photonic metamaterial with conical dispersion and Dirac-like point is achieved by optimizing the structure and material component parameters of dielectric rods with square lattice in air. It is demonstrated that a triply degenerate state can be realized at the Dirac-like point, which relates this metamaterial to a zero-index medium with effective permittivity and permeability equal to zero simultaneously. Electromagnetic (EM) wave can propagate without any phase delay at this frequency, and strong dispersion occurs in the adjacent frequency cone, leading to dramatic changes in optical properties. We introduce a ZIM into photonic metamaterial defect filter to compress the bandwidth to the realization of ultra-narrow bandpass filter. The ZIM is embedded into the resonant cavity of line defect filter, which is also composed of dielectric rods with square lattice in air. In order to increase the sensitivity of the phase change with frequency, the Dirac-like frequency is adjusted to match the resonant frequency of the filter. We study the transmission spectrum of the structure through the COMSOL Multiphysics simulation software, and find that the peak width at half-maximum of the filter decreases as the thickness of ZIM increases, and the peak transmittance is still high when bandwidth is greatly compressed. The zero phase delay inside the slab can be observed. Through field distribution analysis, the zero-phase delay and strong coupling characteristics of electromagnetic field are observed at peak frequency. The comparison with conventional photonic metamaterials filter is discussed. We believe that this work is helpful in investigating the realization of ultra-narrow bandpass filters.

Study and Design of Bandstop Filtres Based on a Multiple Co-Directional Complementary Split Ring Resonator (CSRR)

The objective of this study is to enhance the performance of metamaterial filters. Indeed, the distinctive electromagnetic characteristics of metamaterials, along with their capacity to guide and manipulate electromagnetic waves in ways unattainable by natural materials, have rendered metamaterials progressively appealing in recent years. One type of metamaterial that has been studied extensively is the Split Ring Resonator (SRR) and its complementary counterpart (CSRR), Both of these structures possess dimensions significantly smaller than the wavelength. They demonstrate bandstop characteristics and exhibit negative permeability or negative permittivity in a narrow frequency range centered around their resonant frequencies. These frequency bands can be precisely and easily controlled in terms of frequency selectivity and rejection level by adjusting various parameters. As a result, these resonators are designed and optimized for use in the design of new filters. The obtained results confirm that the proposed metamaterial resonators significantly enhance the performance of the targeted applications.

Electrically tunable graphene-based multi-band terahertz metamaterial filters.

In this study, we have designed an electrically tunable multi-band terahertz (THz) metamaterial filter based on graphene and multiple-square-loop structures. The structure contains multiple metal square loops, and these loops with different sizes correspond to different THz frequencies, achieving our expected efficacy of a multiband wave filter. Furthermore, by sweeping external voltages, we could change graphene's Fermi levels, and thus the high-sensitivity THz filter's capability from single-band to multi-band filtering can be modulated. We expect that this study of a hybrid THz wave filter would be promising for the development of selecting channels in THz and 6 G communications.

Silicon bowtie structure based adjustable nonrigid all-nonmetal metamaterial terahertz filter.

An all-nonmetal metamaterial (ANM) terahertz device with a silicon bowtie structure has been developed, which has comparable efficiency to that of its metallic counterparts, and better compatibility with modern semiconductor fabrication processes. Moreover, a highly tunable ANM with the same structure was successfully fabricated through integration with a flexible substrate, which demonstrated large tunability over a wide frequency range. Such a device can be used in terahertz systems for numerous applications, and is a promising substitute for conventional metal-based structures.

Flexible and Polarization Independent Miniaturized Double-Band/Broadband Tunable Metamaterial Terahertz Filter.

In this paper, the design of a double-band terahertz metamaterial filter with broadband characteristics using a single conducting layer is presented. The design uses a structured top metallic layer over a polyimide material. The proposed design has achieved broadband band-pass transmission characteristics at the resonances of 0.5 THz and 1.65 THz, respectively. The 3-dB bandwidths for these two resonances are 350 GHz and 700 GHz, respectively, which indicates that dual-band resonance with broadband transmission characteristics was obtained. The design has achieved the same transmission characteristics for two different orthogonal polarizations, which was confirmed using numerical simulation. The design was tested for a different angle of incidences and it was observed that this results in angle-independent transmission behavior. In addition, for obtaining tunable resonant behavior, the top conductor layer was replaced by graphene material and a silicon substrate was added below the polymer layer. By varying the Fermi level of graphene, modulation in amplitude and phase was observed in numerical simulation. The physical mechanism of double-band behavior was further confirmed by surface current distribution. The proposed design is simple to fabricate, compact, i.e., the size is λ0/8, and obtained dual-band/broadband operation.

Полосно-пропускающий фильтр на метаволноводе

This article proposes an alternative method for designing miniature waveguide filters based on locally resonant metamaterials. Implementation of ultra-small metamaterial filters (metafilters) using the subwavelength direction mechanism in hollow waveguides loaded with small resonators. In particular, composite pin waveguides are used, constructed from a hollow metal tube loaded with a set of resonant pins that are spaced over deep subwavelength distances. In such structures, multiple resonance scattering leads to the emergence of a sub-λ mode with a tunable bandwidth below the induced hybridization gap. In order to guarantee compatibility with existing technologies, a subwavelength method of matching small filters with standard waveguide interfaces, which can be called a metaport, is proposed. Calculations were performed using the CST Studio computeraided design environment.

Nonlinear-Lamb-wave-based plastic damage detection assisted by topologically designed metamaterial filters

Owing to their high sensitivity to material microstructural defects, nonlinear guided waves (NGWs) show great promise for the early detection of incipient damages, conducive to numerous structural health monitoring (SHM) applications. In an SHM system, however, inevitable non-damage-related nonlinear sources exist, which may overwhelm the damage-induced nonlinear components and in turn jeopardize the practical efficacy of the detection methodology. Therefore, these deceptive nonlinear interferences need to be mitigated or eliminated as much as practically possible. By embracing the concept of metamaterials, a wave filtering device, referred to as a meta-filter (MF), is developed in this paper. The MF is constructed through mounting periodic stubs on the surface of the structure under inspection to condition the probing signals under the second harmonic Lamb-wave-based SHM paradigm. Through topological optimization, the MF enables ultra-wide stop bands to eliminate the second harmonic Lamb waves of the probing waves while preserving their strong fundamental wave components. The band structure and the underlying mechanism, alongside the MF-enabled wave properties, are numerically investigated. Upon tactically introducing deceptive nonlinear sources such as the nonlinear adhesive bonding layers in a piezoelectric transducer-activated SHM system, the performance of the MF is examined from the SHM perspective, which is finally validated experimentally using a metal specimen containing local plasticized incipient damage. Results demonstrate that the designed MF allows for flexible selection of the excitation frequency on one hand, and entails significant enhancement of the detection ability of the NGW-based SHM system on the other hand owing to the customized band features arising from the topological optimization.

Design and Characterization of Wideband Terahertz Metamaterial Stop-Band Filter.

We propose and design a metamaterial broadband stop-band filter with a steep cut-off in the terahertz region. The filter is based on the flexible structure of metal-dielectric-metal-dielectric-metal (MDMDM). Simulation results show that the filter has a center frequency of 1.08 THz, the square ratio reaches 0.95, and the −20 dB bandwidth reaches 1.07 THz. In addition, it has excellent flat-top characteristics with an average transmission rate in the resistive band of no more than 5%. The relative bandwidth has been up to 99%, and stopband absorption rate has reached more than 98%. The effects of the main structural parameters on the transmission characteristics are discussed. The role of each layer of metal in the filter is explored by studying the effect of the variation of the number of metal layers on the filter. The symmetry of the structure ensures the polarization insensitivity of the filter at normal incidence. The correctness of the simulation results was verified by analyzing the effective permittivity and magnetic permeability. To investigate the transmission characteristics of the metamaterial filter in-depth, we analyzed the electric field strength and surface current distribution at the center frequency of the filter. The designed terahertz filter may have potential applications in terahertz communications, sensors, and emerging terahertz technologies.

Design and analysis of terahertz filters based on multilayer metamaterials.

We numerically and experimentally demonstrate a series of multilayer metamaterial filters in the terahertz region. The designed structure consists of multiple metal-polyimide composite layers and cyclic olefin copolymer layers. The transmission spectra of the filters are characterized by terahertz time-domain spectroscopy, and the measured results agree well with simulations. In addition, the mechanism of the multilayer structure is theoretically studied by a thin film multibeam interference model. The proposed filters exhibit high efficiency at passband and can be broadly utilized as compact devices in practical applications at terahertz frequencies.

Design and realization of frequency and mode electronically reconfigurable metamaterial stopband filter for wireless communication systems

ABSTRACT This paper addresses the growing need for multifunctioning microwave components by proposing a miniaturized reconfigurable stopband filter. The compact size is the outcome of the novel metamaterial split-ring resonator used for realizing this filter. Six PIN diodes are used to achieve real-time frequency and mode reconfigurability. This filter is designed to reject three important frequencies in wireless communication systems: 1.9, 2.4, and 3.6 GHz. Moreover, depending on the use, it is possible to reject one, two, or all the three frequencies, which resulted in seven different cases with three possible modes: single, dual, or triple stopbands. A prototype was manufactured for this filter, and strong agreement between simulation and measurement results is observed. Due to its target bands, this reconfigurable stopband filter is suitable for various cellular communication technologies from 3 G and 4 G up to 5 G and many other S-band applications.

Ultrathin flexible terahertz metamaterial bandstop filter based on laser-induced graphene

An ultrathin, flexible metamaterial filter operated at terahertz (THz) frequencies based on one-step C O 2 laser-induced graphene (LIG) is proposed. The effects of laser machining parameters on conductivity and THz transmittance of the LIG are investigated, and a bandstop filter is fabricated from an ∼ 50 µ m polyimide film using the LIG method with relatively high laser power and scanning speed. At a blocking central frequency of 0.67 THz, the planar filter transmission is ∼ 40 % with only small variations for different bending angles. The simple, low-cost LIG method enables fabrication of THz metamaterial devices with great potential for THz imaging, sensing, and detection.

Design, simulation, and measurement of a multiband tunable metamaterial filter

Electromagnetic (EM) metamaterials/metasurface with mode conversion function or active modulation characteristics have broad application prospects in the field of optoelectronics or 6G/Terahertz (THz) communication. The current challenge is to achieve actived modulation and conversion between these modes. An EM metamaterial filter with phase-changing materia GeSb2Te5 (GST) particles is proposed, simulated, and measured. This metamaterial shows the active tunability and mode conversion by changing the temperature (synchronously changing the crystallinity of the GST arrays). At room temperature, the P1 and P2 peaks (with 90% and 52% of amplitude) are achieved at 9.28 THz and 14.18 THz. The P1 and P2 peaks can be controlled by changing the dielectric layer thickness ot the width of GST arrays in experiments. During the temperature sensitivity experiment, intermediate states of the GST arrays can be achieved by controlling the return temperature. As the temperature increases, the resonance position of the P1 peak will be replaced by the trough, thus realizing the metamaterial filter transmission window is switched between “ON” and “OFF” modes. This GST-based EM metamaterial filter provides application potential for actived mode conversion devices or 6G/THz communication.

Polarisation-insensitive and broadband band-stop metamaterial filter for THz waves

Polarisation-insensitive and broadband band-stop metamaterial filter for THz waves

Design, simulation and measurement of a single-band high-transmittance all-dielectric metamaterial filter

Metamaterial filters are widely used in various photo-electronic devices because of the rich resonance behaviors. Transmission and thermal resonance properties are important factors to measure the performance of photo-electronic devices based on metamaterial filters. It is important to accurate measure the thermal resonance performance of the metasurface to improve the thermal management of these photo-electronic devices, reduce heat loss, and delay device damage. In this paper, a metamaterial filter is proposed and verified in the 0.5–3.0 THz range. A transmission peak of 94.3% at resonance frequency 1.43 THz is achieved by the proposed filter samples at room temperature. The thermal diffusivity (8.28) and thermal resistance (7.58) of the metasurface are measured by using thermal pulses, which can be improved effectively by separating the sample and the standard substrate with a resin layer (TIM layer). The intensities of the applied electric field and heat pulse have significant influence on the thermal diffusivity and thermal resistance of the metasurface. When the electric field exceeds 42 km/mm or the heat pulse exceeds 1.3 mJ, the thermal diffusivity and resistance of the metasurface appear obvious divergence phenomenon. Moreover, the thermal resonance properties of the proposed sample is higher than that of the stacked plate structure.

Tunable liquid crystal metamaterial filter with polarization-insensitive characteristic

ABSTRACT In this paper, a systematic design method for polarization-insensitive tunable liquid crystal (LC)-based metamaterial filter with the passband of flat-top and sharp roll-off is proposed. A compact LC-based bandpass metamaterial filter unit cell with a dimension of 0.09 λ × 0.09 λ is designed. The metamaterial filter element consists of a pair of planar pinwheel-shaped metallic structures with four symmetrically distributed isosceles triangular patches. LC is sandwiched in this pair of metallic structures. The metal pattern on each layer is 90° rotationally symmetrical leading that the LC-based metamaterial filter is insensitive to the polarization of incident waves. In order to obtain the transmission characteristics of flat top and sharp roll-off, a double-layer LC-based metamaterial filter structure is proposed. Full-wave simulations show that the central frequency of the metamaterial filter is tuned from 10.88 GHz to 11.78 GHz, and the tuning range reaches up to 7.9%. Meanwhile, under TE and TM polarizations, the double-layer LC-based metamaterial filter structure achieves filtering characteristics with a flat-top passband of not less than 997 MHz and sharp roll-off for arbitrary LC states. Moreover a prototype sample is designed, fabricated, and measured to verify the proposed design method.

Design, simulation and fabrication of a new terahertz cross-shaped metamaterial bandpass filter to obtain a narrow frequency bandwidth

In this study, a new terahertz cross-shaped metamaterial bandpass filter is designed, simulated and fabricated. Among the properties of the proposed filter, compared to built-in filters (especially cross-shaped filters), one may refer to higher output transfer intensity (of almost 100%) and less noise effects (unwanted frequencies) in the whole range of 0.1 to 3 THz. The simulation is conducted by the CST STUDIO software and the FDTD method. Experimental results with THz-TDS system and simulation results with CST STUDIO software that are well matched.

Tunable terahertz metamaterial filter based on applying distributed load

A terahertz metamaterial filter with a tuning capability between the modes of single band and dual band is presented. The filter consists of two metal-array layers and three polymer layers, the latter of which can be tuned by mechanical loads. The proposed structure can afford three neighboring intrinsic resonant transmission peaks (0.59 THz, 0.70 THz and 0.84 THz), in which the first two peaks are different from the last one via the analysis of equivalent permittivity as well as the distributions of surface current and electric field. With the increase of distributed load within 30 N/mm2, the former resonant peaks of 0.59 THz and 0.70 THz approach and merge to form a new pass band, while the former resonant peak of 0.84 THz shows an obvious blue shift and departs from the former neighboring peaks to form a new narrow pass band with high transmittance of 0.95. The transmission characteristics of the structure are analyzed in detail by the distribution of electromagnetic fields and surface current. This work may provide a new tuning method which can realize the switch from single band to dual band.

Subwavelength Metawaveguide Filters and Metaports

This paper proposes an alternative technique for the design of miniaturized waveguide filters based on locally resonant metamaterials (LRMs). We implement ultrasmall metamaterial filters (metafilters) by exploiting a subwavelength (sub-\ensuremath{\lambda}) guiding mechanism in evanescent hollow waveguides, which are loaded by small resonators. In particular, we use composite pin-pipe waveguides (CPPWs) built from a hollow metallic pipe loaded by a set of resonant pins, which are spaced by deep-subwavelength distances. We demonstrate that, in such structures, multiple resonant scattering nucleates a sub-\ensuremath{\lambda} mode with a customizable bandwidth below the induced hybridization band gap (HBG) of the LRM. The sub-\ensuremath{\lambda} guided mode and the HBG, respectively, induce pass and rejection bands in a finite-length CPPW, creating a filter, the main properties of which are largely decoupled from the specific arrangement of the resonant inclusions. To guarantee compatibility with existing technologies, we propose a subwavelength method to match the small CPPW filters to standard waveguide interfaces, which we call a metaport. Finally, we build and test a family of low- and high-order ultracompact aluminum CPPW filters in the X and $K_{u}$ bands (10--18 GHz). Our measurements demonstrate the customizability of the bandwidth and the robustness of the passband against geometrical scaling. The three-dimensional printed prototypes, which are 1 order of magnitude smaller and lighter than traditional filters and are also compatible with standard waveguide interfaces, may find applications in future satellite systems and 5G infrastructures.

Metamaterial optical filter with maximal absorption coefficient

Tunable transmission and absorption coefficients in visible and infrared range have been obtained for metamaterial filters with a structure of metal on dielectric nanocomponents. Materials were stacks of nanoconstituents placed in alternate layers, which present a maximum or a minimum of the transmission coefficient and convenient values of the absorption coefficient when the periodicity is interrupted in a controlled manner. A simulational set-up with the metamaterial sample in a channel waveguide was conceived for obtaining the S parameters at the field propagation, using the HFSS program. The plasmon dispersion at the metal / dielectric interface was taken into account for calculating the frequency dependent surface plasmon wave vector, which can be adjusted by modifying the refractive index and the electric permittivities of the constituents. Transmission coefficients have been calculated for different structure geometries. The optimal metamaterial configurations have been chosen, in function of the desired filtering effect, for a range of incident field wavelength of 600 – 900 nm. A transmission variation of about 50 … 80 % at the central frequency, in comparison with the transmission effects in the side bands has been demonstrated for the proposed metamaterial filters.