1. Introduction

Camouflage has been an important ability of many animals for avoiding predators over millions of years [1,2]. Camouflage technology is essential for hiding key information in human society. The conventional camouflage is designed for a single detector or a narrow operating band. With the continuous upgradation of detection methods, detectors with different operating bands are often combined, as in the case of LiDAR, MWIR cameras, and infrared thermal imagers [3–5]. Therefore, the traditional single-band camouflage has lost its competitive advantage. Multispectral camouflage technology, which can counteract multispectral complementary detection technology has become attractive. In recent years, it has been valued for its wide applications in the aerospace and military fields [6,7]. However, the design of multispectral camouflage is still difficult. This type of design is not only required to take into consideration the working principle and working wavelength of multiple detectors, but should also be as simple as possible.

LiDAR and infrared detectors are indispensable in precision detection techniques. LiDAR emits a laser onto objects and obtains parameters such as the target distance, orientation, and shape by comparing the reflected laser signals (echo signal). In the case of long-distance and large-area detection, LiDAR can quickly acquire a large amount of distance point data, known as point cloud data. These data can be processed by filtering, denoising or a combining algorithm for three-dimensional object reconstruction to obtain a large amount of intuitive information [8–10]. The core strategy of LiDAR camouflage is to cover low reflection materials with the detected objects, which is aimed at reducing the laser echo signal [11,12]. Thermal radiation is widespread and carries a great deal of information about the temperature distribution and surface materials [13,14]. According to Kirchhoff's law, infrared invisibility can be achieved only when the camouflage materials have low infrared absorption in the infrared band [15]. In previous studies, low-absorption materials in the infrared wide band were typically used to cover the camouflaged target [16,17]. However, this affected the radiation heat transfer of the camouflaged target and increase its temperature. Reducing the temperature requires active refrigeration, which places additional load on the camouflaging target. Moreover, the working bands of LiDAR are mostly infrared bands. The design with low-absorption wide band infrared clearly contradicts the LiDAR camouflage. Therefore, effective infrared camouflage should be wavelength selective, while avoiding the introduction of thermal instability, and reducing surface temperatures [18,19]. The combination of multispectral camouflage and passive cooling significantly improves the effectiveness of infrared camouflage. As a passive, continuous and efficient cooling method, radiative cooling is an ideal and compatible with multispectral camouflage [20,21].

Using metasurface to manipulate electromagnetic energy is an effective solution for achieving multispectral camouflage. The metasurface can adjust the band of the spectral response by tuning the periodic structure, exciting one or more fixed resonant modes, and forming a multiband spectral response [15–23]. After determining the geometric parameters, the spectral characteristics of the metasurface cannot be changed. To excite many resonances that meet camouflage requirements in a wide spectral band, combinations of metasurface, 1D-PCs (one dimensional photonic crystals), and multilayer films have been explored for multispectral camouflage [24–26]. Deng et al. proposed a laser, infrared and microwave compatible camouflage structure with a Ge/ZnS multilayer film on the top and a YbF₃/ZnS/Ge/ZnS nanolayer on the bottom [25]. Zhu et al. proposed a combination of a Ge/ZnS multilayer film and Cu-ITO-Cu metasurface to complement the multispectral camouflage and radiative cooling [27]. Although the performance of these structures is superior, their combination inevitably results in a large thickness, poor mechanical stability, and thermal stability. In recent studies, phase change materials have shown great promise for the design of multispectral devices owing to their non-volatile and dynamically adjustable properties. The incorporation of phase change materials can facilitate the design of multispectral camouflage. Moreover, the crystalline and amorphous states of phase change materials do not require additional power to be maintained after the change, such as vanadium dioxide (VO₂), Ge₂Sb₂Tb₅ (GST), Sb₂S₃, and Sb₂Se₃ [28,29]. Among them, Sb₂Se₃ is widely used in reconfigurable photonic devices because of its non-volatility, low loss, and high phase-switching speed [30].

In this study, a multispectral camouflage metasurface is proposed. The metasurface contains Sb₂Se₃, which changes the phase to switch from the high-performance thermal management mode to the camouflage mode. When Sb₂Se₃ is in the crystalline phase (C-Sb₂Se₃), the metasurface is in high-performance thermal management mode. The resonance wavelengths are excited in two extremely high atmospheric transmission bands: the MWIR band (3–5 µm) and atmospheric window (8–13 µm), where heat can be exchanged with the outer space. The cooling performance of the structure is verified by calculating the cooling power under various atmospheric conditions. The maximum cooling powers realized were 65.5 Wm^-2. When the Sb₂Se₃ is in the amorphous phase (A-Sb₂Se₃), the metasurface is in the camouflage mode. The peak wavelength is shifted to the short-wave infrared band (SWIR) and non-atmospheric window (5–8 µm), thus reducing the risk of detection by the mid infrared detector. The absorption in the 1064 nm reaches 95.6% in the simulation. In an experiment conducted to test the camouflage performance of LiDAR, the echo signal reduction rate of the camouflage mode reached 96.3% compared with the blackground plate. Meanwhile, high absorption in non-atmospheric window can also increase natural convection, which facilitates surface temperature control and increases the camouflage capability in the mid-infrared band. The dynamically tunable metasurface proposed in this paper can be used as a coping strategy to satisfy the paradoxical design requirements of radiative cooling and long-wave infrared (LWIR) camouflage in a single structure.

2. Experiment

2.1 Materials

Sb₂Se₃ (target) with a purity of 99.999% was purchased from Jiangxi Ketai New Materials. China. Au and Al (target) with a purity of 99.99% was purchased from Beijing Jingmaiyan Material Technology Co., LTD. China. All the targets purchased were used directly.

2.2 Fabrication

A dynamically tunable metasurface based on Sb₂Se₃ was manufactured using electron- beam lithography (EBL). Figure 1(a) presents the experimental manufacturing process. First, a Au layer was deposited on a silicon dioxide substrate (2 cm × 2 cm) by using EBE (DZDZS500). The Sb₂Se₃ layer was then grown on the Au layer using radio frequency magnetron sputtering (VTC300) at 300 °C, the sputtering power and growth pressure were set at 40 W and 2.0 Pa, respectively. Figure 1(b) presents the X-ray diffraction pattern (XRD) of sputtered Sb₂Se₃ at 300 °C and room temperature. The XRD of sputtered Sb₂Se₃ at 300 °C is consistent with the orthorhombic Sb₂Se₃ (PDF card JCPDS 15-0861). At the same time, scanning electron microscopy (SEM) images of amorphous and crystalline Sb₂Se₃ is shown in Fig. 1(c). Argon gas flow rate was 40 sccm. The radio frequency magnetron sputtering was also used to deposit Al. Subsequently, to form the design pattern of the Al layer, the surface was spin-coated by the polymethyl methacrylate (PMMA) at 4000 rpm for 30 s. The glue on the sample was dried for 90 s at 180 °C. The PMMA was then subjected to EBL (Raith Elineplus). A square pattern array was formed during the development process. After photographic fixing, hardening film 90 s at 180 °C. The Al layer was etched using inductively coupled plasma etcher (ICP, LAM9600). Finally, the photoresist was removed, and the surface of the sample was cleaned with acetone, isopropyl alcohol and water respectively. The dimensions of the fabricated metasurface were 200 µm × 200 µm. The final thickness of Au, Sb₂Se₃ and Al was 130 nm, 152 nm and 110 nm, respectively.

 

Fig. 1. (a) Fabrication technology and morphology of dynamically tunable metasurface based on Sb₂Se_3. (b) the XRD of sputtered Sb₂Se₃ at 300 °C and room temperature. (c) SEM of amorphous and Crystalline of Sb₂Se₃.

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2.3 Measurements

The operating wavelength of the airborne LiDAR system that collects LiDAR echoes was 1064 nm. The repetition rate of the 1064 nm laser used in the airborne LiDAR was 2 kHz. The pulse width was 1500 ps. The beam mode was TEM00, and the pump current of 1064 nm laser was 1 A. The wavelength of the guiding laser (green) was 532 nm.

The reflectance spectra of the metasurface based on Sb₂Se₃ were obtained using a Fourier-transform infrared (FTIR) spectrometer (Thermo Fisher Nicolet iN10). A mercury cadmium telluride detector was used in the FTIR spectrometer. The band of the reflectance spectrum measured using FTIR is 675–4000 cm^-1.

3. Result and discussion

3.1 Device design and working principle

To design a multispectral camouflage metasurface, the absorption characteristics of the metasurface should be determined based on the working principles of different detection methods. The metasurface is tunable, and the spectral characteristics in the camouflage mode are as follows: (1) in the visible band, low absorption can aid in avoiding the obvious infrared characteristics caused by a high surface temperature; (2) in the working bands of common LiDARs (e.g., 1064 nm), high absorption can reduce the echo signal. In addition to the high absorption of 1064 nm, the absorption in the SWIR band should be retained at a low level, to prevent the metasurface from absorbing an excessive amount of solar energy; (3) in the MWIR (3–5 µm), low absorption can limit the thermal radiation signal received by most thermal imagers or thermal search missiles to achieve MWIR camouflage; (4) High absorption in non-atmospheric window (5–8 µm) and low absorption in atmospheric window (8–13 µm). In the thermal management mode, the design requirements conflict with those in the camouflage mode. The key difference is high absorption in the atmospheric window. It enhances the cooling efficiency via heat exchange but is simultaneously harmful for LWIR camouflage at the same time. To satisfy the requirements of different absorption spectra in the same band, the multispectral tunable camouflage strategy of the designed metasurface is presented in Fig. 2(a).

 

Fig. 2. (a) Compatible strategies for multispectral camouflage and radiative cooling. (b) Simulated absorption spectra of C-Sb₂Se₃ and A-Sb₂Se₃. (c) SEM images of MSM metasurface. d) Enlarged SEM image. The geometric parameters of nano structures are indicated.

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A simple structure is ideal for multispectral camouflage over a wide spectral band. For thin films, it is generally difficult to achieve this ideal design with a small number of layers. By designing a metasurface structure on a suitable film, it is easier to realize multispectral camouflage through a simple structure. The MIM (metal-insulator-metal) metasurface exhibits excellent resonance wavelength modulation and wavelength selectivity [31,32]. However, for multispectral camouflage design requirements, the resonant modes excited by the MIM metasurface are limited. Semiconductor materials that are opaque in the visible band and exhibit reversible phase transitions may be useful solutions to this problem. MSM (metal-semiconductor-metal) metasurface not only increase the number of resonance modes, but the resonance modes can also be switched to different bands, thus enhancing the tunability of the metasurface [33]. The metasurface was designed to satisfy the requirements and determine the parameters of the subsequent experimental preparation. Thus, the proposed adaptive selective metasurface has an MSM structure comprising of Au, Sb₂Se₃, and Al. The thicknesses of three materials were 110 nm (t₁), 150 nm (t₂), and 100 nm (t₃), respectively. The period P was 1000 nm. Al is a good material that not only has lower manufacturing costs than noble metal, but also has great potential in terms of industrial compatibility [34]. Moreover, Al exhibits a good metal reaction to compensate for its high loss in the infrared spectrum band. Therefore, Al was selected and lithographed to form a highly symmetrical pattern. The side length w of the pattern was 800 nm. Sb₂Se₃ has the advantages of a low band gap and low energy required for phase switching (electrical, thermal, or optical methods). The Au film acts as a metal mirror to prevent the majority of the electromagnetic waves from passing through the entire structure [35]. Using the equation: A(λ) = 1 - R(λ), the absorption A(λ) of the metasurface can be obtained. In Fig. 2(b), the absorption curves of the amorphous and crystalline phases of Sb₂Se₃ at normal incidence were calculated by using the commercial FDTD Solution software. Periodic boundary conditions were adopted in the x and y directions, and perfect matched layer (PML) conditions were adopted in z directions. The three-dimensional unit cell structure was then established. In the TM mode, the plane wave was incident on the unit cell along the z direction. The transmission monitor was located below the Au layer, and the reflection monitor was located λ/4 above the MSM metasurface. The coefficients n and k of Al and Au are obtained from FDTD database. The n and k coefficients of Sb₂Se₃ are obtained from Ref. [30].

Figures 2(c) and 2(d) present the SEM images of the MSM metasurface. The unit cells with good periodicity were successfully constructed, as shown in the enlarged SEM images. The highly symmetrical structure endows the entire metasurface with a high polarization tolerance, as shown in Figs. 3(a) and 3(b). Moreover, some resonance modes benefit from this and are insensitive to the incident angle. A common operating band of LiDAR is 10.6 µm, which is one of the resonant modes excited by a metasurface. When the incident angle varies from 0 ° to 40 °, the resonance modes in the LWIR band were barely affected by the incident angle, as shown in Figs. 3(c) and 2(d). In contrast, the resonance modes in the SWIR band change significantly with an increase in the incident angle. When the incidence angle is 0 °, the reflectance is 0.012. When the incidence angle is 20 °, the reflectance is 0.98 in Fig. 3(d). As laser-guided equipment and weapons can only detect scattered signals at small collection angles, this phenomenon had little impact on the performance of the 1064 nm LiDAR camouflage.

 

Fig. 3. Reflection spectra for polarization angle from 0-90 ° in (a) C-Sb₂Se₃ and (b) A-Sb₂Se₃. Reflection spectra for the incident angle from 0-40 ° in (c) C-Sb₂Se₃ and (d) A-Sb₂Se₃.

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To analyze the potential applications of the metasurface and directions for optimization, the thickness of Al, side length of the nanobrick pattern, and thickness of Sb₂Se₃ were analyzed. Figures 4(a)–4(c) present the relationship between the metasurface reflectance and wavelength and the three geometric parameters. In Figs. 4(b) and 4(c), the entire structure barely meets the conditions of resonance, and the phase change of Sb₂Se₃ has little effect on the reflection in the visible light band. Al is etched into a the nanobrick on the surface of the Sb₂Se₃ layer. As the thickness of Al and Sb₂Se₃ increases, the number and position of the resonant wavelengths in the SWIR band changed constantly. In Fig. 4(c), the main resonance wavelengths in the MWIR and LWIR bands are significantly red shifted with an increase in the side length. The change in the side length also affects the reflection of the visible range. With the increase in the side length, the distance of the patterns gradually decreases, and the average reflectance increases from 57% to 72%. This phenomenon is not only due to the increase in the metal area, but also because a compact metasurface structure facilitates scattering and enhances the reflection of visible light [36].

 

Fig. 4. Simulation results of optimization of MSM metasurface. (a) Reflectance versus the thickness of Sb₂Se₃ and wavelength for the metasurface. (b) Reflectance versus the thickness of Sb₂Se₃. and wavelength for the metasurface. (c) Width of Al. (d) Three-dimensional diagram of unit cell. (e) Electric field diagram of the x–y cross section. (f) Distribution of electric and magnetic fields in the x–z cross section.

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To further understand how light is captured by the MSM metasurface and determine the resonance mechanism, the electric and magnetic field distributions in different cross sections are analyzed as shown in Fig. 4(d). Three major resonant wavelength modes were analyzed: 1067 nm, 2569 nm, and 6580 nm. In Fig. 4(e), the MSM structure excites dipole resonances on the sides of the nanobrick pattern, and the luminous portion at the edge of the nanobrick pattern can be observed in the x–y cross section of all resonance modes. The dipole resonance limits the incident light to the edge of the nanobrick and makes an important contribution to all three major resonance modes. Figure 4(f) presents the distribution of electric and magnetic fields in the x–z cross section. The resonant resonance at 1067 nm was in the mixed mode. In the electric field distribution of the 1067 nm resonance mode, the incident light is mainly limited to the upper edge of the Al nanobrick, which is a phenomenon of the surface plasmon polaritons (SPPs) excited in the metasurface. Moreover, the resonance mode at 1067 nm was almost unchanged when different metals were used as the top nanobricks. In the presence of a magnetic fields, two different resonance modes can also be clearly observed in the Sb₂Se₃ layer. The properties that are sensitive to the dielectric thickness but not to the period confirm the contribution of the Fabry–Perot resonance to the high absorption of 1067 nm. Therefore, the 1067 nm resonance is dominated by surface plasmon (PSPR) and localized surface plasmon resonance (LSPR). In the electric fields of 3070 nm and 8051 nm, there was an obvious gap in the Sb₂Se₃ layer. This is because the constructive interference of the gap plasmon polariton (pre-GPP) and post-GPP in the Sb₂Se₃ layer forms a standing wave resonance [37]. This strongly coupled magnetic resonance is called the gap surface plasmon (GSP) resonance and can generate anti-parallel currents that oscillate in the Al and Au layers.

The GSP resonance can be described using the Fabry–Perot resonator formula in Eq. (1) [38,39]:

3.2 LiDAR camouflage performance

LiDAR can be used to detect, track, and identify aircraft, missiles, and other targets. The most common operating wavelength for a LiDAR is 1064 nm. The working principle of LiDAR is to compare the emitted detection signal (laser beam) with the signal reflected from the target (echo signal) from the time interval and the strength difference between the signals to obtain the spatial data of the object. Therefore, the key to implementing LiDAR camouflage is to reduce the reception of laser echo signals. These methods primarily include changing the material and shape of the detected object. Figure 5(a) presents the experimental platform and devices used to test the LiDAR echo signals. Because the human eye cannot distinguish the 1064 nm laser, LiDAR is equipped with a 532 nm laser as a guide laser to confirm the specific location of the laser incident on the sample. Figure 5(a) presents the locations of the LiDAR and the sample. The sample was then placed on a background plate. In general, the output power of the LiDAR is high, and the surface of the sample is Al, except for the lithographic region. To prevent LiDAR damage, however, the output power should not be excessively high for close-distance measurements. The power obtained from the sample was 99.2 mW. However, the outgoing laser cannot be directed vertically onto the sample. The angle between the outgoing laser and the reflected lasers was maintained at 20 °. The majority of LiDARs that are engaged in long-distance measurements collect echo signals at a small angle. Therefore, the insensitivity of the incident angle has little influence on LiDAR camouflage [41]. The entire test process was carried out in a dark environment to avoid interference from stray light. For the 1064 nm LiDAR, the pulsed laser ranging method was adopted, and the ranging principle is presented in Fig. 5(b). The 1064 nm LiDAR sampling frequency was 400 ns/time. Therefore, the echo signal period was set as 400 ns. Compared with the echo signal of a conventional surface, the signal intensity of the camouflage mode is much lower than that of the conventional surface, which is demonstrated in Fig. 5(c). As shown in Fig. 5(d), the peak voltage of the blackground plate reaches 557.2 mV, and that of the camouflage mode is 21.8 mV. The signal reduction rate is 96.3%.

 

Fig. 5. (a) Diagram of experiment comparing conventional surface and A-Sb₂Se₃ LiDAR echo signals, the distance D between metasurface sample and 1064 nm LiDAR is 9 m. The illustration on the right presents the output power of the pulsed laser and a blackground plate for comparison. (b) Schematic of pulsed laser ranging principle. (c) Echo signal waveform of A-Sb₂Se₃ and conventional surface. (d) Enlarged waveform comparison of echo signals.

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In addition to the high absorption of 1064 nm, if the space between the Al patterns on the metasurface is considered as nanopores. According to the formula of the scattering efficiency: ${Q_{sca}} = {\sigma _{sca}}(\lambda )/0.25\pi {d^2}$, where σ_sca (λ) is the scattering cross-section, and d is the diameter of a scattering nanopore [36,42]. The scattering efficiency is inversely proportional to the size of the nanopore. The dense porous structure of the metasurface will also enhance the performance of the LiDAR camouflage in terms of improving scattering efficiency.

LiDAR technology can generate high precision digital surface models, utilizing two-dimensional or three-dimensional point cloud data that find applications in various fields such as earth observation and precision guidance [43,44]. To visually demonstrate the camouflage performance of the MSM metasurface, simulations were conducted for two scenarios: forests and grasslands near forests, as depicted in Fig. 6(a) and Fig. 6(d). The two-dimensional photon point cloud data in Fig. 6(b) presents the forest distribution and canopy height of the mountain range. The three-dimensional point cloud data in Fig. 6(e) presents the forest within a 500 m × 500 m region, including the properties, height, and distribution of the trees in the forest. The red lines indicate the areas that are required to be camouflaged. The camouflaged images of simulated in Figs. 6(c) and 6(f) when covering the metasurface. In camouflage mode, the covered area absorbs the 1064 nm laser emitted by the LiDAR, resulting in no echo signal from the camouflage area returning to the LiDAR receiver. In Fig. 6(c), trees are almost completely hidden except for mountain range and a few noisy points (black points). In Fig. 6(f), the camouflage area of the square is consistent with the blackground (the area without the echo signal). In larger point cloud data, it is usually misjudged as invalid data, to achieve the purpose of LiDAR camouflage.

 

Fig. 6. (a) Area scanned using the airborne LiDAR (RIEGL VQ-580II (H2225798), 1064 nm) which was carried by the aircraft [45]. (b) Schematic of two-dimensional forest point cloud data and camouflaged area. (c) Simulated point cloud data of the camouflaged mode. (d) Area scanned using the LiDAR carried by the ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2, 1064 nm) [46]. (e) Schematic of three-dimensional forest point cloud data and camouflaged area. (f) Simulated point cloud data of the camouflaged mode. The upper right illustration presents the point cloud data in camouflage mode.

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2.4 MWIR camouflage and radiative cooling performance

Many detectors work in LWIR and MWIR bands. However, the high absorption in the atmospheric window (8–13 µm) is one of the necessary conditions for radiative cooling. The atmospheric window of 8–13 µm not only has high atmospheric transmittance, but the emitters with high absorption in this range also exhibit a better cooling performance [47]. Figure 7(a) presents the simulation of the reflectance spectra of Sb₂Se₃ with different crystallization fractions from amorphous to crystalline and demonstrates the continuous modulation of the MSM metasurface. The experimentally obtained reflection curve verifies this phenomenon, as shown in Fig. 7(b). In Fig. 7(c), the camouflage performance was estimated in the two detection bands, and the absorption measured in the experiment was used to calculate the thermal radiation. The spectral radiance flux (RF) per unit area emitted from the surface was determined as follows:

 

Fig. 7. (a) Simulated reflection curve represents the crystallization fraction ranging from 0% to 100%. (b) Reflection spectra of experimental data and simulated data (2–13 µm). (c) Comparison of blackbody, conventional surface, and A-Sb₂Se₃ thermal radiation. (d) Cooling power variation with the temperature difference. (e) Different non-thermal radiation gain coefficient q.

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Table 1. Comparison of recent works on multispectral camouflage

View Table

Although the camouflaging performance of a single spectrum is not prominent, the dynamically tunable MSM metasurface is compatible with more spectra and has a wide operating range. In addition, the MSM metasurface can be tuned to the thermal management mode, which is compatible with radiation cooling and enhances infrared camouflage by reducing the temperature. In the crystalline phase, the resonant wavelengths of the MSM metasurface were within the atmospheric window. The experimental reflectance and simulated reflectance were used to evaluate the radiative cooling performance of the metasurface. To calculate the efficiency of radiative cooling accurately, the heat transfer process in an open space should be taken into consideration. On combining the heat transfer processes in sub-ambient radiative cooling, the net cooling power of the radiative cooler can be defined as shown below. We define A as the area of the radiator.

In the thermal management mode, the cooling power evaluated via the simulation and experimental data are presented in Fig. 7(e), and the highest cooling power is 64.3 Wm^-2 and 65.5 Wm^-2, respectively. Under typical outdoor conditions (q = 8 Wm^-2K^-1), the metasurface does not provide actual cooling. If the cooling power is further increased, the method of combining radiative cooling materials can be used. For example, the ZnS/Ge multilayer film [25,27], polydimethylsiloxane (Es-TPU) [42], and polydimethylsiloxane (PDMS) [56].

High absorption in the 5-8 µm range can also reduce the infrared signature of the metasurface. After changing to the camouflage mode, high absorption at 5–8 µm can still participate in heat dissipation. The non-atmospheric window is mainly affected by H₂O, and the radiation generated by the metasurface can be absorbed by H₂O. Therefore, the utilization of high absorption in the 5–8 µm band would promote thermal stability of the metasurface.

4. Conclusion

In summary, an MSM metasurface that can implement radiative cooling, 1064 nm LiDAR, and infrared camouflage was proposed. The multiple resonance modes and phase-changing characteristics of Sb₂Se₃ facilitate the dynamic tunability of the metasurface as well as the thermal management and camouflage modes. In the thermal management mode, the cooling power was up to 65.5 Wm^-2; whereas in the camouflage mode, the infrared radiation reduction rates in the MWIR and LWIR bands were 98.2% and 70.5%, respectively. The reduction rate of the 1064 nm LiDAR echo signal was 96.3%. In addition, point cloud images were demonstrated in the camouflage mode, which intuitively demonstrates the excellent LiDAR camouflage of the metasurface. This study provides a new strategy for improving the camouflage compatibility and promoting the application of optical camouflage technology in thermal management, energy conservation, and military camouflage.