Metamaterials are artificial materials composed of periodic or aperiodic structures to have extraordinary electromagnetic characterizations. By properly tailoring the feature size of metamaterial, it can be realized electro-optic devices spanning the wavelength range from microwave, infrared (IR), visible to ultraviolet (UV) caused from the transformation optics.
In view of metamaterials having unique electromagnetic characterizations that cannot found in nature materials, metamaterials have great potentialities in the development of novel optical devices, such as optical waveguide, filter, switch, nanophotonic, sensor, high-resolution imaging, and so on. Moreover, the development of metamaterials can greatly reduce the volume and weight of conventional optical devices
In the reported literatures, the uses of materials for metamaterials are including silver (Ag), gold (Au), aluminum (Al), and copper (Cu). Among of these materials, Ag and Au are the two most often used for metamaterial applications than aluminum (Al) and copper (Cu) due to their relatively small ohmic losses or high conductivity in IR wavelength range. However, Ag material will suffer the degradation in the fabrication process.
Chinese researchers led by Zefeng Xu, Ruijia Xu have proposed novel IR metamaterial absorber by using chalcogenide glass (ChG) materials. ChG materials are amorphous compound containing the chalcogen elements (S, Se, Te) and exhibit wide IR transparency windows. They are easily synthesized to be a thin-film layer and their compositional flexibility allows the tuning of optical properties making them ideal for optoelectronics applications. The characteristics of ChG materials are zero extinction coefficient, excellent IR transparency, high third-order nonlinearity, adjustable refraction index and energy bandgap in IR wavelength range.
Stealth technology is vital in the military because it facilitates the acquisition of control over strategically important areas and the destruction of key targets to ensure survival and to enable invasions. In general, stealth technology involves the achievement of low observability by reducing signal detection or resending countermeasure signals. The detector observes infrared, radar, and acoustic signals reflected by or emitted from the targets. Therefore, researchers have endeavoured to reduce the scattering and reflection of radar waves from the surfaces of objects that could be detected by radar detection systems
Metal–insulator–metal (MIM) and photonic crystal structures are the most popular configurations for selective absorbers. However, the low peak emissivities of photonic crystal structures limit the spectral properties of absorbers with such designs. On the other hand, MIM structures exhibit exotic properties in terms of absorption and wavelength selectivity. Owing to the plasmonic phenomenon that occurs at the metal-dielectric interfaces, the absorption nearly reaches unity at certain wavelengths, enabling the realization of perfect absorbers.
Research on the development of frequency-selective devices using MIM structures for stealth technology applications has been very popular. Many studies have been conducted on metamaterial perfect absorbers to improve the performance of stealth technology using radar-absorbing surfaces.
Since the IR perfect absorber using an MIM structure increases the IR signature of target objects by enhancing their thermal radiation, the applications of perfect absorbers in IR stealth technology have not been generally accepted. In previous studies, IR perfect absorbers were developed for the application to IR radiation sources rather than to IR stealth technology.
As an alternative, they adjusted the spectral band of the perfect absorber to suppress the IR signature. Currently, IR technology is commonly applied in thermal imaging devices for surveillance systems, which measure the thermal radiation of objects, as well as in laser-guided missiles to improve the accuracy of attacks on military targets by detecting scattered IR laser light to guide missiles to their targets.
The thermal radiation emitted by an object provides the main IR signature that is used for thermal imaging by IR tracking devices, and depends on the temperature, size, and shape of the object. In addition, IR waves scattered from target surfaces provide important IR signatures that are utilized by laser-guided missiles. The wavelength of the IR signature depends on the type of IR detector that is utilized, and can be between 2 μm and 14 μm. Since the IR signature of an object depends on various parameters related to its size and surface properties, such as its temperature, emissivity, and reflection geometry, there are various methods of reducing IR signatures to achieve IR stealth technology.
Decreasing the surface temperature to suppress the thermal radiation from the target surface is the most popular approach. In addition, exhaust plume shielding is adopted to change the sizes and shapes of IR sources. IR homing technology has been developed following the progress in IR detector technology in terms of sensitivity and resolution. IR search and track (IRST) systems detect thermal radiation from the surfaces of military vehicles and combustion exhaust for the purpose of surveillance and to guide missiles to their targets.
Surface property modifications have also been studied to enable the suppression of IR signatures of objects and the development of IR stealth technology. Metal-powder–resin coating was reported to yield low surface emissivity in the long-wavelength IR (LWIR) range of 8–14 μm. To achieve low observability, the contrast radiant intensity was reduced by covering the surface of a tank with tiles, whose temperature was held at ambient temperature to make the tank IR-invisible. Meanwhile, frequency-selective devices with T-shaped MIM structures and silver (Ag) nanoparticles have been reported to attenuate the IR signatures of vehicles, in the atmosphere
Infrared Metamaterial Absorber Shows Promise in Radiative Cooling, reported in Feb 2022
Researchers from the State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS) have proposed a broadband, polarization- and angle-insensitive metamaterial absorber (MA) that covers the longwave infrared (LWIR) band. The design supports radiative cooling applications, as well as infrared imaging and thermal detection.
MAs that are based on subwavelength periodic patterns can achieve perfect absorption at any target wavelength, depending on the design of their geometric parameters. Because they are ultrathin and due to their near-field enhancement, MAs are widely used in photodetection, solar energy harvesting, and gas detection, as well as in thermal emitters and infrared imaging.
However, given the resonant nature of plasmon excitation, common MAs are only able to achieve perfect absorption in a narrow wavelength range. In the current work, the researchers succeeded in inducing antiparallel currents in the upper and lower metal layers of the MA; the incident light, coupled with the metal pattern of the MA, excited localized surface plasmon resonance. The induction ensured that energy was confined in the MA.
Common broadband MAs are based on the combination of multi-localized surface plasmon resonance modes and the intrinsic absorption of lossy dielectric layers. In addition, rationally designed 2D metal grating structures can also excite propagating surface plasmon resonances (PSPR) at the metal-dielectric interface.
The researchers introduced PSPR into the patterned metal-dielectric-metal sandwich structure of MA. The top pattern of the MA consisted of two alternating ring sizes, with the four adjacent rings forming a “super unit,” which made the MA two distinct periods.
Therefore, the MA excited multiple modes of surface plasmon resonance, and the lossy metal enabled the four absorption peaks with wide full width at half maximum. The researchers obtained a perfect broadband absorption covering the LWIR band with this design.
Compared with previous broadband absorbers, the researchers said, the excitation of hybrid modes can achieve broader wavelength band absorption with a more straightforward structure. In addition, the energy of the incident light is all concentrated in the metal layer of the MA , which enabled the researchers to flexibly select the material of the dielectric layer according to the application scenario.