Multifunctional Hierarchical Metamaterials: Synergizing Visible-Laser-Infrared Camouflage with Thermal Management

Abstract

With the rapid development of multispectral detection technology, realizing the synergistic camouflage and thermal management of materials in multi-band has become a major challenge. In this paper, a multifunctional radiation-selective hierarchical metamaterial (RSHM) is designed to realize the modulation of optical properties in a wide spectral range through the delicate design of microstructures and nanostructures. In the atmospheric windows of 3–5 μm and 8–14 μm, the emissivity of the material is as low as 0.14 and 0.25, which can effectively suppress the radiation characteristics of the target in the infrared band, thus realizing efficient infrared stealth. Simultaneously, it exhibits high emissivity in the 2.5–3 μm (up to 0.80) and 5–8 μm (up to 0.98) bands, significantly improving thermal radiation efficiency and enabling active thermal management. Notably, RSHM achieves low reflectivity at 1.06 μm (0.13) and 1.55 μm (0.005) laser wavelengths, as well as in the 8–14 μm (0.06) band, substantially improving laser stealth performances. Additionally, it maintains high transmittance in the visible light range, ensuring excellent visual camouflage effects. Furthermore, the RSHM demonstrates exceptional incident angle and polarization stability, maintaining robust performances even under complex detection conditions. This design is easy to expand relative to other frequency bands of the electromagnetic spectrum and holds significant potential for applications in military camouflage, energy-efficient buildings, and optical devices.

1. Introduction

The widespread adoption of infrared technology in military equipment, including precision-guided weapons and night-vision devices, has greatly improved battlefield monitoring and target identification. Consequently, infrared stealth technology has become essential for modern warfare. Objects with temperatures above absolute zero emit detectable energy [1]. Infrared camouflage technology reduces the target’s infrared radiation to render it indistinguishable from the background, effectively evading detection. According to the Stefan–Boltzmann law (j = εσT⁴), the infrared radiation signal is proportional to surface emissivity (ε) such that lowering the emissivity of the target can weaken the radiation signal. Compared to low-emissivity materials in the full mid-wave infrared band, spectral selective materials, such as metasurfaces [2,3,4,5], photonic crystals [6,7,8,9], multilayer films [10,11,12], hierarchical metamaterials [13,14], etc., possess unique electromagnetic wave manipulation capabilities, which suppress the radiative emission in the mid-wave infrared (MWIR, 3–5 μm) and long-wave infrared (LWIR, 8–14 μm) range while promoting radiative cooling through the non-atmospheric window (5–8 μm) without expending extra energy, balancing the requirements of camouflage and heat dissipation.

Laser detection technology can efficiently and accurately identify targets and realize real-time dynamic tracking by actively emitting laser beams and analyzing the echo signals. Laser camouflage aims to minimize the reflectivity of the target to reduce the detectability of the target. For reducing laser detectability, there are mainly two approaches [15,16]: absorbing incident beams to attenuate reflections or scattering them to disrupt specular returns. Due to the conflicting principles with infrared stealth technology, their compatibility remains a difficult challenge.

Visible light camouflage technology controls the color of the surface and constructs structural colors through methods like electrochromism [17], photochromism [18,19], and mechanical actuation [20], thereby blending the target into the surroundings to achieve camouflage. However, these technologies rely on external activation or physical structure changes, and their opacity limits their application in scenarios such as cockpit windows. In contrast, materials with high visible light transmittance offer unique advantages: not only can they meet the needs of these special scenarios but they can also maintain visible light camouflage in other scenarios and are compatible with other stealth technologies, providing a more optimal solution for special application scenarios.

In this paper, a radiation-selective hierarchical metamaterial (RSHM) is designed to realize the effective synergy of multiple camouflage techniques for infrared, laser, and visible light. This structure exhibits radiative selectivity in the mid-infrared band; low emissivity in atmospheric windows (3–5 μm and 8–14 μm) for suppressing radiation intensity, realizing infrared camouflage; and high emissivity in non-atmospheric windows (2.5–3 μm and 5–8 μm) for efficient thermal management, enhancing thermal stability. Moreover, its high transparency in the visible light band significantly broadens its range of applications. The angle-insensitive and polarization-independent characteristics of the RSHM provide broad applicability in complex and changeable application environments.

2. Design and Method

To simultaneously realize infrared radiation suppression, active detection camouflage, and thermal management and meet the requirements of special light-transmitting scenarios, the structure in this study needs to satisfy the following principles: (1) low absorption in the infrared atmospheric window bands, specifically including mid-wave infrared (MWIR, 3–5 μm) and long-wave infrared (LWIR, 8–14 μm) bands; (2) high emissivity in the non-atmospheric window (2.5–3 μm and 5–8 μm) to realize radiation cooling; (3) low emissivity at 1.06 μm, 1.55 μm, and 10.6 μm to cope with laser detection; and (4) high transmittance in the visible light band. The corresponding ideal emission spectrum is shown in Figure 1a.

Based on the above principle, a radiation-selective, layered metamaterial (RSHM) [Figure 1c] with a checkerboard structure is designed by periodically arranging infrared radiation-selective material (IRSM, which is composed of ITO/SiC/alternately) unit cells [Figure 1b(i,ii)] to realize the precise tuning of the radiation characteristics of a specific band. In order to optimize its performance, we employed the wave optics and optimization module of COMSOL Multiphysics 6.2 to calculate and optimize the parameters of the unit cells and the RSHM in order to ensure that it exhibits desirable radiative selectivity in a predetermined frequency range. The simulated geometric structure is constructed according to the design, and the periodic boundary conditions for the side walls are used. Assuming that the incident wave is a plane wave, the excitation port is positioned at the junction of the top perfectly matched layer (PML) and the air domain, and the corresponding transmission port is located at the interface between the bottom PML and the substrate. The optical constants (refractive index and extinctions) of SiC and Ag are taken from COMSOL’s built-in materials library. Due to the pronounced metallic-like properties of ITO in the IR band, its equivalent permittivity in this band can be accurately described using the Drude model [21]:

$$\epsilon (\omega )={\epsilon }_{\infty }-{\displaystyle \frac{{\omega }_p^2}{\omega (\omega +\mathrm{i}\gamma )}}$$

(1)

where ε_∞ = 3.95 is the relative permittivity at infinite frequency, ω_p = 461 THz is the plasma frequency, and γ = 1.822 × 10¹⁴ Hz is the damping coefficient. ITO film resistance is set to 5 Ω∙sq⁻¹ for high reflection. The temperature dependence of the optical parameters is not considered.

To evaluate the performance of the optimized structure and obtain the best parameters, the following evaluation function is constructed:

$$\mathsf{F}\mathsf{O}\mathsf{M}={\displaystyle \sum \left|{\overline{\epsilon }}_{[{\lambda }_1,{\lambda }_2]}-{\epsilon }_{\mathsf{i}\mathsf{d}\mathsf{e}\mathsf{a}}\right|}$$

(2)

$${\overline{\epsilon }}_{[{\lambda }_1,{\lambda }_2]}={\displaystyle \frac{{\int }_{{\lambda }_1}^{{\lambda }_2}\epsilon (\lambda )I_{\mathsf{B}\mathsf{B}}(\lambda ,T)\mathsf{d}\lambda }{{\int }_{{\lambda }_1}^{{\lambda }_2}{I}_{\mathsf{B}\mathsf{B}}(\lambda ,T)\mathsf{d}\lambda }}$$

(3)

where $\overline{\epsilon }$ is the average emissivity between wavelengths λ₁ and λ₂, ε(λ) is the emissivity at the wavelength, and I_BB is blackbody irradiance. According to Planck’s blackbody radiation law, I_BB(λ, T) = (2hc²/λ⁵)∙[exp(hc/λk_BT) − 1] is the radiation energy of a blackbody at wavelength λ and temperature T (when not specified, T is 300 K), and ε(λ) is the film’s normal spectral emissivity. According to Kirchhoff’s law, in thermal equilibrium, the absorptivity of any object relative to the input radiation of the blackbody is equal to the emissivity of the object at the same temperature: that is, α = ε. After optimization, the corresponding thickness of each layer from the bottom to the top is as follows: 300 nm, 800 nm, 5 nm, and 400 nm.

3. Results and Discussion

IRSM (unit cell1) exhibits significant wavelength-selective absorption characteristics in the infrared band [as shown in Figure 2a, the radiation characteristics of cell2 are shown in Figure S1]. It has a peak emissivity of 0.98 in the 5–8 μm band, which is close to the theoretical limit of blackbody radiation. Such a high emissivity significantly improves the potential application of the device in thermal radiation management. In contrast, at 3–5 μm and 8–14 μm bands, emissivity is suppressed to 0.14 and 0.25, showing excellent spectral selectivity. This selective absorption behavior stems from the multi-order standing-wave modes excited within the F-P resonant cavity. In the 5–8 μm wavelength range, the incident electromagnetic waves experience resonance enhancement through the multi-beam interference effect inside the cavity. Consequently, the highly efficient spectral filtering of electromagnetic waves in the atmospheric window band is achieved. Based on the relationship between the resonance wavelength and cavity length of the Fabry–Pérot cavity, a second absorption peak emerges in the 2.5–3 μm band, which can further expand the spectral regulation range of RSHM, enabling effective thermal management in both the 2.5–3 μm and 5–8 μm non-atmospheric window bands. In addition, at 1.06 μm, the structure’s absorptivity reaches 0.87, reducing echo signals and enabling camouflage against 1.06 μm laser detectors. Furthermore, the selective emissivity remains stable even with increasing incidence angles, which is attributed to the high refractive index of SiC, effectively mitigating the effects of angle variations [Figure 2b].

To investigate and verify the physical mechanism of selective emissions, the electric field distributions at 1.06 μm, 6.1 μm, and 10.6 μm [the blue line in Figure 2c] were analyzed, and the corresponding loss distributions were calculated based on Ohm loss theory [the red line in Figure 2c]. At 1.06 μm, the electric field’s intensity decreases significantly as it propagates, and SiC dominates the absorption, accounting for 79% of the total absorption. Meanwhile, at 6.1 μm, electromagnetic waves enter the asymmetric F-P cavity and generate resonance, where the ultra-thin Ag loss layer and ITO reflective layer repeatedly absorb the resonant waves, achieving up to 80% absorption. In contrast, at 10.6 μm, most of the electric field is reflected with minimal absorption. Additionally, IRSM exhibits high visible light transmittance [Figure 2a], preserving the original visible camouflage and supporting special requirements.

While IRSM realizes radiation selectivity, its low absorption in the LWIR is accompanied by high reflection, rendering it vulnerable to laser detection at 10.6 μm. Figure 3a illustrates the spectral characteristics of the RSHM. There is no obvious change in the transmittance of the visible light band and the selective emission of the infrared band, while specular reflectivity (R_s) in the LWIR band is greatly reduced. The average reflectivity decreases from 0.72 (solid green line) to 0.06 (dashed green line) [Figure 3a], realizing ultra-wideband low echo signals to counter tunable broadband laser detection and enabling dual-band infrared camouflage compatible with LWIR laser stealth. Concurrently, specular reflectivity at 1.55 μm is reduced, decreasing from 0.69 to 0.005 and enhancing the NIR laser’s stealth performance.

To verify the underlying mechanism behind the sharp decrease in specular reflectivity, the reflection phases of unit cell1 (φ₁) and unit cell2 (φ₂) under different incident wavelengths were simulated, and the difference (Δφ) between them was calculated [Figure 3b,c]. At 1.55 μm and 10.8 μm, the phase differences are 3π and π, satisfying the condition of destructive interference. Notably, for effective destructive interference, ∆φ is not strictly limited to odd multiples of π but can fall within the range of odd multiples of π ± 37° [22], as displayed by the yellow region in Figure 3b,c. Furthermore, under the same optical path difference, the phase variation amplitude diminishes with increasing wavelength, leading to broadband scattering and low specular reflectivity within the 8–14 μm range, which effectively counters tunable broadband laser detection. By analyzing the z-component of the electric field (E_z) distribution of the RSHM subgroup, composed of unit cell1 and unit cell2 at 1.55 μm and 10.6 μm [Figure 3d], a significant deviation from the electric field distribution of the IRSM is observed (Figure S2). Distinct diffraction fringes are identified, confirming the presence of diffraction. Moreover, the RSHM exhibits consistent properties in the TM mode (Figure S3), indicating polarization insensitivity.

For a periodic chessboard structure, the relationship between the propagation directions of the incident light (θ₀ and ϕ₀) and reflected light (θ and ϕ) can be expressed using the following formulas [23]:

$$\mathsf{t}\mathsf{a}\mathsf{n}\varphi ={\displaystyle \frac{\mathsf{s}\mathsf{i}\mathsf{n}{\theta }_0\mathsf{s}\mathsf{i}\mathsf{n}{\varphi }_0\pm (2n+1)\lambda /2P_y}{\mathsf{s}\mathsf{i}\mathsf{n}{\theta }_0\mathsf{c}\mathsf{o}\mathsf{s}{\varphi }_0\pm (2m+1)\lambda /2P_x}}$$

(4)

$$\begin{gathered} \begin{array}{l}\mathsf{s}\mathsf{i}\mathsf{n}\theta ={\displaystyle \frac{\mathsf{s}\mathsf{i}\mathsf{n}{\theta }_0\mathsf{s}\mathsf{i}\mathsf{n}{\varphi }_0\pm (2n+1)\lambda /2P_y}{\mathsf{s}\mathsf{i}\mathsf{n}\varphi }}\\ \\ \hspace{1em}\hspace{1em}={\displaystyle \frac{\mathsf{s}\mathsf{i}\mathsf{n}{\theta }_0\mathsf{c}\mathsf{o}\mathsf{s}{\varphi }_0\pm (2m+1)\lambda /2P_x}{\mathsf{c}\mathsf{o}\mathsf{s}\varphi }}\end{array} \end{gathered}$$

(5)

For RSHM, with unit cell1 and unit cell2 designed as square structures, the reflected light exhibits an azimuth angle of 45° and a scattering angle of 22° at 10.6 μm. As illustrated in Figure 3e, the RSHM demonstrates insensitivity to variations in incident angles, maintaining stable emissivity and reflectivity levels across a wide range of incidence angles (0° to 50°) owing to its polarization-insensitive design. This enables robust multi-band camouflage performances in both infrared and laser bands under non-normal incidence detection.

Figure 4a presents the radiation intensities of RSHM and blackbodies in atmospheric and non-atmospheric windows at different temperatures. The results show that RSHM greatly reduces the radiation intensity in the atmospheric windows, while the radiation intensity in the non-atmospheric window is close to that of the blackbody, effectively realizing radiation suppression and thermal management. To evaluate the multiband compatible camouflage performance (MCCP) of the designed RSHM, the following evaluation functions are defined [2]:

$$\mathsf{M}\mathsf{C}\mathsf{C}\mathsf{P}={\displaystyle \frac{P_{5–8\hspace{0.17em}\mu \mathsf{m}}^2}{P_{3–5\hspace{0.17em}\mu \mathsf{m}}\cdot P_{8–14\hspace{0.17em}\mu \mathsf{m}}}}\times {\displaystyle \frac{1-R_{\mathsf{s}10.6\hspace{0.17em}\mu \mathsf{m}}}{{\overline{\epsilon }}_{\mathsf{L}\mathsf{W}\mathsf{I}\mathsf{R}}}}\times (1-R_{\mathsf{s}1.06\hspace{0.17em}\mu \mathsf{m}})\times (1-R_{\mathsf{s}1.55\hspace{0.17em}\mu \mathsf{m}})$$

(6)

$$P_{[{\lambda }_1,{\lambda }_2]}={\displaystyle {\int }_{{\lambda }_2}^{{\lambda }_1}\epsilon (\lambda )}I(\lambda ,T)\mathsf{d}\lambda$$

(7)

where $P_{[{\lambda }_1,{\lambda }_2]}$ represents the radiation intensity of the corresponding band calculated by Equation (7); R_{s1.06 μm}, R_{s1.55 μm}, and R_{s10.6 μm} are the specular reflectivity values at 1.06 μm, 1.55 μm, and 10.6 μm, indicating the echo signal of the RSHM at the corresponding wavelength; ${\overline{\epsilon }}_{\mathsf{L}\mathsf{M}\mathsf{I}\mathsf{R}}$ is the averaged emissivity in the LWIR band.

Since blackbody radiation intensity varies with temperature, the MCCP is temperature-dependent. As shown in Figure 4b, the MCCP of the designed RSHM decreases as the temperature rises from 300 K to 900 K. The higher the MCCP, the better its camouflage performance. Compared to traditional broadband low-emissivity materials (ε = 0.1), RSHM exhibits significantly higher MCCP, demonstrating that multiband compatible stealth materials possess enhanced stealth capabilities. The thermal images from infrared cameras map the target’s temperature distribution and its contrast with its surroundings, which is the key quantitative metric for evaluating effectiveness. Infrared cameras capture the target’s emitted infrared radiation power and derive temperature data using the Stefan–Boltzmann law and inversion algorithms:

$$T_{\mathsf{I}\mathsf{R}}=P_{\mathsf{I}\mathsf{R}}^{-1}({\epsilon }_{\mathsf{I}\mathsf{R}},T_{\mathsf{I}\mathsf{R}})$$

(8)

where ε_IR is the default emissivity built into the IR detector (usually ε_IR = 1). P_IR(ε_IR, T_IR) represents the radiation intensity detected from the object surface, which includes the true radiation intensity of the sample surface (P_obj) and environment radiation intensity reflected (P_env) by the object:

$$P_{\mathsf{I}\mathsf{R}}(\epsilon ,T)=P_{\mathsf{o}\mathsf{b}\mathsf{j}}({\epsilon }_{\mathsf{o}\mathsf{b}\mathsf{j}},T_{\mathsf{o}\mathsf{b}\mathsf{j}})+P_{\mathsf{e}\mathsf{n}\mathsf{v}}(R_{\mathsf{o}\mathsf{b}\mathsf{j}},{\epsilon }_{\mathsf{e}\mathsf{n}\mathsf{v}},T_{\mathsf{e}\mathsf{n}\mathsf{v}})$$

(9)

$$P_{\mathrm{obj}}({\epsilon }_{\mathsf{o}\mathsf{b}\mathsf{j}},T_{\mathsf{o}\mathsf{b}\mathsf{j}})={\displaystyle {\int }_{{\lambda }_1}^{{\lambda }_2}{\epsilon }_{\mathsf{o}\mathsf{b}\mathsf{j}}(\lambda )I_{BB}(\lambda ,T_{\mathsf{o}\mathsf{b}\mathsf{j}})\mathrm{d}\lambda }$$

(10)

$$P_{\mathrm{env}}(R_{\mathsf{o}\mathsf{b}\mathsf{j}},{\epsilon }_{\mathsf{e}\mathsf{n}\mathsf{v}},T_{\mathrm{env}})={\displaystyle {\int }_{{\lambda }_1}^{{\lambda }_2}{R}_{\mathsf{o}\mathsf{b}\mathsf{j}}(\lambda )\cdot {\epsilon }_{\mathsf{e}\mathsf{n}\mathsf{v}}(\lambda )\cdot I_{BB}(\lambda ,T_{\mathsf{e}\mathsf{n}\mathsf{v}})\mathrm{d}\lambda }$$

(11)

where ε_obj(λ) and ε_env(λ) are the radiation spectra of the object and environment, respectively; T is the surface temperature of the sample; T_a is the ambient temperature; [λ₁, λ₂] is the detection band of the IR camera. In the present study, the ambient temperature is 300 K with its emittance at ε(λ) ≈ 1, which can be considered a blackbody. The results in Figure 4c show that radiant temperatures follow actual temperature trends, but T_IR (thermal infrared radiation) is significantly lower. At T_actual = 900 K, radiative temperatures are 693 K (3–5 μm) and 561 K (8–14 μm).

Here, we summarize some research reports in the field of multi-band camouflage in recent years, as detailed in

Table 1. Comparison of multifunctional compatible camouflage research efforts.

Structure	Material	Camouflage Band (μm)						Thermal Management (μm)		Ref.
		Visible	Laser			Infrared
			1.06	1.55	10.6	3–5	8–14	2.5–3	5–8
9-layer film	SiO2/Ge/TiO2	√	√	×	√	√	√	×	√	[24]
7-layer film	Au/ZnS/Ge/Pt/SiO2	×	√	√	√	√	√	×	√	[11]
Lambert	Al	×	√	√	√	√		×	×	[25]
Meta-surface	Ge/ZnS	×	√	√	√	√	√	×	√	[26]
Meta-surface	Au/Ge/Ti/Ge	×	√	√	√	√	√	×	×	[16]
Meta-surface	ZnS/Ge/SiO2/Pt/Au/Ag	×	√	√	√	√	√	×	√	[27]
Meta-surface	Al/Ge/Ag	√	√	×	×	√	√	×	×	[28]
Meta-surface	SiC/Ag/ITO	√	√	√	√	√	√	√	√	This work

. Compared to others, in this work, the RSHM is not only compatible with visible, infrared, and laser multi-band stealth capabilities but also realizes thermal management in the dual non-atmospheric window bands of 2.5–3 μm and 5–8 μm while maintaining a relatively simple structure. It successfully integrates multiple camouflage requirements, laying a solid foundation for its outstanding multifunctionality and adaptability in various operational scenarios.

4. Conclusions

In summary, this study proposes a SiC/Ag/ITO-based RSHM that highly integrates visible light-, laser-, and infrared-compatible camouflage while realizing thermal management. The RSHM exhibits spectrally selective emissivity profiles, showing low emissivity values in the 3–5 μm and 8–14 μm atmospheric windows, attenuating the IR radiation signal while demonstrating enhanced thermal emission with high emissivity in non-atmospheric transmission bands (2.5–3 μm and 5–8 μm) for efficient radiative heat dissipation. For laser camouflage, the RSHM has high absorbance at 1.06 μm (α = 0.87) due to its intrinsic absorption, which is useful for countering Nd:YAG laser detectors. The periodic chessboard-like structure utilizes the destructive interference to reduce the specular reflectance at 1.55 μm (R_s = 0.005) and within the 8–14 μm range (R_s = 0.06), enhancing NIR laser camouflage and mitigating threats from tunable broadband laser detectors in the LWIR. The structure maintains high optical transparency to preserve its intrinsic visible light camouflage functionality while exhibiting angle-insensitive responses and polarization-insensitive performances to meet requirements under various conditions. This multifunctional design is compatible with existing etching and sputtering fabrication processes and offers significant potential for various applications, providing a theoretical basis for the subsequent research and development, optimization, and engineering application of multispectral camouflage materials and devices.