3. Applications
Metamaterial absorbers have found numerous applications across various fields, due to their unique electromagnetic properties and absorption capabilities. The applications also vary, depending on the single-peak, multi-peak, or broadband absorption characteristics. Narrowband absorbers can be applied for sensing and detection by using the sensitive characteristic of resonance peaks with the surrounding environment, while broadband absorption is more powerful for various applications. Some of their notable applications are as follows.
Antenna Design: Broadband metamaterial absorbers are utilized in antenna design to enhance performance. They can be used to suppress unwanted reflections, reduce the side lobes, and improve the overall efficiency and bandwidth
[38]. By incorporating the metamaterial absorbers, researchers could achieve improved antenna characteristics and optimize the antenna performance for specific applications.
Figure 4a shows a proposed metamaterial antenna structure in the simulation layout and photo of the prototype
[39]. The antenna employed a configuration of ten metamaterial unit cells, each of which consisted of a pair of series interdigital capacitors and a short-circuited inductive stub by using a metallic interconnection. A lossy substrate FR-4 with a thickness of 0.8 mm, loss-tangent of 0.025 was used to implement a prototype of the proposed planar antenna. The effect of a metamaterial antenna with a substrate-integrated waveguide (SIWs) is demonstrated in
Figure 4b by the simulated and measured reflection coefficient (
S11) and transmission one (
S21).
S11 quantifies the impedance matching, and
S21 presents the isolation between the two ports. The results illustrate that the reflection and transmission coefficients in the cases with SIWs are much better than those without SIWs. This demonstrates that the reflection and transmission coefficients are significantly improved by employing the SIWs.
Figure 4. (
a) Simulation layout and photo of the fabricated prototype. (
b) Simulated and measured S-parameters of the metamaterial-inspired antenna before and after applying the SIWs
[39].
Radar Cross-section Reduction: Broadband metamaterial absorbers play a crucial role in stealth technology by reducing the radar cross-section (RCS) of objects or structures. By absorbing and selectively dissipating incident electromagnetic waves, the metamaterial absorbers can effectively minimize the radar signature of objects, making them less detectable by radar systems
[40]. This application has significant implications for military and defense research. A coding metasurface is a cutting-edge technology which manipulates electromagnetic waves, based on the cleverly-designed properties of the unit cell called meta-atoms. These meta-atoms were engineered to control the phase, amplitude, and polarization of reflected waves. A broadband coding metamaterial was designed for the RCS reduction, as presented in
Figure 4a
[41]. The RCS was manipulated by the different metamaterial coding sequences, as shown in
Figure 4b.
Figure 4. (
a) Fabricated coding metasurface and (
b) transmission coefficient
[41].
Sensing and Detection: Broadband metamaterial absorbers have been explored for sensing and detection applications by using the tailored absorption-frequency characteristic. They can be designed to selectively absorb specific frequencies, enabling us to have enhanced sensitivity and accuracy in various types of sensors. This includes applications in areas such as gas sensing, malaria, glucose, and environmental monitoring
[42][43]. In
[44], a metamaterial sensor based on a rectangle enclosing adjacent triple split-ring resonators (SRRs) is presented to recognize various oils, fluids and chemicals within the X band.
Figure 5 shows the experimental setup for measuring the liquid using the metamaterial sensor, and includes the following details: front view of the fabricated metamaterial-based sensor (
Figure 5a), back view (
Figure 5b), liquid insertion process (
Figure 5c), experimental setup of the waveguide (
Figure 5d), sensor attached with the X-band waveguide (
Figure 5e), and sample holder attached with an extended guided wave (
Figure 5f).
S11 results for palm and sunflower oil are in
Figure 5g, and for benzene and carbon tetrachloride are in
Figure 5h. Depending on the liquid, the resonance frequency of the metamaterial sensor was shifted in both simulation and measurement. Based on this characteristic, the liquid could be detected and classified. In other work, the tunable metamaterial absorber was also applied for the sensing application. Zhang et al. described how the absorption resonances of split-disk metamaterials could be tailored to cover a wide wavelength range of 1.5 to 5.0 µm by adjusting the geometrical configurations
[45]. The sensor devices, based on these configurations, exhibited high sensitivities of 3312, 3342, 3362, and 3567 nm/RIU, respectively. This finding suggests a promising avenue for the development of optical-gas sensors and biosensors with a high sensitivity.
Figure 5. (
a) Front view of the fabricated metamaterial-based sensor, (
b) back view, (
c) liquid insertion process, (
d) experimental setup of waveguide, (
e) sensor attached with the X-band waveguide, and (
f) sample holder attached with an extended guided wave. (
g)
S11 results for palm and sunflower oil, and (
h) benzene and carbon tetrachloride
[44].
Energy Harvesting: Broadband metamaterial absorbers have shown promise in energy harvesting applications. By efficiently absorbing incident electromagnetic waves across a broad frequency range, they can convert this absorbed energy into usable electrical power. This application has potential in areas such as wireless power transfer, self-powered sensors, and energy harvesting from the ambient electromagnetic radiation
[18][46]. Fowler et al. presented a radio-frequency (RF) energy harvesting device that exhibited a high efficiency
[47]. The device utilized a metamaterial perfect absorber integrated with Schottky diodes to achieve the efficient conversion of captured RF waves into DC power. The experimental results revealed that the rectenna was capable of generating a power output of 100 µW when subjected to an incident intensity of 0.4 µW/cm
2. The design of unit cell of the rectenna-based metamaterial was constructed from an SRR array with Schottky diodes embedded in the gaps. The metamaterial design and current analysis are presented in
Figure 6. In other previous work, a tunable perfect metamaterial absorber was introduced for electromagnetic-energy harvesting
[48]. The absorber consisted of a circular ring with a groove and varactor diodes, providing flexibility and reconfigurability. The proposed tunable structure, despite its simple geometry, achieved a near-perfect absorption of 99.91% and 84.79% at frequencies of 5.01 and 4.79 GHz, respectively. The objective of this study was to demonstrate the collection of electromagnetic energy without polarization concerns by employing the tunable dual-band metamaterial absorber. This kind of absorber can serve as a guide for designing new absorbers at higher frequencies and be applied in various electromagnetic-energy harvesting applications, including power transfer and wireless power transmission.
Figure 6. Design of a rectenna-based metamaterial. (
a) SRR array with diodes embedded in the gaps, (
b) sketch model of the unit cell, (
c) equivalent circuit model of the Schottky diode, and (
d) surface-current distribution at the resonance frequency
[46].
Imaging: Broadband metamaterial absorbers have been investigated for imaging applications. By tailoring the absorption properties of the absorber, researchers can obtain improved contrast and resolution in imaging techniques such as MRI (magnetic resonance imaging), terahertz imaging, and microscopy
[49]. The metamaterial absorbers also find applications in optical devices, such as filters and modulators. In
[50], a metamaterial structure used to enhance the performance of THz reflectance imaging was investigated. This THz reflectance imaging with metamaterial was used to resolve different regions of mouse brain tissue due to the difference in their refractive index.
Figure 6a illustrates the schematic of the THz-reflection measurement process. The metamaterial with polydimethylsiloxane sample was mounted onto a two-dimensional moving stage for a raster scan, and the reflectance images of 60 × 60 pixels with a resolution of 250 µm were obtained by using a THz time-domain spectroscopy system.
Figure 7b presents a reflectance image at an incident angle of 65 degrees. This image at a resonance frequency of 1.1 THz exhibited the highest contrast and sensitivity.
Figure 7. (
a) Experimental process for THz-reflection measurement with a motorized raster-scanning stage, and (
b) reflectance images at 1.1 THz by using the metamaterial
[50].
These are just a few examples of the diverse applications of broadband metamaterial absorbers. The ongoing research and developments in this field continue to expand the potential of metamaterial absorbers in various scientific disciplines.