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Abstract
Visible-infrared compatible camouflage is significant to enhance the equipment survivability through counteracting the modern detecting and surveillance systems. However, there are still great challenges in simultaneously achieving multispectral camouflage with high transmittance in visible, low emissivity in the atmospheric windows and high emissivity in the non-atmospheric window, which can be attributed to the mutual influence and restriction within these characteristics. Here, we proposed an optically transparent infrared selective emitter (OTISE) composed of three Ag-ZnO-Ag disk sub-cells with anti-reflection layers, which can synchronously improve the visible transmittance and widen absorption bandwidth in the non-atmospheric window by enhancing and merging resonance response of multi-resonators. Test results reveal that low emissivity in infrared atmospheric windows, high emissivity in the 5-8 µm non-atmospheric window and high optical transparency have been obtained. In addition, the radiative flux of OTISE in 3-5 µm and 8-14 µm are respectively 34.2% and 9.3% of that of blackbody and the energy dissipation of OTISE is 117% of that of chromium film. Meanwhile, it keeps good optical transparency due to the ultrathin Ag film. This work provides a novel strategy to design the optically transparent selective emissive materials, implying a promising application potential in visible and infrared camouflage technology.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
According to Planck’s law, any object with a temperature higher than absolute zero (0 K) will radiate electromagnetic energy to the surrounding by virtue of their atomic and molecular oscillations, and the radiation wavelength is mainly located in the infrared (IR) range in most cases [1]. Based on this principle, IR detection and imaging technology have been developed and used in spying on tanks, aircrafts and soldiers [2–4]. Suppressing infrared signatures of object surface in the two atmospheric windows (3–5 µm and 8–14 µm) are of substantial significance for equipment camouflage, which could reduce the probability of being detected [5]. Stefan-Boltzmann law shows that IR camouflage can be achieved by reducing either the emissivity or the temperature of the material. To decrease the emissivity over the whole IR range, some materials such as metal film [6], micro-powder [7] and metasurfaces [8] are proposed and demonstrated. Unfortunately, materials with low emissivity covering on the objects will hinder its heat radiating to the periphery, thus enhancing the surface temperature and weakening the camouflage performance. For ideal IR camouflage, the material should possess low emissivity in the atmospheric window and high emissivity in the non-atmospheric window, which can not only reduce IR signature, but also decrease the surface temperature of the object by radiating part of the energy [9–12]. Infrared selective emitter is an artificial engineering structure that can tailor emissivity in a specific IR band has been widely studied in the past [13]. The ISE with high emissivity in non-atmospheric has been realized by metamaterials [10,11,14–25], photonic crystals [26–28], and multilayer structures [9,12,29–31]. Generally, the ISE is composed of metal film, dielectric film and (or) patterned structure, which can realize high emissivity in the non-atmospheric window and keep low emissivity in the atmospheric window. Whereas these methods mentioned above are usually opaque and cannot be used for optically transparent IR camouflage. In some real applications, the optically transparent IR camouflage are needed. For example, cockpit window as a component of equipment plays an important role in signal communication. Recently, transparent metal and dielectric film such as Indium Tin Oxide (ITO) are used to achieve an optically transparent infrared selective emitter with high emissivity in the non-atmospheric window [16,32]. However, due to the electrical conductivity of ITO being much lower than metal, either the resonant peak value of emissivity in the non-atmospheric window is low when keeping low emissivity in the atmospheric window, or emissivity in the non-atmospheric window is high while the emissivity in the atmospheric window tends to be higher. In addition, the bandwidth of resonant peak in the non-atmospheric window is narrow for the structure with a single resonant peak [16]. It remains a challenging issue to achieve OTISE with high transmittance in visible, low emissivity in the atmospheric windows and high emissivity in the non-atmospheric window as they are interferential with each other [11].
In this paper, we proposed a visible-infrared compatible camouflage structure composed of three Ag-ZnO-Ag disk sub-cells with anti-reflection layers to improve visible transmittance and widen absorption bandwidth in the non-atmospheric window by enhancing and merging resonance response of multi-resonators, which not only provides low emissivity in the atmospheric windows of 3-5 µm and 8-14 µm but also possesses high emissivity in the non-atmospheric window of 5-8 µm and high optical transparency. To verify the performance of the designed OTISE, visible and IR cameras were used for qualitative analysis by capturing the visible and thermal images. In addition, quantitative spectral measurements are taken of the transmittance in the visible light and the emissivity in the IR waves by using UV/VIS/NIR and FTIR spectrometers. The experiments demonstrate that the proposed OTISE has more excellent IR camouflage performance and high optical transparency in comparison with traditional low emissivity material Cr film and optically transparent material.
2. Design and optical characteristics analysis
2.1 Structure design
In order to achieve multispectral camouflage with high transmittance in visible, low emissivity in the atmospheric windows and high emissivity in the non-atmospheric window, we proposed a five-layers visible-infrared compatible camouflage structure as illustrated in the schematic in Fig. 1(a) and Fig. 1(b), which consists of anti-reflection layer (ZnO), metal disk layer (Ag), dielectric spacer (ZnO), infrared reflector layer (Ag) and anti-reflection layer (ZnO), from top to bottom. Herein, we chose ultrathin Ag film as a metal layer for enhancing electromagnetic resonance as it processes high visible transmittance and high conductivity compared with ITO commonly used for transparent electrodes [33,34]. ZnO is selected as a visible light anti-reflection layer and dielectric layer due to almost lossless in visible and IR bands. Three sub-cells with adjacent resonant central wavelength are applied to realize broadband absorption within the non-atmospheric window. The proposed structure was simulated by using the commercial solver of CST Microwave Studio. Unit cell conditions were set along the x and y direction in the simulation. We applied the Lorentz-Drude dispersion model for analyzing the electromagnetic behavior of Ag in the IR band [35], and the permittivity of ZnO in the IR band was taken from [36]. The permittivity of ZnO and Ag in visible light were measured using a spectroscopic ellipsometer as shown in Fig. 1(c).
Fig. 1. Designed and simulated results. (a) Schematic of the proposed OTISE. (b) Super-cell diagram of designed OTISE and the corresponding geometry parameters as follows: P = 2.6 µm, D1 = 1.48 µm, D2 = 1.3 µm, D3 = 1.6 µm, t1 = t5= 40 nm, t2 = t4= 10 nm, t3 = 300 nm. (c) Permittivity of Ag and ZnO in visible band. (d) Simulated visible light transmittance. (e) Simulated IR emissivity of three different diameter disks and their combination (OTISE).
The geometry parameters of the multi-resonators super-cell are shown in the caption of Fig. 1(b). The visible transmittance of the OTISE is shown in Fig. 1(d), and the average visible transmittance is about 77%, which indicates the structure possesses high optical transparency in the full-visible band. According to Kirchhoff’s law, spectral emissivity is equal to absorptivity at certain temperatures and wavelengths at thermal equilibrium. Figure.1 (e) summarizes the simulated emission spectra, corresponding to a disk of 1.3 µm, 1.48 µm and 1.6 µm diameter centered in the sub-cell and their combination (OTISE). The single-disk emission spectrum shows a narrow emission peak, while the proposed OTISE not only keeps a strong emission peak, but also possesses broadband absorption within the non-atmospheric window. The bandwidth of emissivity over 0.8 for the OTISE is 1.36 µm, which is two times that of a single disk (the bandwidth of emissivity over 0.8 is 0.67 µm). The simulated band emissivity is 0.12 and 0.09 in the atmospheric window of 3-5 µm and 8-14 µm, respectively. In the non-atmospheric window, the simulated band emissivity is 0.65. The simulated band emissivity is 0.24 in IR band 3-14 µm. The band emissivity for wavelength band [λ1, λ2] at temperature T can be expressed in terms of spectral emissivity ɛ(λ, T) and spectral irradiance of blackbody Ebλ(λ, T) as Eq. (1) [9]:
2.2 Optical characteristics of the OTISE in visible and IR band
In order to interpret the mechanism of multi-resonators for widening absorption bandwidth in the non-atmospheric window, we plot the magnetic field distributions (the absolute magnitude of the vector sum of x, y and z components) of the cross-section of the x-z plane (y = 1300 nm) as shown in Fig. 2(a)–(c), which correspond to three resonance wavelengths of λ1= 7.56 µm, λ2= 7.1 µm and λ3= 6.44 µm. The magnetic resonance is confined in the region between the Ag disk and the Ag reflector layer and the strongest resonance region for three resonance wavelengths in Fig. 2(a)–(c) correspond to three sub-cells with a disk diameter of 1.6 µm, 1.48 µm and 1.3 µm, respectively. The results show the magnetic resonant wavelength depends on the metal disk diameter. The magnetic resonance attributes to the induced current caused by the incident electric field. A resonance cavity is formed between the Ag disk and the Ag reflector resonators, it is indicated that the merging of adjacent resonance peaks can widen absorption bandwidth in the non-atmospheric window.
Fig. 2. Simulated magnetic field distribution. (a), (b) and (c) magnetic field distributions (x-z plane, y= 1300 nm) of resonant wavelength are λ1= 7.56 µm, λ2= 7.1 µm and λ3= 6.44 µm, respectively.
To further analyze the influence of geometrical parameters on the performance of OTISE, we have simulated the visible transmittance and IR absorption of individual sub-cell, and illustrated in Fig. 3(a)–(c). The top and bottom ZnO layers as anti-reflection layer are used to suppress visible reflection and increase transmittance. The period of sub-cell and disk diameter were set to P and D, respectively. Under the normal incidence of transverse electric (TE) electromagnetic wave, the visible transmittance varies with the thickness of anti-reflection layers as shown in Fig. 3(a). Uniform and high transmittance occurs in the whole visible band when the thickness is 40 nm, and the results indicate that the visible transmittance is sensitive to the thickness of the top and bottom ZnO layer. Figure. 3(b) is the infrared absorption variation with the period of single sub-cell P while disk diameter is fixed (D = 1.3 µm). The maximal emissivity shifts to short wavelength with an increase of period P when P is not more than 2.5 µm, but it hardly changes after P is over 2.5 µm. As shown in Fig. 3(c), the absorption peak moves to long wavelength as disk diameter increases, which agrees with the prediction based on the inductance−capacitance model that the resonant center wavelength is proportional to the disk diameter [37]. Thus the diameter of the disk can be used to adjust resonant wavelength in the undetected band. In addition, the emissivity of multi-resonators changing with the thickness of the middle ZnO dielectric layer is demonstrated in Fig. 3(d), which is used to analyze the effect of the resonant cavity in the metal-dielectric-metal structure. With the increase in thickness of the dielectric layer, the emissivity in the non-atmospheric window increases. Nevertheless, surface plasmon resonance occurs when the thickness exceeds 300 nm.
Fig. 3. Simulated results. (a) Influence of thickness of top and bottom ZnO layer (t1 and t5) of individual sub-cell on visible transmittance. (b) Diameter of disk was fixed (D =1.3 µm), emissivity of individual sub-cell changed with length (P) of sub-cell changed. (c) P is constant (P = 4.5 µm), diameter of disk dependence of emissivity of individual sub-cell. (d) Influence of dielectric layer (t3) of thickness on emissivity of proposed OTISE. (e) and (f) Incident angle dependence of proposed OTISE on emissivity of TE and TM polarizations, respectively.
The emissivity bandwidth in the non-atmospheric window can be extended by enhancing and merging the resonance response of multi-resonators compared with the single sub-cell structure, meanwhile, the multi-resonators can keep low emissivity in the atmospheric windows of 3-5 µm and 8-14 µm. In the practical application of IR camouflage materials, oblique incidence and insensitive polarization are often required. Figure. 3(e) and Fig. 3(f) present the relationship between emissivity with the incident angle of TE and TM polarizations. The emissivity in the non-atmospheric window does not change with the incident angle for the TE and TM polarizations. The resonant peak of surface plasmon mode occurs at the atmospheric windows of 3-5 µm when the incident angle reaches 35°. The simulated results in Fig. 3(e) and Fig. 3(f) demonstrate that the proposed OTISE processes good performance of IR camouflage with polarization-independent and wide-angle response.
3. Evaluating visible and IR camouflage performance
To verify the visible and IR camouflage performance of the designed OTISE, an OTISE sample was prepared on a silica substrate and the fabrication processes are as follows. First, we prepared a quartz substrate by heating it in Piranha solution (H2SO4:H2O2 = 3:1) to remove organic matter on the surface and washing clear up in deionized water. Next, the ZnO(bottom), Ag (bottom), ZnO, Ag (top) and ZnO (top) were deposited on the prepared substrate in turn, respectively. All layers were used magnetic sputtering (power 200 W) in a vacuum (pressure 8.7×10−4 Pa), and argon (flow speed 80 sccm) acts as protective gas. Then, a 1µm-thick layer of positive photoresist (AZ1500) was spin-coated (2200 rpm, 30 s) onto the sample, and multi-resonators were patterned on top of the layer structure by using laser direct writing lithography (power 30 mW, light intensity 55%, focal length -10%) to create an appropriate photoresist mask. Finally, we transferred the pattern to the top Ag and ZnO layer by using ion beam etching (power 200 W) and the resist mask was removed by soaking in acetone. The area of production is 2 cm*2 cm, and an SEM image of the part is shown in Fig. 4(a). The transmittance of the sample in the visible region (400-800 nm) was measured by using UV/VIS/NIR spectrometer (Lamda 1050, PerkinElmer).
Fig. 4. Visible performance. (a) Scanning electron microscope (SEM) of fabricated OTISE. (b) Measured visible transmittance of OTISE, Cr film and quartz substrate. (c) Visible image. (d), (e) and (f) Visible camouflage performance at three typical backgrounds.
Measured visible transmittance of OTISE as shown in Fig. 4(b), and the average measured visible transmittance was about 63% that lower than simulated about 14%, which attributes to the differences in permittivity between experiments and simulation and imperfection of the fabrication. Figure. 4(c) shows an image out of the window through the OTISE, and confirms that the OTISE has high transparency. In order to study the performance of the fabricated OTISE compared with traditional IR low emissivity material, a chrome-based (Cr) film with an average emissivity similar to OTISE was employed as the reference sample. A visible transmittance of OTISE, quartz and Cr film are shown in Fig. 4(b). Quartz has uniform transmittance which values about 92% and Cr film is opaque, both of them are not compatible with visible and infrared camouflage. The OTISE with optical camouflage coating underneath is helpful to prevent optical detection, and three typical backgrounds (sky, jungles and deserts) are selected to print on pictures to demonstrate optical camouflage performance as shown in Fig. 4(d)–(f). Compared with the original visible camouflage coating, the OTISE can preserve visible camouflage.
To evaluate the IR camouflage performance, we used FTIR (A513, Bruker) to measure reflectivity, and absorptivity can be obtained by subtracting reflectivity from the unit because the metal film of the OTISE results in zero transmittance in infrared as shown in Fig. 5(a) (green curve). Measured IR emissivity spectra of the OTISE are shown in Fig. 5(a), and the band emissivity in the atmospheric window of 3-5 µm and 8-14 µm are 0.24 and 0.09, respectively. In the non-atmospheric window, the band emissivity is 0.57. The band emissivity is 0.22 in the IR band 3-14 µm. There are two ways to evaluate IR camouflage, direct measurement of radiative temperature and calculation of emissivity energy. We assess both to demonstrate the IR camouflage performance. Measured emissivity spectra of the OTISE, Quartz and Cr film are shown in Fig. 5(a). Quartz has high absorption in the IR band and the band emissivity of Cr film is 0.17 in 3-14 µm. The OTISE, quartz and Cr film were placed on a graphite heat plate, and thermal images of two temperatures are shown in Fig. 5(b) and Fig. 5(c). The IR image was obtained from IR thermal camera (FTIR, T650sc, 8-14 µm). When the actual specimen temperature of the heat plate is maintained at 69.6 °C, the surface average temperature of quartz, Cr film and OTISE substrate are 65 °C, 34.1 °C and 29.1 °C, respectively. Under the condition of higher temperature, the tested surface average temperature of quartz, Cr film and OTISE are respectively 91.9 °C, 43.3 °C and 34.7 °C while the heat plate temperature rises to 98.4 °C. The radiative temperature of the OTISE is lower than quartz, and it is suggested that the proposed OTISE possesses outstanding IR camouflage performance compared with quartz. Meanwhile, the radiative temperature of the OTISE is significantly lower than that of Cr film, due to the radiation cooling in the undetected band and lower emittance in the detected band. In general, the proposed OTISE can reduce surface temperature by dissipating energy in the non-atmospheric window compared to low emissivity materials used in traditional IR camouflage. The reason why the emissivity in the atmospheric window of the OTISE is lower than others is that the ultrathin Ag film is used as the metal layer in the metal-dielectric-metal structure. Mostly, the OTISE possesses high optical transparency, and it is hoped to be used in visible-IR compatibility camouflage applications, such as aircraft canopies. Furthermore, the beneath coating can be observed when applying a visible camouflage coating and realizing arbitrary visible camouflage.
Fig. 5. IR camouflage performance. (a) IR emissivity of OTISE, Cr film and quartz substrate. (b) and (c) Thermal images of OTISE, Cr film and quartz substrate in 8-14 µm band. (d) Spectral irradiance of the OTISE compared to a blackbody and Cr film (T = 310 K). (e) Evaluation of total camouflage performance (TCP) for the proposed OTISE, reported multilayer structure [32], reported metamaterial [16] and Cr film.
To analyze the magnitude of IR signal reduction and energy dissipation quantitatively for IR camouflage, we calculated the radiation flux by integrating the spectral irradiance of the atmospheric window and the non-atmospheric window. The spectral irradiance of blackbody, OTISE and Cr film are shown in Fig. 5(d). Blackbody radiator represents idealized energy dissipation, while Cr film as traditional low emissivity material suppresses IR signature. Radiative flux was calculated by Eq. (3) [38]:
Here, $\int {d\varOmega = 2\pi \int_0^{{\raise0.7ex\hbox{$\pi $} \!\mathord{/ {\vphantom {\pi 2}}}\!\lower0.7ex\hbox{$2$}}} {d\theta \sin \theta } }$ is the angular integral over a hemisphere, A is the area, λ1, λ2 are the shortest and longest wavelength of the integrating band, ɛ(λ, T, θ) is spectral and angular emissivity. The measured spectral and angular emissivity of OTISE and Cr film are shown in Fig. 6(a) and Fig. 6(b). We considered room temperature (T = 310 K) and unit area. The radiative flux of blackbody in the atmosphere window of 3-5 µm and 8-14 µm are 8.39 W/m2 and 200.06 W/m2, respectively. The radiative flux of OTISE are 2.87 W/m2 and 18.67 W/m2, which are 34.2% and 9.3% of the blackbody’s radiative flux. These results indicate that the OTISE can reduce IR signature significantly in the detected band. It is important for the reduction of the lock-on envelope by considering the IR detector in previous research [11]. In order to study energy dissipation, we calculated the radiative flux of Cr film and OTISE in the undetected band of 5-8 µm. The radiative flux of OTISE is 34.15 W/m2, which is a 117% increase in dissipated energy compared with that of Cr film (15.71 W/m2). This means that the OTISE can dissipate part of the accumulated energy caused by the reduction of the IR signature through the undetected band compared with the conventional low emissivity material. Compared with previous work, the OTISE’s vertical direction radiative flux in the detected band of 3-5 µm (2.17 W/m2) and 8-14 µm (17.01 W/m2) decreased by 27.6% and 67.1% than that of metamaterial [16], and 12% and 68.9% than that of multilayer structure [32].
In order to consider IR signal reduction and energy dissipation synthetically, and evaluate the total performance of our designed OTISE, a factor called total camouflage performance (TCP) [19] is introduced and defined as Eq. (4):
4. Summary
In summary, we proposed a visible-infrared compatible camouflage structure composed of three Ag-ZnO-Ag disk sub-cells with an anti-reflection layer, which has high optical transparency and IR selective emission. Anti-reflection with appropriate thickness is selected to reduce visible reflection and increase transmittance by impedance matching. The combination of three Ag-ZnO-Ag disk sub-cells not only keeps a strong emission peak, but also possesses broadband absorption within the non-atmospheric window. To verify the performance of the designed OTISE, visible and IR cameras were used for qualitative analysis by capturing visible and thermal images. The visible image shows the OTISE has sufficient transmittance when taken the photo out of the window through the OTISE and covered it on visible camouflage coating. Thermal images indicate IR signature reduction of OTISE better than Cr film because of lower surface temperature with radiation cooling in the undetected band and lower band emittance in the detected band. In addition, quantitative analysis by using UV/VIS/NIR and FTIR spectrometer was conducted, and the average visible transmittance is 63% of the proposed OTISE, which achieved 34.2% and 9.3% of the IR signature of a blackbody in 3-5 µm and 8-14 µm, simultaneously, increasing the energy dissipation of a Cr film by 117% in the undetected band of 5-8 µm.
Funding
National Natural Science Foundation of China (61975210, U20A20217); National Key Research and Development Program of China (2021YFA1401000).
Acknowledgments
The authors gratefully acknowledge Bing Song and Jin Tang for their assistance in the sample fabrication.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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