Original research articleSelective metamaterial perfect absorber for infrared and 1.54 μm laser compatible stealth technology
Introduction
Since been experimentally demonstrated by Landy et al. [1], metamaterial perfect absorbers have attracted considerable interest in applications such as thermal emitting [2,3], thermophotovoltaics (TPV) [4,5], and sensors [6]. Metal-insulator-metal (MIM) is the typical configurations for selective absorbers. Various type of metallic structures including square patches [7], cross resonators [8], and split-ring resonators [9], are adopted in MIM structures. By artificially optimizing the geometry of metallic structures, the MIM structures can achieve perfect absorption at certain wavelength, which exhibit exotic performance in terms of absorption and wavelength selectivity [10,11]. Photonic crystal [12,13] also have wavelength selective characteristics, however, the peak emissivity of such design is lower than MIM structure, which limit the spectral properties of absorbers.
Stealth technology plays an important role in the warfare, because it provides a way to ensure survival and enable invasions. With the development of advanced detection system and precision guidance technology, all the acoustic, radar, and infrared signals reflected or emitted by the targets may be detected [14,15]. Therefore, efforts have been devoted to reduce the scattering or reflecting radar waves and suppress the thermal radiation of the surfaces. Due to the prominent absorption and frequency selective performance, metamaterial perfect absorbers have been applied for stealth technology to reduce the cross section of electromagnetic waves from GHz to THz [[16], [17], [18]]. The thermal radiation of target surface provides the main detection IR signal that related to its size and surface characteristics, such as geometry, temperature, and emissivity. The usual way is to reduce the surface temperature for effectively suppressing the thermal radiation of target. In addition, the special design of exhaust plume shielding can change the sizes and shapes of IR sources. Surface property modification is also a potential method for IR stealth technology. In previous work, one-dimensional Ge/ZnS photonic crystal [19] was reported to yield low surface emissivity in the band of 3–5 μm and 8–14 μm, the metal/resin composite coatings [20] were able to achieve low infrared emissivity in the band of 8–14 μm. Although the IR perfect absorbers enhance the thermal emissivity of object surface and have always been developed for IR radiation sources rather than IR stealth technology, some researchers [21,22] have developed wavelength selective devices to decrease the infrared radiation of vehicles in the atmospheric windows. This indicates that properly adjusting the absorption spectrum of MIM structures can also achieve infrared stealth.
In this paper, a selective metamaterial perfect absorber with MIM structure for passive and 1.54 μm laser compatible infrared stealth is proposed. This structure yield a perfect absorption peak at 1.54 μm and thereby attenuate the reflection signals detected by laser-guided missiles. Meanwhile, the emissivity in 3–5 μm and 8–14 μm is suppressed below 0.1 and 0.06 respectively. Moreover, the emissivity is higher than 0.75 in 5.9–7 m which can improve the capability of radiative cooling notably. To exhibit the practical usability of this structure, we analyzed the absorption behavior for different polarization waves in the case of oblique incidence.
Section snippets
Design and analysis
First, we designed a narrow-band selective absorber. As depicted in Fig. 1, the unit structure has five layers and form two MIM structures with different period. The unit cell has periodic dimensions of 1.8 μm in the direction of x and y. The width of square metal patches in the third and top layer is 1.32 μm and 0.26 μm respectively. Silver was chosen as the metal material and its dielectric constants was obtained from Ref. [23]. The second layer is Polyimide spacer with dielectric constant of
Thermal emission property
Before the analysis, the radiative properties of proposed structure are described by a spectral and angular emissivity and the temperature is assumed to be T. The emissivity is calculated by the ratio of the radiant exitance of object and blackbody at the same temperature. The radiant exitance of the structure with unit area can be expressed by Ref. [26]:Here, is the angular integral over a hemisphere.
Angle stability
In practical applications, it is necessary to consider the performance at oblique incidence. As can be seen from Fig. 7, when the incident angle reaches 50°, the 1.54 μm laser light for TE polarization and TM polarization can be absorbed about 72.8% and 66.8% respectively. The absorption of TE wave is more than 70% in the range of 6–6.95 μm, and higher than 75% in the range of 5.8–7 μm for TM wave. When the TM wave is oblique incidence, a second order resonance absorption peak [27] occurs near
Parameters effect
The actual measured absorptivity tend to deviate from the theoretical values due to the fabrication imperfections and the deviation of the simulation data for the dielectric constant from the actual material parameters. Therefore, it is essential to estimate the parameters effect on performance.
First, the absorption spectrum was calculated as the width w of metal patch M2 in single resonator set as 1.32 μm, 1.41 μm and 1.51 μm. Fig. 9(a) shows that the resonant wavelength is mainly determined
Conclusions
In this paper, MIM structures with different sizes were combined as a dual-band narrowband absorber of which the absorption reaches 99.3% and 98.7% at 1.54 μm and 6.1 μm respectively. Then the superposition principle of multiple resonant peaks was employed to expand the absorption bandwidth in 5–8 μm. Simulation results show that the absorptivity/emissivity is higher than 75% in the range of 5.9–7.1 μm, while suppressed under 0.1 and 0.06 in the middle and far atmospheric windows, respectively.
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