Metamaterials have been in the frontiers of research owing to the amazing electromagnetic features these exhibit, which are not found in naturally occurring mediums. These composites achieve exotic effects through a structure that is smaller than the wavelength in the spectral region of interest. For example, at optical frequencies, molecules of metamaterials would be nanoparticles of diameter much less than the visible light wavelength, whereas at radar frequencies, the component units can be as large as a few millimeters across. The primary feature of metamaterials remains that the dimension of material components must satisfy the sub-wavelength requirement. By exploiting chemistry and microstructure, materials can be produced with properties those are non-existing in the nature. As such, metamaterials can be engineered to have a wide range of electromagnetic characteristics at desired frequencies and can have great potentials in technological applications. These tailored structures can manipulate incoming waves in fantastic ways, because their electromagnetic properties are derived mainly from the resonating elements.
The current trend of metamaterial research involves designing and fabricating nanostructures that are capable of manipulating electromagnetic waves at the terahertz spectrum. Studies of terahertz metamaterials involve standard characterization for effective medium properties, e.g., transmission, permittivity, and permeability. With this viewpoint, we made a timely decision to bring up a Special Issue of the Journal of Electromagnetic Waves and Applications (JEMWA), entitled “Complex Metamaterial Mediums and THz Applications,” wherein the conversant authors report their recent research results highlighting the different aspects of metamaterials and applications.
Metamaterials can be used as perfect absorbers of different electromagnetic radiations. In line with this, a paper included in this Special Issue reports the design and spectral performance of broadband metamaterial perfect absorber that operates in the ultraviolet and the visible regions of the electromagnetic spectrum. In particular, the proposed absorber is a three-layer nanoscaled structure comprised of U-shaped resonators (made of gold) of sub-wavelength size embedded over a nanolayer of dielectric medium (silica glass), and backed by a copper nanolayer surface, which yields absorptivity over 99%. This work also analyzes the angular dependence (of the incident light) of absorption characteristics, and the results of investigation suggest prudent use of such perfect absorbers in integrated heat absorbing mediums that would include sensors, integrated photodetectors, thermal imaging and solar cells.
In the context of absorption by metamaterials, Bie et al. presented three-layer metamaterial-based microwave absorber containing patterned magnetic sheet as the middle layer, and compared the results with those obtained with the three-layer absorber structure with complete sheet as the middle layer, instead of the patterned one. Their simulations and experiments indicate the achievement of huge expanded absorption bandwidth of over 12 GHz in the former structural condition of the absorber. As the authors argue, the effect due to scattering caused by the patterned magnetic sheet plays vital role in obtaining the expanded bandwidth, thereby making the use of patterned magnetic sheet in the microwave absorber structure as an efficient way to increase the absorption bandwidth.
The electromagnetic energy harvesting application based on tunable dual-band metamaterial absorber comprised of ring circle-shaped with gap resonator, operating in the microwave frequency regime, is also discussed by Dincer in this Special Issue of JEMWA. The application of the proposed structure as capacitive sensors is emphasized in order to show an additional feature of the model. It is expected that the absorber can serve as a model guide to design other new forms of metamaterial absorbers for the frequency range of interest and can be used in several energy harvesting related applications.
Apart from the implementation of circular rings in metamaterial structure, the performance evaluation of perfect metamaterial absorber based on square resonator with square gap at the middle is also presented in this Special Issue highlighting the flexibility in design to adjust the metamaterial properties for usage in the frequency spans of interest. The structure is analyzed numerically for the GHz and the THz frequency regimes and experimentally for the GHz frequency regime only. The investigation reveals the absorptivity of perfect metamaterial absorber around 99.99% at 5.48 GHz and 99.92% at 0.865 THz frequencies. The applications of the proposed absorber are expected to be in the areas such as defense systems, stealth technologies.
In the context of absorbers, Baskey et al. reported the design, synthesis, characterization, and performance evaluation of the kinds of metamaterial absorber comprised of periodic arrays of metallic hexagonal closed rings and octa-star structures printed over the dielectric substrate to achieve multiple absorption peaks at 4.10, 6.15, 10.05, and 15.52 GHz with the respective absorptivity of 0.98, 0.99, 0.99, and 0.99. The authors anticipate the proposed metamaterial structure for absorber applications in radar cross-section reduction, thermal detectors, and thermal imaging.
In the context of thermal tuning of semiconductor-based metasurface, subwavelength strips of indium antimonide (InSb) can be considered to enhance the carrier density in semiconductor. The plasmonic operation would result in alteration of the dielectric constant of material in the THz regime and would stimulate to conceptualize devising thermally tunable THz metamaterial modulator, as reported by Luo et al. in this Special Issue. As far as the depth of modulation of the device is concerned, it can be enhanced by incorporating ferroelectric sheets of strontium titanate, because the dielectric constant of ferroelectric medium would alter due to the variation in temperature. In conclusion, the authors attempt to demonstrate the operation of thermally tunable THz modulator comprised of semiconducting resonators – an interesting and efficient application of metamaterials in the THz regime.
The use of metamaterials in designing sophisticated antennas is a common practice in the present day technological need. In this stream, this Special Issue of JEMWA incorporates a paper by Kumar and Gupta on microstrip patch antenna that implements metamaterial slab of the form of metallic split-ring resonators. The results are compared with the situation when a dielectric material is placed in-between the metamaterial cover and the patch antenna. It has been found that the proposed design of antenna yields improved bandwidth and gain.
In the context of split-ring resonator-based metamaterials, one of the papers in this Special Issue describes the design, fabrication, and characterization of quad-band THz frequency selective surface using such mediums. It has been reported that the structure exhibits band-stop response with attenuation higher than 20 dB in all the operating frequency bands and a band rejection ratio higher than 15 dB.
Implementation of helical structure is common in guides to support slow-wave propagation. In one of the papers of this Special Issue of JEMWA, Sharma and Pathak presented studies of helical waveguide having the loading of metamaterial medium. The dispersion characteristics of the guide are investigated in the THz frequency regime of the electromagnetic spectrum using the appropriate boundary conditions. The obtained results indicate slow-wave features of the guide over a wide bandwidth from THz to optical spectral range, which throws the illuminating idea that the resulting guide would be useful for various engineering applications such as realizing optical buffer and memory.
Array of carbon nanotubes (CNTs) may also be categorized in the class of metamaterials. These nanotubes are of the kinds of single-walled and multi-walled, and metamaterials comprised of CNTs would be greatly useful in developing a wide range of electronic device applications including imaging or high data rate transmission links. Furthermore, CNT-based metamaterials may also be used in designing antennas; some fundamental studies in this direction are discussed in one of the papers incorporated in this Special Issue of JEMWA raising the transient electromagnetic analysis of CNT-based dipoles. In this context, numerical results related to transient currents on each CNT dipole are reported, and the electromagnetic coupling analysis between the CNT dipoles is touched upon depending on the separation and the number of CNT dipoles with the concept that the developed formulations can be expanded in the case of 2D CNT antennas.
Finally, the themes of all the papers included in this Special Issue of JEMWA are centered, in some way or the other, to the design and application of metamaterial mediums in certain frequency regime. The Editors-in-Chief are hopeful that the readers will find these interesting and useful in steering their own research in the metamaterial-related areas.
Joint Editors-in-Chief