Introduction

Metamaterials are synthetic materials designed to possess characteristics that are not commonly present in natural materials1. The main idea behind metamaterials is to create structures with specific electromagnetic properties to influence the behavior of electromagnetic waves. This is attained through the precise arrangement of subwavelength components such as dielectric particles, split-ring resonators, and metallic wires2,3,4,5. The ability of metamaterials to bend light in the opposite direction from that of regular materials is one of its most remarkable features, known as negative refractive index6. This property has important implications for cloaking, good absorption, and other applications where light path control is essential. Metamaterial absorbers play a significant role in these applications in terms of stealth characteristics. When it comes to bandwidth engineering, metamaterials dominate in their adaptability, providing exact manipulation of electromagnetic waves over both wide and short frequency ranges7,8. Metamaterials are transforming several technical domains by offering customized solutions to satisfy particular bandwidth needs for applications that need selective frequency modification9,10.

Stealth technology is essential in defense and security systems because it makes it easier to seize crucial areas of interest and destroy vital targets to ensure both survival and also allow invasions. The main goal of stealth technology is to make military vehicles, such as ships and planes, invisible to radars and other sensors11. MIMstructure is one kind of metamaterial that is employed in stealth technologies12. There has been a lot of interest in the development of frequency-selective devices for stealth technology applications using MIMarrangements13,14. Such devices have been considered as unique filters that only allow some wavelengths to pass through while blocking others. In stealth technology, it is all about concealing and masking from radars and sensors. Military vehicles and installations might become even more difficult to detect by the enemy’s radar and detection systems with the help of improved stealth technology. Several studies have shown that the use of metamaterial perfect absorbers as radar-absorbing surfaces improves the efficiency of stealth technologies15,16. Furthermore, to get a polarization-invariant performance, research in this area has been extended to symmetric MIM structures, which include stacked layers, multiple rings, and composite unit cells. Due to its high absorption efficiency, most of the research on stealth technology using MIM structures has now been focused on reducing the cross-section of electromagnetic waves in the gigahertz (GHz) to terahertz (THz) frequency range17,18.

In previous work, IR perfect absorbing MIM structures using dielectric (MgF2 spacer between silver layers) were developed for their applications in the field of IR stealth technology. In this paper, the plasmonic-based metamaterial with MIM structure has been designed using GaAs as dielectric, it provides less absorption at important infrared wavelengths, perhaps improving transparency and lowering object detectability in infrared stealth applications. MgF2 is one of the materials that contains fluoride and might absorb moisture. At some point, MgF2 optical qualities may change due to environmental moisture exposure. This will reduce the material’s transparency in the infrared spectrum and decrease the efficiency of IR stealth technologies. When compared to MgF2, the material GaAs showed better resistance to moisture absorption. Owing to its natural characteristics, it is less affected by surrounding moisture, which lowers the possibility of deterioration and increases the optical transparency.

Fig. 1
figure 1

(a) Design MIM structure having different layers. (b) The front view of the MIM structure consists of a silver disk array and a square pitch layer.

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Design, simulation, and dimension of the proposed two mim structure

Figure 1 shows the design of the plasmonic-based metamaterial with a metal-insulator-metal (MIM) structure for the application of infrared stealth technology. The designed two MIM unit cell structures composed of a five-layer stack of Silver - Fr4 - Silver - Gallium Arsenide – Silver structure. Silver (Ag) is chosen as a lossy metal in the first, third, and fifth layers as shown in Fig. 1(a). The thickness of each silver layer inside the selective MIM structure is the same t1 = t3 = t5 = 0.1 μm. Fr4 is chosen as dielectric material in the second layer as shown in Fig. 1(a) having permittivity of 4.3 and thickness t2 = 0.2 μm. The top fourth layer in Fig. 1(a) consists of Gallium arsenide (GaAs) used as a dielectric spacer having a permittivity of 12.94 and thickness t4 = 0.1 μm. Figure 1(b) shows the front view of a unit cell having a periodic dimension along X and Y direction Sx=Sy=2.4 μm. The top layer in the unit cell consists of a two-dimensional Ag disk layer having a radius r1 = 0.3 μm also the length “w” along (X&Y) of the square Ag layer is 2.1 μm. “L = 2.2 µm” represents the length of the square GaAs and Ag layers in the third and fourth layers.

Fig. 2
figure 2

Resonance behavior of the designed MIM structure to incident electromagnetic radiation along the negative z-axis.

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The optical resonant behavior of the designed plasmonic-based MIM structure can be predicted by employing the Finite-Difference-Time-Domain (FDTD) in a computer simulation technology (CST)microwave studio19,20,21. Figure 2 shows that the incident electromagnetic radiation coming from the Z-direction hit the surface of the MIM structure. This interaction leads to the collective oscillation of free electrons at the surface of silver layers to excite surface plasmon resonance inside the MIM structure which enhances the absorption and scattering of the designed structure. To evaluate the absorption characteristics of the designed MIM structure, it is crucial to consider both the scattering parameters of reflection and the scattering parameters of transmission. The absorption of the designed MIMstructure can be calculated by using the equation22.

$$\:A=1-T\left(\omega\:\right)-R\left(\omega\:\right)$$
(1)

Where \(\:T\left(\omega\:\right)\) denotes the transmission and \(\:R\left(\omega\:\right)\) denotes the reflection, \(\:R\left(\omega\:\right)={\left|{S}_{11}\right(\omega\:\left)\right|}^{2}\) and \(\:T\left(\omega\:\right)={\left|{S}_{21}\right(\omega\:\left)\right|}^{2}\), where S11 and S21 are the reflection and transmission coefficient of the absorber, respectively. Due to the presence of a silver disk layer and silver square layer on the top of the designed MIM structure, the transmittance of the structure is eliminated across the entire near-infrared frequency ___domain when incident electromagnetic (em) radiation hits the surface of the MIM structure. Therefore, Eq. 1 can be written as,

$$\:A=1-R\left(\omega\:\right)=1-{\left|{S}_{11}\right(\omega\:\left)\right|}^{2}$$
(2)

Results and discussion

In this work a class of artificial material has been designed to present plasmonic-based metamaterial with MIM structure which manipulates electromagnetic (em) radiation that fulfills the requirement necessary for IR stealth technology as depicted in Fig. 3. Surface plasmon resonances occurred when the frequency of incident emradiation at the Silver-dielectric interface coincided with the natural frequency of the collective electron oscillations23,24. These plasmonic-based metamathe terials exhibited unique optical properties. The absorption spectra between wavelength and absorption intensity are shown in Fig. 3. The wavelength of the incident em radiation is represented in micrometers (µm). The absorbance of incident em radiation at each wavelength is plotted on the Y-axis of an absorption graph. The absorption spectra shown in Fig. 3 are investigated by numerical simulation in the context of CST software. In both X and Y directions, the periodic boundary conditions were applied. Because of the structure’s periodicity, we included only one unit cell in our simulation. The numerical simulation showed that the designed MIM structure exhibits three different absorption peaks at different wavelengths. Due to multiple excitations of surface plasmon at the silver-dielectric interface inside the MIM structure, the absorption spectra ___domain was broadened from 1 to 2 μm and archive dual-band absorption peaks at wavelengths of λ1 = 1.3 μm and λ2 = 1.6 μm as depicted in Fig. 3. The designed MIM structure absorbed a large amount of incident em radiation at a wavelength of λ1& λ2 and the absorptivity rate of both peaks (λ1& λ2) reached to 98% which reduced the scattering of IR laser beams from surfaces of the unit structure, thereby suppressing the IR signatures that would guide laser-guided device to their targets. The absorption spectra are greatly affected by the geometrical characteristics of MIM, which include the size and configuration of a nanostructure and also the thickness of Ag and dielectric layers. Another absorption peak appeared at wavelength λ3 = 6.7 μm in absorption spectra whose absorptivity reached to 96% due to resonant behaviors of incident em radiation and collective oscillations of electrons inside the unit structure. The designed structure at 6.7 μm reduced heat waves from the surface which is a necessary condition for IR stealth technology. There are some other absorption peaks at 3.1 μm and 4.2 μm in between these target wavelengths. The absorptivity of these two peaks reached 70% and 72%.

Fig. 3
figure 3

The absorption peaks at (λ1, λ2,λ3) as a function of wavelength of the design of two MM plasmonic-based Metamaterial perfect absorber.

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Fig. 4
figure 4

(a and c)show electric field distribution along the positive x-axis at resonant wavelengths λ1&λ2 and (b and d) show magnetic field distribution along the positive y-axis at resonant wavelength λ1 &λ2.

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Figure 4 represents the distribution of electric and magnetic fields inside the MIM structure at resonant wavelength. When incident em radiation strikes the MIM structure, it induces surface currents in the Ag layers of the MIM structure due to the displacement of free electrons and produces a highly intense electric field inside the MIM structure. These highly intense electric fields along the X-axis having a magnitude of (3e + 4 v/m and 4.4e + 7 v/m) at λ1& λ2 provided concentrated em energy domains in GaAslayer that act as localized hotspots, concentrating and amplifying the energy of incoming light25,26,27. Characteristic peaks were formed in the absorption spectra in Fig. 3 at wavelengths of λ1 = 1.3 μm and λ2 = 1.6 due to the higher absorption of incident em radiation caused by this energy concentration. Additionally, these induced surface currents formed a closed loop and can be viewed as a magnetic dipole whose magnetic moment can interact significantly with the magnetic field of the incident em radiation, leading to the occurrence of magnetic resonance. By applying Ampere’s law \(\:(\oint\:B.dl={\mu\:}_{o}I)\) to closed loops within the designed structure, the magnetic field distribution, resulting from the induced surface currents, was calculated. The numerical simulation result in Fig. 4 shows a very weak magnetic field distribution compared to the electric field at resonant wavelength λ1 = 1.3 μm and λ2 = 1.6 μm.

Fig. 5
figure 5

Simultaneously excitation of electric and magnetic field (a) Electric field Exat λ3 = 6.7 μm (b) Magnetic field Hy at λ3 = 6.7 μm.

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To investigate the dependence of the absorptivity and resonant wavelength on the geometrical perimeter, we set the dimension of the designed MIM structure such that L = 2.2 μm and W = 2.1 μm to achieve another absorption peak at λ3 = 6.7 μm due to the simultaneous excitation of the electric and magnetic field at the silver-dielectric interface. Numerical simulation showed the excitation of a highly intense electric field and electric dipole along the positive X-axis as shown in Fig. 5(a), having a magnitude of (6.26e + 7 v/m) and phase angle (315oand 90o). Similarly, from Fig. 5(b) excitation of a very strong magnetic field having a magnitude of (3.38e + 5 A/m) and phase angle (78oand 303o) along positive Y-axis is due to the magnetic polariton (MP)resonance28.

Fig. 6
figure 6

Surface current distribution at the resonant wavelength.

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Figure 6 represents surface current distribution at different resonant wavelengths. When em radiations interacted with the MIM structure, it induced oscillations of free electrons at the interfaces between the Ag and dielectric layers. These oscillations were responsible for generating surface currents that circulate in closed loops along the Ag-dielectric interfaces as depicted in Fig. 6. The distribution of these surface currents resembled an inductor-capacitor (LC) circuit inside the MIMstructure, where the closed-loop currents acted as an inductor, and the dielectric layers acted as a capacitor29,30. This inductor-capacitor behavior of the designed structure resulted in the generation of localized electric field along the positive X-axis and magnetic fields along the positive Y-axis inside the MIM structure, which interacted with the incident em radiation, thus enhancing the absorptivity rate at the resonant wavelength (λ1, λ2, and λ3). The surface current density in the context of LC-like behavior may be expressed using an equation similar to the current in an electrical circuit. Additionally, the interaction of the electric and magnetic fields has caused the surface current to oscillate. The surface current’s LC-like behavior may be described by the following simplified equation,

$$\:\nabla\:\times\:\nabla\:\times\:J+{\omega\:}^{2}\epsilon\:\mu\:J=-{\omega\:}^{2}\mu\:\frac{{d}^{2}E}{d{t}^{2}}$$
(3)

From Eq. 3, the curl of the surface current density \(\:\nabla\:\times\:\nabla\:\times\:J\) shows its inductive behaviors, and \(\:\omega\:\) is the angular frequency of incident em radiations, and the oscillations of the surface current are driven by an electric field denoted by E, and also \(\:\epsilon\:\:\&\:\mu\:\) is the permittivity and permeability of the material, respectively. From Fig. 6 the magnitude the of surface current at resonant wavelength was 377,411 A/m having phase angles 90o and 236o at λ3 and 539,550 A/m with phase angles 56.25o and 236.25o at λ2. Similarly, the magnitude of surface current at λ1 was 315,847 A/m with phase angles 101.25o and 315o.

Figure 7 shows the far-field directivity of a designed plasmonic-based metamaterial with MIM structure which plays a crucial role in manipulating the propagation of emradiations to achieve stealth objectives31,32. To reduce the amount of IR radiation that IR sensors can detect, a silver-dielectric two-MIM structure designed for IR stealth usually aims to control the reflection, transmission, and absorption of IR radiation. One common strategy involves designing two MIM structures that the excitation of the electric field along the positive x-axis and magnetic field along the positive y-axis at a resonant wavelength that absorbs or scatters IR radiation in desired directions while minimizing backward scattering towards the source. Notably, Fig. 7 shows that at resonance wavelengths (λ1, λ2, λ3), the designed two MIM structures exhibited 0%backward scattering19. The main lobe orientation which is marked 180° in Fig. 7, indicates that all electromagnetic radiations are scattered in the forward direction, which fulfills the requirement of IR stealth technology. The magnitudes of the main lobe at resonant wavelength are (7.66 dBi, 8.65 dBi, and 9.41 dBi) which show the amount of light scattering in the primary direction, as depicted in Fig. 7.

Fig. 7
figure 7

Far-Field Directivity at resonant wavelength (λ1, λ2, λ3).

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Conclusion

In conclusion, plasmonic-based metamaterial with MIM structure has been presented for the application of IR stealth technology which kept invisibility in front of laser-guided devices at resonant wavelengths 1.3 μm and 6.7 μm. The first absorption peaks at λ1 = 1.3 μm were due to the excitation of the highly intense electric field which effectively suppressed the scattering of the laser signal employed by lasers-guided devices. The second absorption peak at λ3 = 6.7 μm was due to the simultaneous excitation of both electric and magnetic fields which enhances the designed MIM structure’s stealth capabilities by reducing the IR signal generated by the target body through thermal radiation. Under normal incidence of em waves on the metamaterial surface, this MIM structure performed very well throughout a wide frequency range of 1 THz to 300 THz, reaching absorptivity levels of 98% and 96% at the corresponding target wavelengths. The proposed structure composed of multiple layers of silver dielectric has the potential to provide laser stealth capabilities by minimizing backward scattering, which is usually detected by laser-guided devices.