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Tunable Metamaterials
Published in Pankaj K. Choudhury, Metamaterials, 2021
Ecoflex is a highly stretchable silicone elastomer and exhibits good performance in skin safe, tear strength and elongation break. Therefore, Ecoflex is suitable for the design of tunable metamaterials. Using liquid metal encased in Ecoflex, Liu et al. [82] proposed a tunable meta-atom in the X-band frequency range to provide a structural basis for reconfigurable metamaterials. The meta-atom includes a liquid metal SRR and the external flexible elastomer (Ecoflex). Figs. 4.12a and 4.12b illustrate the measured and simulated transmission spectra of the meta-atom with different stretching ratios. With normal incidence, the resonance frequency of the meta-atom shifts from 10.54 GHz to 7.67 GHz, as the stretch ratio alters from 17% to 72%. In the case of another measurement configuration (see the inset of Fig. 4.12b), the resonance frequencies also exhibit obvious red shifts when the stretch level change from 17% to 72%. Three kinds of measurement configurations can excite resonance. Similar tunable characteristics can be observed in the last measurement configuration. During the changing of surrounding elastomer, the liquid metal remains continuous, and the tunable range of resonance frequency can cover 70% of the whole X-band. These features can provide a simple and effective structure for tunable metamaterials and obtain potential applications in wearable and cloaking devices.
Terahertz MEMS metamaterials
Published in Guangya Zhou, Chengkuo Lee, Optical MEMS, Nanophotonics, and Their Applications, 2017
Prakash Pitchappa, Chengkuo Lee
Initially, a photoconductive semiconductor was used as a substrate material on which SRR meta-atoms were fabricated [15]. When the substrate with SRR meta-atoms was pumped with an external optical beam, the conductivity of the overall substrate increased. This change in the surrounding medium of the metamaterial causes both the LC resonance and dipole resonance of the SRR to dynamically modulate with increasing pump power. Furthermore, the photoconductive material was selectively placed at the gap of ESRR meta-atoms and the electrical LC resonance of the ESRR metamaterial was modulated through optical pumping [16]. This selective placement of photoconductive materials in a meta-atom geometry allowed for the probing and manipulation of THz properties that are achievable through multi-resonator systems, such as electromagnetically induced transparency (EIT) analogue [17–20]. The most interesting feature of optical pump-based tunable metamaterials is the ultrafast response, which is in the order of picoseconds and is critical for the realization of high performance THz devices. Alternatively, thermally controlled phase changing vanadium oxide (VO2) integrated into the meta-atom geometry was also demonstrated for ultrafast switching of THz resonances [21]. Thermal control was also used to actively modulate the conductivity of superconductors to realize active control of the THz metamaterial response, and is critical for realizing high Q resonator devices [19,22–24]. Electrical tuning of the THz metamaterial response is widely reported, owing to the ease of integration of the control signal lines with the metamaterial geometry. Some of the electrically tunable approaches involve the conductivity change of doped semiconductors [25], 2D materials such as graphene [26,27], molybdenum disulfide (MoS2) [28], and tungsten disulfide (WS2) [29], and the electrically tunable refractive index of liquid crystals [30–33]. Electrically tunable approaches provide a means of achieving highly miniaturized systems due to the ease of integration with ICs. Hence, the active materials-based tunable THz metamaterials provide a wide range of options with varying performance features to choose from, based on the application needs of the THz devices. However, they possess limitations that would seriously hinder their usage in specific applications. The frequency dependent properties of these active materials limit the scalability of tunable metamaterial over a wide spectral range. Additionally, the use of exotic materials demands for sophisticated fabrication processes and cannot be readily manufactured using batch processes. Furthermore, some of these approaches require bulky setups for providing the control signals, thereby making them not very attractive for miniaturized THz systems.
Theoretical and numerical analysis of the effective medium properties of a ferromagnetic microwire lattice
Published in Waves in Random and Complex Media, 2022
Tarun Kumar, Natarajan Kalyanasundaram, Geetam Singh Tomar
A generalized method for the evaluation of all three diagonal entries of effective permittivity and permeability tensors was presented in this work for a 3D-lattice consisting of ferromagnetic microwires. Numerical results have shown that the dielectric and magnetic properties of the medium are coupled with each other and these properties are controlled by the operating frequency, radius of microwires, microwire spacing and the applied magnetization. Numerical results have confirmed that temporal as well as spatial dispersion takes place in the medium. We have also observed through the numerical results that the effective medium behaves like a bulk medium. The response of medium is like an ENG, MNG or DNG, depending upon the microwire spacing. It may also be noticed that a drastic change takes place in the effective medium properties at FMR frequency. Below the FMR, the medium behaves like an ENG medium however, above the FMR the medium behaves like a DNG. In the light of above discussion, we may conclude that the spatial characteristics of the proposed medium are like an electric plasma as well as a magnetic plasma along the particular directions. Proposed analysis may find its applications in the fabrication of magnetically tunable metamaterials.
Deflected beam pattern through reconfigurable metamaterial structure at 3.5 GHz for 5G applications
Published in Waves in Random and Complex Media, 2022
Bashar A. F. Esmail, Mohamad K. A. Rahim, Huda A. Majid, Noor Asniza Murad, Noor Asmawati Samsuri, Osman Ayop, Adeeb Salh, Najib Al-Fadhali
The reconfigurable/tunable metamaterials, on the other hand, have gained considerable attention in recent years since they exhibit a variable response for upcoming electromagnetic waves. Several approaches have been proposed in the literature to achieve this property, such as electrical and mechanical [26]. The electrical method based on switching components such as varactor diode and PIN diode is the most common method due to its simple integration into many metamaterial shapes and the ease of bias distribution. The tunable metamaterials based on PIN and varactor diodes are introduced in the literature for different purposes [27–33]. The tunable liquid metamaterials based on PIN are utilized to enhance the performance of the microstrip antenna [27–29]. In [27], the reconfigurable high-gain and multiband metamaterial microstrip antenna are demonstrated based on liquid and copper split-ring resonator (SRR). The muti-layer of the SRR and complementary SRR were used to enhance the gain. The tunability is achieved using the PIN diodes, where three different states of switches are used to tune the frequency and bandwidth. Also, in [28], the authors used the liquid and copper SRR based PIN diodes for multiple bands tunability. Further, the gain is improved by adding teeth to the outer ring of SRR. The wire and thin circular wire liquid metamaterial structures were arranged in layers above the patch antenna for enhancing the gain, reflection coefficient, and bandwidth [29]. The authors in [30] used two PIN diodes to reconfigure the polarization of the microstrip antenna. By changing the states of the diodes, both the right-hand and left-hand circular polarization can be obtained. In [31], the PIN diode is incorporated into the metamaterial unit cell to change its functionality. The states on/off of the PIN diode can dynamically tune the resonance frequency of the proposed structure, thereby altering the reflection phases, resulting in significant changes in those phase-related functionalities. This switchable device can be used to demonstrate a helicity dynamical modulator for circular polarization waves and reconfigure the cavity’s resonance frequency at the microwave regime. On the other hand, the varactor diode is used to tune the metamaterials for high-performance operations in [32,33]. The tunable gradient H-shape metamaterial based on varactor diode within the band 4.1–6.6 GHz is presented in [32]. The geometry of the gradient metamaterial is obtained by different six widths of the H-shape, where their characteristics are controlled by applying an external voltage. The tunability is used to overcome phase distortions of the counterpart passive materials within broadband. Besides, metamaterial functionalities can be dynamically switched via changing the external voltage. In [33], the meta-lens is constructed with tunable property based on varactor diodes. By adjusting the external voltages of the diodes, the phase profile can be controlled, resulting in correcting the aberration and changing the focusing functionality.