Graphene-Based Tunable Metamaterial-FSS RAS

Method of measuring the amounts of electromagnetic radiation absorbed and controlled by metamaterials in anechoic chamber

Metamaterials (MMs) are artificial materials fabricated to have electromagnetic (EM) properties, which do not exist in nature. Among MMs, MM perfect absorbers (MMPAs) are promising candidates for the practical and rather immediate applications of MMs. In general, MMPA is composed of three layers. The first layer is periodically-arranged metallic patterns, whose structure and geometrical parameters should be carefully adjusted to fulfill the impedance-matching condition with the ambient, suppressing the reflection of incident EM waves. The second layer is a dielectric layer, which allows a space for the EM waves to be dissipated, and sometimes plays a role of resonance cavity to prolong the time taken by the EM waves inside the second layer. The third layer is a continuous metallic plate, blocking the remnant transmission. The properties of general MMPA are the absorption at specific frequency, the narrow absorption band and so on. Therefore, recently many researchers on MMPAs have focused on multi-band, broadband and tunable absorption. In this chapter, various researches so far about multi-band, broadband and tunable MMPAs are presented and reviewed.

Frequency Selective Surface (FSS) is a specialized structure used in the field of electromagnetic waves and radio frequency (RF) engineering. It is designed to exhibit selective transmission or reflection properties based on the frequency of the incident electromagnetic waves. This article describes the design, construction, and analysis of a textile-based band-stop frequency selective surface for use in the highly EM-polluted GSM, Wi-Fi, LTE, and WiMAX bands. A full-wave EM solver called CST Microwave Studio was used to develop and simulate the unit cell of the proposed FSS at the relevant frequency. In this study, embroidered and screen-printed textile based FSSs were designed. According to the results of this study, it was demonstrated that both embroidery and screen printing FSSs exhibit resonance at a frequency of 3.5 GHz. The screen printing method yielded the best results in terms of resonance frequency sensitivity, while the embroidery method showed a resonance frequency shift. It was observed that the stitch directions and density are important parameters in the embroidery method. Gaps between the embroidery paths in the production of embroidered FSSs resulted in differences from simulations due to the disruption of the structural integrity of the unit cell. Consequently, textile-based FSSs offer advantages over traditional FSSs. This study highlights the potential of textile FSSs as an effective means of reducing electromagnetic pollution, and suggests that further improvements in the design and production processes of textile FSSs can be made.

Stealth and Radar Absorbing materials play important role in several applications including Defense. Such applications require a certain type of coating or paint or film to absorb the incoming electromagnetic (EM) radiation. To achieve stealth at certain band of frequencies, reflection coefficient has to be minimized which essentially means there has to be perfect impedance matching between the incoming wave and material used. Therefore, both complex permittivity $(\epsilon_{r}) (\epsilon_{r}= \epsilon^{\prime} + \mathrm{i}\epsilon$”) and complex permeability $(\mu_{r}\,= \mu^{\prime} + \quad i \mu$”) of the material plays important role in stealth applications. Electromagnetic Metamaterials (MMs) are artificial materials which are, effectively homogeneous, periodic structures with unusual electromagnetic and material properties not readily available in nature. The constitutive electromagnetic parameters (electric permittivity $\varepsilon$ and magnetic permeability $\mu$) of such materials depend on the nature of the unit cell and are essentially uniform along the direction of propagation [1]. Recently, Metamaterial Absorbers (MMAs) have attracted wide interest among researchers due to their exotic properties and possibilities. Since the first prefect MMA was proposed in 2008, many researchers have proposed different topologies and methodologies to produce high absorption of electromagnetic energy. This has been possible by eliminating the reflection and transmission of electromagnetic radiation and by realizing the absorption due to the electrical and magnetic resonances engineered to coincide at the same frequency and the absorption in the intervening dielectric substrate [2]. These MMAs have been engineered to operate at specific frequency spectrum ranging from microwave to ultraviolet. They can be single band, multiband, broadband, polarization insensitive and wide angle [3].

Frequency selective surface (FSS) is a spatial filter that has frequency selection characteristics for electromagnetic waves with different operating frequencies, polarization states and incident angles. At the same time, frequency selective surface has the advantages of simple structure and easy processing compared with the traditional filters. FSS have been investigated for many decades and extensively utilized in various applications. It can used in radar radomes to reduce the radar cross section (RCS) of an array antenna, as a reflector to transmit and reflect electromagnetic waves in different frequency bands. Normally, electromagnetic waves are incident at an oblique Angle onto the target surface. Therefore, the research of FSS absorbers with stable absorbing performance under oblique incidence has tremendous practical application value. This paper proposes a new design scheme for designing a FSS absorber with stable oblique incident response range from 0° to 70°. A single-square-loop array with lumped resistors and square patch printed on the dielectric substrate is proposed, and it exhibits a characteristic impedance matching for wide incident angles from 0° to 70°. The design scheme of the FSS layer under the condition of oblique incidence is introduced in detail, and the equivalent circuit is adopted to make the design clearer and more direct. The results show the absorption bands with oblique incident angles from 0° to 30°, 30° to 50°, and 50° to 70° reflection coefficients below –10 dB are 3.44 to 9.03 GHz, 4.49 to 9.06 GHz, and 4.87 to 8.96 GHz for TE polarization, respectively. The absorber proposed in this paper will contribute much to the development of ultra-wide-angle oblique incidence.

A simple closed-form approximate solution is given to the problem of transmission of a low-frequency electromagnetic wave through a frequency selective surface (FSS). FSS are periodic metal plates (or their complimentary apertures) sandwiched between dielectric slabs. At low frequencies, the induced currents on the metal plates may be approximated by a known function with a constant coefficient to be determined by the boundary conditions. Based on such a approximation, we derive a closed-form solution for the scattered field for FSS with multiple narrow rectangular slots, with a single wide rectangular aperture, and a circular aperture. When compared with the available exact solutions, we find that the one-mode approximation is valid when the period a of the FSS is such that a/\lambda is small enough that no grating lobe appears, e.g., a for normal incidence.

We try to thin the thickness of a λ/4 electromagnetic(EM) wave absorber with inserting a frequency selective surface (FSS) into the spacer in order to control the phase in the spacer. For this reason, we focus on the impedance characteristics of the FSS. Furthermore, we design and fabricate the λ/4 wave absorber using the FSS and measure its absorption characteristics. The result shows that the absorber with the spacer of half thickness of the general absorber is obtained. We have also provided the useful data of the frequency and oblique characteristics of the proposed absorber.

In this article, a via‐based 2.5‐dimensional (2.5D) frequency selective surface (FSS) is proposed to realize a quasi‐elliptical bandpass response with high frequency selectivity. Each FSS element is composed of two parts. Part A is a double‐layer metallic circular patches, which are etched on both sides of a thick substrate and connected by a central big metallic via and several small vias. Part B is a single layer metallic circular patch etched on a thin substrate. The proposed FSS features several transmission/reflection zeros, exhibiting a flat transmission in the passband and fast roll‐off out of band. By investigating the electric field distributions, two paths of electromagnetic wave (EMW) propagation are observed, and the physical mechanism of the transmission/reflection zeros of the FSS is obtained. A prototype of the FSS is fabricated to verify the design, and the measured transmission responses agree with the simulated ones.

Frequency and bandwidth modulation of a wide band-stop metamaterial for EMI shielding applications

This thesis deals with electromagnetic inspired acoustic metamaterials, enabling sound-matter interactions in different wave scenarios that include propagation, guided-waves, radiation, refraction, reflection and transmission. To this end, a particular emphasis is placed on introducing novel applications for acoustic metamaterials operating on each one of the aforementioned wave scenarios. A few years ago, metamaterials have been introduced as a new class of composite artificial materials, engineered to produce unusual effective material properties not readily available in nature. Electromagnetic metamaterials are probably the oldest class of metamaterials, being nowadays in the process of reaching maturity and being proposed for interesting commercial applications. On the other hand, as in most young but not yet mature emerging fields of science, acoustic metamaterials are still providing lots of fertile and unexplored ground for research and study. Despite many inherent physical differences, the propagation of electromagnetic and acoustic waves are both governed by a similar mathematical model, the so-called Helmholtz wave equation. The main purpose of this work is to leverage this amazing similarity by translating recent advances in electromagnetic metamaterials into their corresponding, previously unseen, applications in acoustics. The first contribution of this thesis is to adapt the classic electromagnetic transmission-line theory to allow the design of acoustic metamaterials. The proposed circuit-based theory finds a direct application in the design of composite right/left hand transmission-line metamaterials, which yield novel guided-wave applications for acoustic metamaterials. Then, the developed theory is leveraged to model and achieve optimal design of different acoustic metamaterial-based devices, such as leaky-wave antennas and reflector-type metasurfaces. The second part of the thesis spins around acoustic leaky-wave antennas and their different functionalities, showing that they are able to act as acoustic dispersive prisms in the refraction mode and as acoustic single sensor direction finder in the radiation mode. Studying reflection phenomena in sound-metasurface interactions constitutes the next part of this thesis, where a membrane-capped cavity is introduced performing as an ultra-thin unit-cell for reflective acoustic metasurfaces. This leads to exciting applications of the concept, like acoustic reflectarray antennas and acoustic metasurface skin-cloaks Finally, the last part of this thesis deals with transmission phenomena in acoustic metasurfaces and, especially, in orbital angular momentum metasurfaces. This concept results in the design of an innovative labyrinthine-type helicoidal unit-cell that is used for phase coding a surface and transforming impinging acoustic wavefronts into transmitted helical wavefronts.

Spatially different arrangements of identical metaatoms in a unit cell bring about distinct properties in stereometamaterials. Integrating the stereometamaterial into a perfect metamaterial absorber, we designed, fabricated, and characterized a device with single or double band absorption responses and an absorption/reflection switching characteristic dependent upon the polarization of the incident THz wave. Two non-concentric Cu rings tightly coupled via a polyimide layer form the stereometamaterial frequency selective surface (FSS). The FSS is placed on another polyimide layer deposited on a Cu backplane. Despite the rotational symmetry of the rings, non-centricity breaks the reflection symmetry of the FSS, leading to a unique interaction between the rings and a Cu backplane. Interaction between dipoles on an FSS and their mirror images due to Cu backplane is studied to gain a better understanding of this device.

Tunable propagation properties of graphene asymmetrical bowtie metamaterials (MMs) structures have been investigated in the terahertz regime, including the effects of graphene Fermi levels, structural parameters and operation frequencies. Because of its thin film thickness, the strong resonant curves of the proposed graphene MMs structures are dominated by the plasmonic mode instead of the Fabry-Perot mode for the metal structures. Compared with existing tunable graphene devices, the sharp Fano resonant curve manifests a large Q-factor of more than 40. In addition, as the width of graphene bowtie aperture increases, the resonant frequency dip shifts low frequency, and the resonant amplitude and figure of merit increase. The results are very helpful in order to understand the tunable mechanisms of graphene components and design high sensitivity functional devices, sensors, modulators, and antenna.

A metamaterial (MM) structure composed of a graphene layer, a SiO2/Si substrate, and a metal pattern layer, which is dynamically tunable is numerically proposed at terahertz (THz) frequencies. This study shows that the modulation of the proposed structure mainly depends on the transmission band between two neighboring resonance dips. Based on this property, a fairly wide modulation bandwidth is realized. Moreover, by adjusting the Fermi level of the graphene layer, a modulation depth of the transmission can reach 91.63% for y-polarized wave and 78.66% for x-polarized wave, respectively. Most importantly, the fabrication of the proposed structure is much easier than reported graphene MM modulators as the graphene monolayer is directly deposited on a substrate. This work may provide a further step in the development of tunable devices, such as broadband transmission modulators and sensors.

When a metamaterial (MM) is embedded in a one-dimensional photonic crystal (PC) cavity, the ultra-strong coupling between the MM plasmons and the photons in the PC cavity gives rise to two new polariton modes with high quality factor. Here, we investigate by simulations whether such a strongly coupled system working in the terahertz (THz) frequency range has the potential to be a better sensor than a MM (or a PC cavity) alone. Somewhat surprisingly, one finds that the shift of the resonance frequency induced by an analyte applied to the MM is smaller in the case of the dual resonator (MM and cavity) than that obtained with the MM alone. However, the phase sensitivity of the dual resonator can be larger than that of the MM alone. With the dielectric perturbation theory - well established in the microwave community - one can show that the size of the mode volume plays a decisive role for the obtainable frequency shift. The larger frequency shift of the MM alone is explained by its smaller mode volume as compared with the MM-loaded cavity. Two main conclusions can be drawn from our investigations. First, that the dielectric perturbation theory can be used to guide and optimize the designs of MM-based sensors. And second, that the enhanced phase sensitivity of the dual resonator may open a new route for the realization of improved THz sensors.

Tunable perfect absorber and multifunctional logic device based on graphene metamaterial