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Abstract
Infrared (IR) stealth with thermal management is highly desirable in military applications and astronomy. However, developing selective IR emitters with properties suitable for IR stealth and thermal management is challenging. In this study, we present the theoretical framework for a selective emitter based on an inverse-designed metasurface for IR stealth with thermal management. The emitter comprises an inverse-designed gold grating, a Ge2Sb2Te5 (GST) dielectric layer, and a gold reflective layer. The hat-like function, which describes an ideal thermal selective emitter, is involved in the inverse design algorithm. The emitter exhibits high performance in IR stealth with thermal management, with the low emissivity (ɛ3–5 µm =0.17; ɛ8–14 µm =0.16) for dual-band atmospheric transmission windows and high emissivity (ɛ5–8 µm =0.85) for non-atmospheric windows. Moreover, the proposed selective emitter can realize tunable control of thermal radiation in the wavelength range of 3–14 µm by changing the crystallization fraction of GST. In addition, the polarization-insensitive structure supports strong selective emission at large angles (60°). Thus, the selective emitter has potential for IR stealth, thermal imaging, and mid-infrared multifunctional equipment.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Infrared (IR) stealth technology aims to reduce the IR signature of objects against IR detectors. The application scenarios of IR stealth include mid-infrared (MIR) photodetectors, thermal imaging systems, and heat-seeking missiles [1–5]. According to the Stefan-Boltzmann law, the radiated thermal energy per unit of a hot surface is related to the temperature of the surface and the infrared emissivity from the surface. Therefore, IR stealth with thermal management can effectively satisfy the requirements of reducing the surface temperature and realizing low IR emissivity. This strategy involves achieving low emissivity in dual-band atmospheric transmission windows (ATWs), which include the mid-wavelength infrared (MWIR; wavelength (λ) ≈ 3–5 µm) and long-wavelength infrared (LWIR; wavelength (λ) ≈ 8–14 µm), IR stealth, and high emissivity in the wavelength ranges of 5–8 µm for radiative cooling. In this case, the ideal thermal selective emitter is a “top-hat-like” function, which shows a unit value of emittance from 5 to 8 µm and zero in dual-band MIR ATWs [6].
Over the past decade, several thermal photonic structures with wavelength-selective abilities have attracted increasing attention. The proposed photonic structures are employed to realize several applications, including solar cells, radiative cooling, and IR stealth [7–12]. These thermal photonic structures are expected to find a wide range of applications, such as in industry, environment observation, and military. Particularly, in military applications, IR stealth plays an important role in avoiding IR signal tracking and identification. Pan et al. [13] designed and fabricated different photonic grating structures to implement MIR-compatible stealth. Zhu et al. [14] proposed a wavelength-selective emitter based on a multilayer metal structure for applications in high-temperature infrared stealth with efficient thermal management. Song et al. [15] used a metal (insulator) metasurface structure to realize multifunctional thermal stealth. Due to the limitations of traditional structural design methods, it is challenging to implement the top-hat-like function in selective emitters.
The innovative field of inverse design has recently been transforming conventional nanophotonics and discovering digital nanophotonics rather than engineering, primarily by manually selecting parameters or employing semi-analytical models [16,17]. Therefore, the design process of inverse design is different from that of traditional design methods [18]. In some problems, spectral characteristics are directly related to the application of ideal spectral curves, including but not limited to sunlight absorption, radiative cooling, nanophotonics devices, and multi-spectral camouflage problems [19–23]. Algorithms that improve the performance of nanophotonics and consider the material distribution for processing technology have emerged, such as direct binary search (DBS) algorithms [24,25], genetic algorithms [26,27], topology optimization [28–30], and neural network [31–33]. An inverse design that considers the “top-hat-like” function of spectral characteristics as the objective function is expected to achieve IR stealth with thermal management. In addition, tunable thermal emitters with spectral selectivity are required for multifunctional applications such as IR stealth [34] and thermal imaging [35].
Phase-change materials (PCMs) have attracted significant attention, owing to their tunability without changing the structural parameters. Ge2Sb2Te5 (GST) is a PCM widely used in optical storage media [36,37], tunable photonics computing [38], and infrared thermal emission [39]. In the infrared band, GST has a dielectric permittivity of ∼16.0 and ∼34.0 in its amorphous and crystalline states, respectively. Some studies have illustrated that the crystalline state of GST can be converted using methods such as heating, laser pulses, and applied voltage [40–42]. Once crystallization or reamorphization is accomplished, it can be maintained for many years at room temperature [43]. In addition, crystalline GST (c-GST) exhibits a high absorption in the MIR region, which can generate resonance at different wavelengths by changing the thickness of GST, whereas amorphous GST (a-GST) is transparent [44]. Based on these merits, GST is an ideal candidate for tuneable MIR multifunctional devices.
In this study, we demonstrate a PCM-based metasurface selective emitter (MSE) that combines the inverse design algorithm with IR stealth for the first time. The DBS algorithm was employed to topologically optimize the top single-layer nanostructured metallic pattern of the MIM metasurface. A GST dielectric layer was introduced to maximize the emittance of the 5–8 µm spectrum and implement the switching function. Similar to the conventional MIM structure, a layer of optically thick gold mirrors was used as the bottom layer. The simulation results reveal a low emissivity (ɛ3–5 µm =0.17; ɛ8–14 µm =0.16) in the dual-band MIR ATWs and a high emissivity (ɛ5–8 µm =0.85) in the non-window band, achieving a performance close to that of the ideal thermal selective emitter. According to the principle of thermal imaging, the proposed MSE can be disguised as an object at a temperature of 70 °C and a real temperature of 200 °C. Compared to other reported thermal stealth structures, this structure has good compatibility between IR stealth and thermal management. In addition, based on the phase-changing characteristics of GST, the proposed metasurface can realize continuous conversion of IR stealth to thermal imaging. The angular and polarization insensitivities of the metasurface were verified. Based on the above advantages, MSE has great potential for applications in IR stealth, thermal imaging, and multifunctional infrared detection.
2. Structure and model
2.1 Background and MIM structure for IR stealth
As shown in Fig. 1(a), a realistic atmospheric transmittance model plotted for Ref. [45] is exhibited as a light-blue shaded area at 3–14 µm. Owing to the low loss transmission in the wavelength ranges of 3–5 µm and 8–14 µm, dual-band MIR ATWs are used in IR detectors and IR imaging [46–48]. For the wavelength range of the non-atmospheric window, it is difficult to implement long distances. Therefore, a non-atmospheric window was adopted to dissipate heat without influencing IR stealth [49]. According to the theory of blackbody radiation, it is approximately 876 W/m2 at a temperature of 473 K in the wavelength range of 5–8 µm, as shown in Fig. 1(a) by the red solid curve. According to Kirchhoff's principle, for objects in a steady state, the absorption spectrum is equivalent to the emission spectrum. Consequently, an ideal “hat-like” function with unit absorption in the non-atmospheric window band and zero absorption in the dual-band MIR ATWs was considered for realizing IR stealth with thermal management. Owing to their superiority in processing and absorption performance, MIM structures are widely used in the design of perfect absorbers. Patterned metal gratings have been developed to excite different surface plasmon (SP) resonances and realize strong spectral absorption, such as strips, crosses, and rings [50–52]. Moreover, the MIM structure with PCMs can realize functional conversion and modulation of the working wavelength without changing the structural parameters [53,54].
Fig. 1. (a) Atmospheric transmittance (light-blue shaded area) with detected band (3−5 µm and 8−14 µm) and undetected band (5−8 µm) indicated and radiative heat flux at 473 K. (b) Top view of one unit cell of the design has been divided into 50 × 50 pixels. (The yellow pixel represents gold, and the purple pixel represents air, and the GST is exposed) (c) Schematic of the PCM-based MSE. The MSE is developed with inverse design gold grating coated on the GST and gold mirror. The periodic structural parameters are P = 1 µm. The thicknesses of inverse design gold grating (t1), GST dielectric layer (t2) and gold mirror (t3) are 40, 230 and 300 nm, respectively.
2.2 Algorithm optimization and simulation
The top periodic unit is divided into 50 × 50 pixels, as shown in Fig. 1(b), and each is a square of 20 nm × 20 nm, which can be fabricated by electron beam lithography followed by the Au lift-off process. The pixels have a binary state of dielectric properties, i.e., Au or air. To obtain a polarization-insensitive metasurface, we improved the inverse design to obtain an 8-fold symmetrical unit cell. The option of initial structure is discussed in the Fig. S1. And we verify that all-air structure is a appropriate solution to research the locally optimal structure. Therefore, the algorithm optimization process starts from an “all-air” structure comprising a bare thin-film GST and gold mirror. Figure 1(c) shows the schematic of the PCM-based MSE that comprises an inverse-designed gold grating, a GST dielectric layer and a gold reflective layer.
As shown in Fig. 2, we describe all steps for designing the metasurface using the DBS algorithm. In the dashed frame in Fig. 2, we show the top views of the first switching pattern, which chooses the first pixel as the starting point, and switches the pixel state of the first pixel and the final optimized top gold pattern of the proposed metasurface. We switched one pixel state (Au or air) and compared its performance in terms of the objective function with that of the unchanged structure to determine the state of the pixel. In an iteration, each pixel is scanned by a column to determine the state distribution of different pixels. This algorithm-based optimization method provides enhanced flexibility for generating complex and nonintuitive nanostructure features, which enables the designed metasurface to be closer to the objective function and surpass traditional resonant structural elements. Subsequently, a figure-of-merit (FOM) that can approximate the hat function of the ideal emitter is defined as follows:
Fig. 2. Designed schematic of the DBS algorithm, initial structure, and optimal metasurface. The steps of the DBS algorithm design include choosing pixel, switching the state of pixel, calculating FOM, judging the spectral characteristic according to the FOM, and outputting the optimal topological structure. The initial structure and the optimal metasurface are given in dashed frames. The initial point position and scanning order are indicated in the dashed frame initial structure diagram.
Compared with some other optimization methods, DBS method has the characteristics of orientation, simplicity, and scalability. We also make a comparison of computational cost between the DBS algorithm and genetic algorithm (GA). In the Fig. S4, the comparison illustrate that the DBS method has the advantages of strong orientation and low computational cost. Due to the strict orientation, this makes it challenging to explore the global optimum. There are also some studies to strengthen the DBS method by improving on the starting structure and the scan path.
The finite-difference time domain (FDTD) is a method of approximately solving Maxwell's equations, and has been used for numerical simulations. The commercial software Lumerical FDTD Solutions was employed in a simulation where IR plane waves propagated to the metasurface along the z-direction. Both the boundary conditions in the x- and y-directions are periodic layers. The upper and lower boundary conditions of the z-direction are perfectly matched layers and metal layers, respectively.
The spectral transmittance T(λ) and reflectance R(λ) of the proposed metasurface were monitored during the simulation. The absorptivity A(λ) was calculated as follows: A(λ) 1 − T(λ) − R(λ). According to Kirchhoff’s law, absorptivity A(λ) equals emissivity ɛ(λ). Consequently, the emissivity spectrum ɛ(λ) of the proposed metasurface was obtained from the simulation results. Combining the FDTD methods and DBS algorithm, it takes about 70 hours (6 iteration) to get the optimized metasurface on a computer with a 6-core central processing unit (Intel Core i7-9750H). The total memory of the computer is 32 GB (SAMSUNG DDR4 2666 MHz).
2.3 Material optical characterization
As shown in Fig. 3, the dielectric constants of the GST in the crystalline and amorphous states are plotted for Ref. [55,56]. In the IR band, amorphous GST is a low-loss material, whereas crystalline GST is a lossy material, which allows it to achieve intrinsic absorption in the MIR spectrum with changes in thickness [39]. In addition, GST materials have several intermediate phases that can be controlled by various methods such as heating time, laser irradiation, and gating voltage. The effective dielectric constants of GST with different degrees of crystallinity were calculated using the Lorentz–Lorenz relation [57–59]:
3. Results and discussion
3.1 Simulation result and physical mechanism
First, we consider the case in which IR plane waves are normally illuminated on an MSE, as illustrated in Fig. 1(c). Figure 4(a) shows the emissivity of the proposed PCM-based MSE in the IR band. Owing to the intrinsic absorption of GST and the coupling mode of the patterned gold grating, the absorption of the proposed structure was 85.3% in the wavelength range of 5–8 µm. Low emissivity is achieved in the dual-band atmospheric windows (ɛ3–5 µm =0.17; ɛ8–14 µm =0.16). The IR stealth with thermal management of the proposed inverse-designed metasurface was fully verified by comparison with a dual-layer stacked structure (GST thin layer and gold optical layer). The blue line in Fig. 4(a) shows the emission spectrum of the stacked structure without the patterned gold grating. The ideal hat-like function, which is compatible with IR stealth with thermal management, is shown in Fig. 4(a) by the red dashed line. It is evident that the emissivity spectrum of the metasurface is closer to the ideal hat-like function than that of the bare GST thin film and gold mirror.
Fig. 4. (a) Simulated spectral emissivity at normal incidence and spectral emissivity of ideal thermal selective emitter. (b) Electric field distribution in the indicated xy- and xz- cross sections at the two peak wavelengths.
To explain the physical mechanism of the high average emissivity in the wavelength range of 5–8 µm, we provide the electric field distribution of the two resonance peaks of the proposed metasurface, as shown in Fig. 4(b). The emission peak at 5.5 µm wavelength is mainly due to the SP resonance between the patterned gold gratings. The other emission peak at 7.0 µm wavelength is realized by the coupling between the SP resonance and Fabry-Pérot cavity of the c-GST and Au thin film [39]. It is worth noting that the high emissivity of wavelengths ranging from 5 to 8 µm has no effect on thermal imaging because of high atmospheric absorption. Therefore, this part of the energy was quickly dissipated through the high emissivity of the metasurface. For a 473 K object, the radiation power of this part of the spectrum was as high as 876 W/m2.
3.2 Performance of IR stealth with thermal management
Most natural objects have a blackbody-like radiation spectrum. For the IR detector, the temperature of the object is determined by the detected radiation power [13,14]:
where Tr represents the temperature obtained using the inverse function of the blackbody radiation spectrum, P represents the radiation power detected by the IR detector, which corresponds to the radiation power radiated by the black body at temperature T. ɛIR is the emissivity of the IR detector in the working wavelength (7.5–14 µm), and ɛIR = 1 for the infrared camera. The detection power of an infrared camera is generally determined by two factors: the black-body radiation power and the environmental reflection power of the object [13–15] asAccording to the emissivity of the MSE, we plotted the detection power under an infrared camera working in the wavelength range of 7.5–14 µm. Figure 5(a) shows that when our structure is at 200 °C, the observation power under the infrared camera is 406 W/m2, which corresponds to the black body radiation power at 70 °C. Therefore, our metasurface is expected to be disguised as an object at a temperature of 70–200 °C. The measured radiation temperatures (Tr) of the objects are significantly lower than their real temperatures (T), and the radiation temperatures increase with increasing real temperatures. In addition, we provide the radiative heat flux curve of the MSE at 473 K. Compared with an ideal blackbody at 473 K, the proposed metasurface has obvious wavelength-selective emission, which has a high heat dissipation in the wavelength range of 5–8 µm, as shown in Fig. 5(b).
Fig. 5. (a) The relation between radiation temperatures and real temperatures of the MSE: the lines are calculated with the integrated power. (b) The spectral radiation intensity of the blackbody and MSE at 473 K, the black and red lines represent the blackbody and the MSE, respectively.
To evaluate the performance of our designed metasurface structure, we defined the efficiency of the thermal stealth compatible with thermal management. The performance of the MSE should be determined in two parts: IR stealth, including dual-band ATWs, and thermal management. Therefore, we define the efficiency as the product of the dual-band MIR efficiency and the out-of-band efficiency.
Here, $eff_{MWIR}^{in}$, $ef{f^{out}}$, and $eff_{LWIR}^{in}$ represent dual-band MIR and out-of-band efficiencies, respectively, and ɛ(λ) is the spectral emissivity of the optimized emitter. The emissivity in the dual-band MIR ATWs reflects the stealth performance of the proposed structure under infrared imaging. Out-of-band emissivity determines the thermal management performance of the proposed selective emitter, which is related to radiative cooling.
We also provide a comparison of these results with other works [13,14,49,61], as shown in Table 1. It can be seen that the designed structural performance has certain advantages, and it has good compatibility with the IR stealth and thermal management performance. For the FOM evaluation, we used equal weights for normalized α1, α2, and α3(α1= 0.5, α2= α3= 0.25). The definition of compatible efficiency (eff), which is another evaluation standard for the compatibility between IR stealth and thermal management inspired by a previous study [19], is given by Eqs. (5) and (6), respectively. We also compare the designed metasurface with the initial structure (dual-layer film). The comparison shows that the optimized metasurface significantly improves the performance of infrared stealth with thermal management.
Table 1. Performance of the selective emitters for IR stealth with thermal management under two types of evaluations
3.3 Switchable thermal management and thermal imaging
In the previous study, a dual-layer PCM-based structure has been fabricated and realized the tunable emissivity in the LWIR [62]. Notably, the PCM-based structure is expected to achieve functional transformation without changing the structural parameters [63]. Inspired by these, we discuss the tunable properties of the designed metasurface in the MWIR. This will enable the designed selective emitter to achieve a light-dark-like transition that can transform IR stealth into a thermal imaging configuration that has the potential to be used in the aerospace and military fields. Figure 6 shows the emissivity spectral characteristics under different crystalline fraction parameters. According to the Eq. (2), we obtained the dielectric permittivity of GST under different crystallization fractions. With an increase in the crystalline fraction, the selective emission spectrum exhibited a red shift, which was related to the resonance of the MIM structure and the properties of the GST material. We also provide a color map of the metasurface with MWIR ATWs (the integral transmittance is 67% in the wavelength range of 3–5 µm) as the background. Owing to the IR stealth performance of the MSE, it is difficult to distinguish the environment from the proposed MSE with c-GST. As the crystalline fraction decreases, the proposed PCM-based MSE applied to the IR stealth transforms into a thermal imaging structure. Furthermore, the PCM-based MSE can achieve tunable absorption for the range of approximately 2 µm in the wavelength range of 3–8 µm, including selective emission of 3–5 µm, which has potential applications in the field of thermal imaging [38], encryption with IR color [64], and adaptive thermal management [5,65]. In addition, because the phase change material GST can achieve rapid switching within the nanosecond/picosecond range [66,67], the proposed MSE can achieve rapid functional switching.
Fig. 6. Simulated spectral emissivity of the plasmonic thermal emitter gradually changes as crystallization fraction of GST increases from 0% to 100%. The colormaps of PCM-based MSE with different crystallization fractions comparing the MWIR ATWs (the integral transmittance is 67% in the wavelength range of 3–5 µm) have been marked in each emissivity spectrum.
3.4 Dependence of the IR stealth performance with polarization and incident angle
It is necessary for the emitters to support a wide range of angles so that light waves entering from different directions can be well emitted by the nanostructure [68–70]. In order to obtain a polarization-insensitive MSE configuration, an 8-fold symmetrical unit cell was designed using the improved DBS algorithm. The emissivity values for different polarization angles are shown in Fig. 7(a). Because all polarized light waves can be decomposed into the sum of the x-direction and y-direction light waves that respond to a symmetrical structure, the proposed metasurface exhibits perfect polarization insensitivity. In addition, we discuss the incident-angle insensitivity of the designed metasurface. As shown in Fig. 7(b), when large-angle light (60°) is incident, the designed metasurface can still maintain good performance.
Fig. 7. (a) Emissivity of the MSE in the range of 3–14 µm, and (b) MSE is insensitive to the angle and polarization.
4. Conclusion
In conclusion, we demonstrated a PCM-based MSE for IR stealth using thermal management. The MSE is based on an MIM structure, which includes an inverse-designed gold grating, GST dielectric layer, and gold mirror. Simulation results show that low emissivity in the dual-band MIR ATWs and high emissivity in the non-window wavelength range of 5–8 µm were achieved. By combining inverse design and ideal thermal selective emitter theory, the proposed selective emitter exhibits better performance than traditional IR stealth with thermal management. Owing to the introduction of PCM materials, the proposed metasurface can realize tunable control of thermal radiation in the wavelength range of 3–14 µm. In addition, the proposed selective emitter has good insensitivity to polarization and supports large-angle incidence (60°), which is crucial to IR stealth equipment. Our study provides an approach and a reference for inverse-design IR stealth with thermal management.
Funding
National Natural Science Foundation of China (60907003, 61805278); Foundation of NUDT (JC13-02-13, ZK17-03-01); Natural Science Foundation of Hunan Province (13JJ3001); Program for New Century Excellent Talents in University (NCET-12-0142).
Acknowledgements
The authors gratefully thank Yifei Xiao for her help with the schematics of the configurations, thank Jie Huang and Zheng Peng for their useful advice.
Disclosures
The authors declare that there are 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.
Supplemental document
See Supplement 1 for supporting content.
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