1 Introduction

The increasing prevalence of the thermal imaging technologies in military applications has necessitated the development of effective thermal camouflaging techniques. According to Planck’s law (Baranov et al. 2019; Zhu et al. 2020), every object that has a temperature above absolute zero, which is -273.15 °C, continues to radiate heat. This is crucial for such targets as aircraft, ground vehicles, and naval ships since some of their components can reach hundreds of degrees (Baranwal and Mahulikar 2016; Mahulikar et al. 2012; Thompson et al. 1998). Once the infrared trace created by components that belong to those are detected or tracked by thermal detection systems, thus they can be targeted by weaponry systems. It may seem straightforward to achieve thermal camouflage by blocking all emission wavelengths, such as using metallic foils or stainless steel. But according to the Stefan-Boltzmann Law (P = εσT4), the emission of an object is intrinsically tied to its surface emissivity (ε) (Buhara et al. 2021; Luo et al. 2019; Zhu et al. 2021), and Kirchoff’s law further establishes that the absorptivity α(T, λ) of an object is equal to its emissivity ε(T, λ) (Salihoglu et al. 2018; Tang et al. 2020). In contrast to these laws, since different atmospheric windows exist with different characteristics, using a full emission suppression method causes heat accumulation and instability issues (Buhara et al. 2021; Kim et al. 2019; Zhu et al. 2020). Thereby, a pathway for manipulating the absorption spectrum of nanoantennas to design efficient thermal emitters is needed. This can be achieved with an approach that emitted power must be selectively released into the atmosphere through specific wavelengths. The infrared region notably has two such regions: MWIR range of 3–5 μm and the long-wave infrared (LWIR) range of 8–12 μm. The NTIR is situated between these ranges, at 5–8 μm wavelengths, which is a region where high emission is needed to provide radiative cooling, different from the MWIR and LWIR. Additionally, SWIR cameras, which operate in the 1.4–3 μm range, are increasingly being used for target tracking (Cao et al. 2023; Lee et al. 2021; Sun et al. 2022). Therefore, for an object to ideally be infrared camouflaged, it should function as a good absorber in SWIR and NTIR, and as a reflector in MWIR and LWIR, which means the emission in MWIR and LWIR ranges must be suppressed. At the same time, absorption should be maximized in SWIR and NTIR atmospheric regions (Buhara et al. 2021; Huang et al. 2021; Ji et al. 2018; Kim et al. 2019). To achieve this, different types of metamaterials (Kang et al. 2024; Kim et al. 2017; Lee et al. 2019; Li et al. 2023). and multi-layer film structures (Salihoglu et al. 2018) have been presented, including MIM (Metal-Insulator-Metal) formations and phase-changing materials such as VO2 and GST (Kang et al. 2021a, 2023; Kocer et al. 2021; Qu et al. 2018; Şanlı et al. 2022). In this study, we represent a simple, low-cost, easy-to-fabricate MIM nano infrared emitter device, which stands for metasurface with Diamond-shaped grating. The selection of materials for the nanoantenna structure, which includes silicon photonic technology, was made considering scalability, cost-effectiveness, and widespread availability, as well as compatibility with known CMOS fabrication methods. This approach provides ease of supply and lower production costs. Additionally, the relatively simple geometry of the nanoantenna structure suggests that its simplicity and standard lithography techniques can be used for fabrication. Although the claim of low cost and easy fabrication for nanoscale structures is generally debatable, the specific design choices made in this research, particularly the choice of CMOS-compatible materials and the simplicity of the geometry, support the feasibility of achieving cost-effectiveness. Detailed information on the effective fabrication of the presented MIM nanoantenna structure and the specific advantages and potential limitations of existing thermal camouflage technologies is provided in the supplementary document.

Emitters that have diamond-shaped gratings offer improved directionality of emission due to their pointed ends, which help focus emitted infrared light radiation along specific directions. Additionally, compared to circular shapes of the same footprint, diamond-shaped emitters boast a larger surface area, facilitating better heat dissipation crucial for maintaining efficiency. The sharp corners and edges of these emitters also support more plasmonic resonances, enhancing the local electromagnetic field and consequently increasing emission intensity.

Diamond-shaped Nano Emitter (DNE) consists of a unique design of Ag grating, Si3N4 dielectric, and Ag substrate. With this design, a promising approach is obtained to provide a suitable thermal camouflaging. To provide a more comprehensive understanding of the material selection process and its impact on the performance of the proposed structure, simulation studies were conducted with various metal and dielectric materials using them in the existing MIM structure. The results of these studies are given in the supplementary document. As can be understood from the results obtained, the most effective thermal camouflage was obtained with the recommended materials. Thanks to its amazing characteristics for radiative cooling, Ag is chosen as the grating material. This feature has an important role in achieving broadband absorption in the NTIR range. Si3N4 (Kischkat) is a dielectric used and referred to often in infrared camouflage applications (Lee et al. 2022; Li et al. 2023; Xiao et al. 2023). Characteristics of the presented DNE are simulated using a three-dimensional (3D) finite-difference time-domain (FDTD) tool. In the simulations, three narrow resonance peaks in SWIR at λ1 = 2.20 μm, λ2 = 2.54 μm, λ3 = 2.99 μm, and one in the MWIR window which is at λ4 = 3.99 μm, are observed. Also, a broadband absorption is achieved in the NTIR window at λ5 = 7.4 μm. These narrow resonance peaks in SWIR and MWIR provide more than 90% absorption, also more than 97% absorption is observed in the NTIR window. As of our knowledge, this is the first study that employs this much narrow resonance peaks, located in SWIR and MWIR ranges simultaneously, with a wide absorption band located in the NTIR window.

2 Theory and design

Figure 1(a) shows the single unit cell of DNE and Fig. 1(b) is the array sheet of DNE structure. This structure is based on an Ag substrate with a height of “H1” and the dielectric layer which is Si3N4 has a height of “H2”. Lastly, the grating part of the structure with diamond-shaped Ag comes, with a thickness of “t” and a side length of “N”. Structure is periodic in the x and y axis with the value of “P”. To achieve optimal values, single parameter sweep analysis for grating thickness, grating side length, and structure periodicity are performed first, as shown in Fig. 2(a-c) as a function of absorption. After the single parameter analysis, dual parameter sweeps are performed for related wavelength resonance peaks which are λ1, λ2, λ3, λ4, and λ5 respectively. Figure 3(a-e), represents the contour plots of grating thickness and periodicity of the structure. With that, compatibility of the parameters with each other and with atmospheric absorption wavelengths was examined. Thus, parameters are determined as, H1 is 100 nm, H2 is 350 nm, t is 20 nm, N is 900 nm and P is 2090 nm. Boundary conditions are chosen as periodic in the x and y-axis, and PML (Perfect Match Layer) in the z-axis. TM polarized plane wave light source in the wavelengths between 1 and 12 μm is placed above the structure. To obtain reflection and transmission data, reflectance and transmission monitors are placed above the light source and below the structure, respectively.

Fig. 1
figure 1

Schematic design of the presented nanoantenna. (a) Perspective view of the presented nanoantenna unit cell, (b) 3D view of the nanoantenna array sheet

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Fig. 2
figure 2

(a-c) Contour plots of the single parameter sweep analysis of designed DNE calculated for various grating thickness, grating side length N, and periodicity P against wavelength

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Fig. 3
figure 3

(a-e) Contour plots of the dual parameter sweep analysis of designed DNE calculated for various grating thickness t against periodicity P for given wavelengths λ1, λ2, λ3, λ4, and λ5 respectively. Color bars represent the absorption

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As it can be seen from Fig. 2(a), value of side length of grating “N” directly affects the position of wide band NTIR resonance peak (λ5), causing a shift to higher wavelengths. Also, similar shift observed for resonance peak in MWIR (λ4). Meanwhile, thickness of grating and periodicity does not directly affect the position or the amplitude of the resonance peaks as a function of absorption, individually.

After observing the direct effect of grating side length, thickness and periodicity of DNE, we decided to further investigate the dual parameter affects as function of absorption, staring from thickness of grating and the periodicity of DNE simultaneously. Figure 3(a-e) illustrates contour plots of the grating thickness and the values of periodicity of DNE varying with each other, as a function of amplitude of absorption. For NTIR resonance peak (λ5), DNE shows approximately %90 absorption amplitude for almost every case where there is a grating exist. On the other hand, for other resonance peaks, DNE shows different amplitudes for different thickness and periodicity values.

Figure 4 (a-e) illustrates contour plots of the grating side length and the periodicity of values of periodicity of DNE varying with each other, as a function of amplitude of absorption. Similarly, for NTIR resonance peak (λ5), DNE shows approximately %95 absorption amplitude for almost every case. For the other resonances, DNE shows near-unity absorption for a wide range of side length and periodicity values. This is also another privilege that presents a flexible design. On the other hand, despite of the high absorption amplitudes, in some cases, it is observed that the resonance peaks may shift out of the desired infrared windows. Thus, in order to achieve the highest absorption amplitudes in the desired infrared windows, the parameters are optimized and selected as t is 20 nm, N is 900 nm and P is 2090 nm.

Fig. 4
figure 4

(a-e) Contour plots of the dual parameter sweep analysis of designed DNE calculated for various grating side length N against periodicity P for given wavelengths λ1, λ2, λ3, λ4, and λ5 respectively. Color bars represent the absorption

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3 Results and discussions

Figure 5(a) shows the different infrared regions in the investigated wavelength range. After all the optimizations mentioned in the previous section, represented absorption behavior of DNE has been achieved as shown in Fig. 5(b). There are four narrow peaks at λ1 = 2.20 μm, λ2 = 2.54 μm, λ3 = 2.99 μm, and at λ4 = 3.99 μm with more than %90 of absorption, is observed. Also, a broadband resonance peak with more than %97 absorption is achieved in the NTIR window at λ5 = 7.4 μm. These results not only point out multiple atmospheric absorption cases of DNE but also show a good cooling character.

Fig. 5
figure 5

(a) IR atmospheric regions, (b) absorption behavior of DNE related with the IR atmospheric regions at given wavelengths λ1, λ2, λ3, λ4, and λ5 respectively

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Although MWIR is typically the area where low emissivity is needed, the placement of λ4 within the atmospheric absorption window (Buhara et al. 2021) enhances the material’s ability to blend with the background thermal signature, contributing to effective camouflage (Yao et al. 2023). The emissivity value obtained in the LWIR region is a value that can be seen in the studies presented in the literature (Buhara et al. 2021; Qian et al. 2023; Zhang et al. 2021), and in the article presented, the average emissivity value is seen to remain 0.33 and below even at 1000 K temperature as seen in Fig. 6 (a). Therefore, the stated value is considered to be negligible. The strategic design of materials with tailored emissivity profiles, such as the proposed DNE, can deceive IR signatures and regulate thermal appearance for enhanced camouflage performance.

Fig. 6
figure 6

(a), (b). MWIR and LWIR signature analysis of DNE by calculating its average thermal emission

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To examine the physical mechanisms of the nanoantenna, electrical field and magnetic field distribution profiles for represented resonance wavelengths, involving a plane wave illumination, have been extracted as shown in Figs. 7 and 8, from the top and front view. In the case of every resonance wavelength, one can observe that there is a significant electrical field enhancement, located between plasmonic (Ag) grating and dielectric interface, mostly focused on the corners of grating. This represents the existence of surface plasmon polaritons (SPPs) (Aslan et al. 2017; Lee et al. 2021; Zhang et al. 2012).

Fig. 7
figure 7

(a) – (e) E-field distribution plot of nanoantenna from front view, (f) – (j) E-field distribution plot of nanoantenna from top view for λ1, λ2, λ3, λ4, λ5, respectively

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Fig. 8
figure 8

(a) – (e) H-field distribution plot of nanoantenna from front view, (f) – (j) H-field distribution plot of nanoantenna from top view for λ1, λ2, λ3, λ4, λ5, respectively

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Also, in some cases, it is possible to observe a portion of light trapped inside a dielectric layer, such as in Fig. 7(b). By looking at it, one can see that Fabry-Perot mode also occurs at λ2, which further provides a bulk absorption in the Si3N4 dielectric layer. Furthermore, inspection of the intensity values, as represented by the color bar plots, indicates varying field intensities across the grating and the whole structure. These field distribution analysis not only highlight the magnitude of the fields at specific regions but also provide insights into the different optical phenomena, occurring at the grating and inside of the dielectric layer.

Next, the effective impedance of the nanoantenna is calculated by using S-parameters (scattering parameters), which are obtained from port analysis simulations, done with the 3D FDTD tool. Absorption can be expressed as \(A = 1-T-R\), thus, using the S - parameters reflection can be represented as \({R = \left|{S}_{11}\right|}^{2}\) and transmission can be represented as \(T = {\left|{S}_{21}\right|}^{2}\). Since we use a metal substrate, nearly zero transmission is observed, so \({S}_{21}\) is considered as zero. As a result, the absorption can be determined as \(A = 1-R\), which also can be written as \(A = 1- {\left|{S}_{11}\right|}^{2}\), so the Eq. (1) (Xiao et al. 2023) can be simplified to Eq. (2). After clarifying these expressions, it is possible achieve the effective impedance using the Eq. (2):

$${Z}_{eff}= \sqrt{\frac{{{(1+S}_{11})}^{2}-{S}_{21}^{2}}{{{(1-S}_{11})}^{2}-{S}_{21}^{2}}}$$
(1)
$${Z}_{eff}= \sqrt{\frac{{{(1+S}_{11})}^{2}}{{{(1-S}_{11})}^{2}}}$$
(2)

Figure 9(a) illustrates the effective impedance of the nanoantenna for the inspected wavelength region of 1–12 μm. The effective impedances of observed resonance wavelengths are almost matching completely with the effective impedance of the surrounding air. This means that at these wavelengths there is almost no reflection, and since the nanoantenna also shows no transmission, it is possible to see a significant absorption at the represented wavelengths.

$${\epsilon }_{eff}\left(T,\lambda \right)={\sum }_{J=1}^{3}{\epsilon }_{j} \left(T,\lambda \right)$$
(3)
$${\epsilon }_{1}\left(T,\lambda \right)=1-R\left(T,\lambda \right)-T\left(T,\lambda \right)$$
(4)
$${\epsilon }_{2}\left(T,\lambda \right)=\left(1- {\epsilon }_{blackbody}\right) {T}_{back}\left(T,\lambda \right){ \epsilon }_{1}\left(T,\lambda \right)$$
(5)
$${\epsilon }_{3}\left(T,\lambda \right)= {T}_{back}\left(T,\lambda \right) {\epsilon }_{blackbody}$$
(6)
$$BBE \left(T,\lambda \right)= \frac{2\pi h{c}^{2}}{{\lambda }^{5}({e}^{\frac{hc}{\lambda kT} }-1)}$$
(7)
$$TE\left(T,\lambda \right)= {\epsilon }_{eff}\left(T,\lambda \right) \times BBE \left(T,\lambda \right)$$
(8)
Fig. 9
figure 9

(a) Effective Impedance values calculated against wavelength. The effective impedance of air (377Ω) and the peak wavelengths of the presented DNE structure are indicated. (b) Comparison of thermal emission of DNE and blackbody radiation at room temperature

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Figure 9(b) shows the theoretical calculation of the thermal emission of DNE and the blackbody radiation at 300 K (For high and ultra-high temperatures, see Fig. S1 in the supplementary document). The thermal emission of DNE and blackbody emitter (BBE) is calculated using Eqs. (3–8), where “T” is the temperature, “λ” is wavelength, “\(\epsilon\)” is emission (Buhara et al. 2021). With this approach, thermal emission of an application, which contains the BBE and DNE can be calculated. Three emissions should be calculated to get the total thermal emission of potential application. The first one is the emission of DNE, calculated in Eq. (3). Second is the emission that transmits into the DNE, reflects from the BBE and transmits backward to the surface of, calculated in Eq. (4). Lastly, the third emission is the blackbody emission which is created in the hot object and transmits through the DNE, calculated in Eq. (5). Sum of these three emissions (Eqs. 3–5), is the effective emission (Eq. 2). After calculating the blackbody radiation using Eq. (6), and multiplying it with Eq. (2), it is possible to calculate the total thermal emission of the application, which is shown in Eq. (7). As result of these calculations, one can observe from Fig. 9(b), DNE structure has a significant signature reduction in MWIR (3–5 μm) and LWIR (8–12 μm) where the calculated thermal emission remains under the BBE’s thermal emission, and also has a strong cooling characteristic, which can be observed from the amount of thermal emission in NTIR (5–8 μm).

Lastly, thermal signature reduction abilities of DNE for various temperature levels have been investigated by calculating average thermal emission between a depicted wavelength range using Eq. (8) (Wang et al. 2021)

$${\epsilon [{\lambda }_{1},{\lambda }_{2}]}_{avg}= {\int }_{{\lambda }_{1}}^{{\lambda }_{2}}\epsilon \left(\lambda \right){I}_{BB}\left(\lambda,T\right)d\lambda / {\int }_{{\lambda }_{1}}^{{\lambda }_{2}}{I}_{BB}\left(\lambda,T\right)d\lambda$$

\({I_{BB}}\left( {\lambda,T} \right) = 2h{c^2}/{\lambda ^5} \cdot {[exp(hc/\lambda {k_B}T) - 1]^{ - 1}}\) is the radiated energy of a blackbody at a wavelength of λ and temperature of T according to Planck’s law. Average thermal emission of DNE in MWIR and LWIR regions is shown in Fig. 6. Temperature values start from 300 K and up to 1000 K. Despite the increasing temperature, DNE shows suitable signature reduction for both MWIR and LWIR regions.

Here in, we list some recently reported results in the field of multi-band camouflage in Table 1. While Kim et al. 2022; Zhu et al. 2021; Pan et al. 2020 and Jiang et al. 2023 provide relatively effective signature reduction in MWIR and LWIR, the majority of the reported works do not achieve the compatibility of narrow band SWIR camouflage characteristics and other camouflage bands. The simple DNE structure in this work, however, provides SWIR camouflage by multiple narrow band absorption peaks, and also compatible with other camouflage requirements, and provides excellent performance in each band.

Table 1 Comparison of the recently reported results for multi-band camouflage with this work, MFS: Multi-layer Film Structures
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4 Conclusions

In conclusion, the research presented in this letter introduces a novel MIM nanoantenna structure with a distinctive Diamond-shaped Ag grating, optimized for thermal camouflaging applications. By exploiting the unique properties of Ag and Si3N4 in the design, the structure achieves significant absorption rates of over 90% in SWIR and MWIR with narrow resonance peaks and above 97% in the NTIR window with a wide band resonance peak. The simulations using the 3D FDTD tool validate the efficacy of this design, showing negligible transmission and near-zero effective impedance at resonance peaks, indicating strong absorption. Field distribution profiles show the significant enhancement of fields and physical mechanisms contributing to these results, such as SPPs and Fabry-Perot modes are also evident. These findings suggest that the presented DNE can effectively manage thermal signatures by selective emission, opening new opportunities for thermal camouflage that could significantly impact military and surveillance strategies and other infrared-based technologies. This innovative technology offers several advantages over existing solutions, including enhanced spectral control, dynamic adaptability, reduced thickness and weight, and integration with existing materials. The nano-scale design allows for precise control over spectral characteristics, improving camouflage effectiveness, while also enabling dynamic changes in thermal emission properties for real-time adaptability. Furthermore, the development of thinner, lighter materials is crucial for applications in wearable technology and UAVs. However, potential limitations include fabrication complexity, durability concerns due to susceptibility to damage, scalability challenges in large-area production, and integration difficulties with existing materials, highlighting the need for further research and development to address these issues.