Metamaterial-Selective Emitter for Maximizing Infrared Camouflage

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Metamaterial Selective Emitter for Maximizing Infrared Camouflage Performance with Energy Dissipation Namkyu Lee, Taehwan Kim, Joon-Soo Lim, Injoong Chang, and Hyung Hee Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04478 • Publication Date (Web): 16 May 2019 Downloaded from http://pubs.acs.org on May 18, 2019

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ACS Applied Materials & Interfaces

Metamaterial Selective Emitter for Maximizing Infrared Camouflage Performance with Energy Dissipation

Namkyu Lee1, Taehwan Kim2, Joon-Soo Lim1, Injoong Chang1, Hyung Hee Cho1* 1. Department of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea 2. Semiconductor R&D Center, Samsung Electronics Inc, Hwaseong, Korea * Corresponding author Tel.: +82 2 2123 2828 Fax: +82 2 312 2159 E-mail: [email protected]

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Abstract Camouflage is a method evading predators in nature by assimilating into the environment. To realize an artificial camouflage surface for displays and sensors, many researchers have introduced several concepts including a metamaterial selective absorber/emitter (MSAE). When MSAE is adopted for camouflage at infrared wave (IR), the energy dissipation of reduced emitting energy, as well as the reduction of emitting energy to deceive the IR signature from the surface, must be considered from the viewpoint of energy balance due to thermal instability. The integrated investigation of radiative heat transfer characteristics and IR signature control of MSAE remains, however, poorly understood. Here, we investigate MSAE for IR camouflage by considering the energy balance in terms of reduction of emitting energy and dissipation of reduced emitting energy. Based on the atmospheric transmittance at an IR band, we designate the detected band as having wavelengths of 3-5 μm and 8-14 μm, and the undetected band as having a wavelength of 5-8 μm. We investigate, via experiments and simulations, the optical characteristics required for IR camouflage and extract the factor that controls the emissive power. Furthermore, we suggest an integrated factor for evaluating the camouflage performance based on concept of energy balance, and propose design guideline for MSAE with the aim of maximizing the camouflage performance at the IR band. This study will help to expand the range of applications (such as energy harvester and sensors) and others that are based on selective absorption/emission.

Keywords: Infrared camouflage, Metamaterials, Selective emitter, Thermal instability, Energy balance,

Optimization


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1. Introduction Camouflage is a method for assimilating into the environment in response to a detected wavelength (which may correspond to visible, infrared, microwave and acoustic waves) to reduce the threat from predators in nature (e.g. chameleons and cephalopods adjust their color in the visible regime)1. Many researchers have tried to realize the artificial camouflage surfaces (similar to this natural camouflage) for displays1, sensors2, color changes3,4 and camouflage coating5. Recently, several researchers have focused on infrared wave (IR) camouflage for energy harvesting6,7, radiative cooling8-10 and thermal cloaking11. For IR camouflage12-16, you should understand the IR signature from the surface based on the Stefan-Boltzmann equation17, which is a function of the temperature and emissivity. Temperature control of IR camouflage represents a direct means of achieving IR signature control, but the realization of this control is difficult, owing to the additional cooling and heating devices required18. Emissivity control is easier than temperature control. When you attach an emissivity control material to a target surface (such as metal powders19, metal arrays20 (ε ≈ 0.1), which controls the emissivity in the entire IR band because the surface coated with military paint has high emissivity (ε ≈ 0.8-0.9) not to hide, you can achieve the IR signature control for camouflage. However, since the emissivity is an intrinsic property, tuning of the spectral emissivity as you wanted is difficult. This is especially true when you reduce the IR signature of the target at the IR band, where the reduced emitting energy yields a temperature increase in the material and, consequently, a thermal instability due to the accumulated energy17,21. Therefore, for IR camouflage material, you should consider the IR camouflage requirements with appropriate optical characteristics, which reduces the emitting energy and dissipates the reduced emitting energy through specified wavelength, simultaneously. Metamaterials represent a suitable candidate for satisfying the requirements of an IR camouflage material. Metamaterials is the artificial materials for realizing the designed optical behaviors by utilizing the interactive behaviors of the unit structures such as the plasmonics. Through the metamaterials, we can achieve the eccentric properties, owing to tunable optical properties, such as a negative refractive index22-24, high refractive index25, near zero permittivity26, photonic crystal27 and polarization control28. In addition, other researchers expanded the research field toward thermal field29-31. Among these metamaterials, a metamaterial selective absorber/emitter (MSAE) based on a metal-dielectric-metal (MDM) structure32-35 is appropriate for IR camouflage due to its optical characteristics, i.e., low emission except for the resonant wavelength occurring during high emission36,37. For this reason, MSAE are



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widely used for thermophotovoltaic38, radiative cooling9, sensors39-41, and other applications42,43. Several attempts have also been made to apply for the optical characteristics of MSAE to IR camouflage by controlling the emissivity44,45, however, despite to the understanding of the radiative behavior represented by changing the emissive energy of MSAE on the thermal instability46, the investigation of this behavior has stymied the used of MSAE for IR camouflage material. Here, we investigate MSAE for IR camouflage considering energy balance in terms of reduction of emitting energy and dissipation of reduced emitting energy. Considering the atmospheric transmittance and IR detector, we designate a target IR band consisting of a detected band of 3-5 and 8-14 μm and undetected band of 5-8 μm47. The experimental and simulation results revealed that the optical characteristics of MSAE depend on metal disk diameter and pitch of MSAE, which is the main parameters regarding the localized and delocalized plasmonic behavior, and demonstrated the IR camouflage using an IR camera of 8-14 μm. Additionally, we deduce the control factor for changing the radiative characteristics and determine the designed factor considering the detected and undetected bands. We then suggest an integrated factor for evaluating the camouflage performance as a concept of energy balance that includes the reduced emitting energy, dissipated energy and temperature variation. From the integrated factor, we confirm that MSAE exhibits superior performance to existing materials such as an Au surface and a blackbody. Furthermore, the designed factor of MSAE should yield the optimal IR camouflage performance. The results of this study will help to expand the range of applications including energy harvesters and sensors that are based on selective absorption/emission.


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2. Materials and Methods 2.1. Background and MSAE for IR camouflage Figure 1a shows the atmospheric transmittance as light-blue shaded area at 3-14 μm used as the IR detection band48. As shown in Figure 1a, the transmitted band occurring at wavelengths of 3-5 and 8-14 μm and non-transmitted band occurring at 5-8 μm correspond to the atmospheric transmittivity. For this reason, the bands of the IR detector are divided into 3-5 μm and 8-14 μm regions for measuring high temperature and observing ambient temperature, respectively. This indicates that if you want to gather the IR camouflage against the detector, you should decrease the emissive energy of the target using the 3-5 μm and 8-14 μm bands as a type of detector. However, when you reduce the IR signature from the target, the emissive energy from the target decreases, leading an increase in the temperature due to the accumulated energy17. The increase in temperature leads to thermal instability of the mechanical system and a reduction in the thermal reliability of target. Thus, to prevent this instability, you should ensure simultaneous reduction of the emissive energy from the target for IR camouflage and dissipation of the reduced emitting energy of 3-5 μm and 8-14 μm through the undetected (e.g. 5~8 μm) band. Considering the IR camouflage requirements which are the reduction of IR signature at the detected band and the dissipation of reduced emitting energy through the undetected band, MSAE is an attractive metamaterial for meeting these requirements. This results from the fact that MSAE is capable of selective absorption/emission, i.e., the absorption is the same as emission based on Kirchhoff’s law, at the specified wavelength and the emissivity at the other band remains low17. For this reason, we adopt MSAE as an IR camouflage surface (see Figure 1b). Figures 1b and c show a unit cell of MSAE as a metal-dielectric-metal (MDM) structure, i.e., a metamaterial structure36,49 and the fabricated surface (metal disc diameter: 1.9 μm, pitch: 4.0 μm). The metal ground acts as the conducting part which induces the current and perfect reflector with no transmittance. The dielectric layer induces the electromagnetic behavior through the separation of the metal ground and disc, which generate the induced current through an incident electromagnetic wave. The size of the metal disc has a significant effect on the resonance wavelength36. In this study, we choose Au as a metal ground, owing to its stable optical properties, having the thickness (t3) of 200 nm which is sufficient for optical properties without transmittance39. ZnS, which is a transparent material for bands of 3-14 μm50, is selected as the dielectric layer with a thickness (t2) of 200 nm. The metal disc, with a thickness (t1) of 200 nm, is composed of the same material as the metal ground (Au).



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2.2. Fabrication For realization of MSAE, firstly, a (100) silicon (Si) wafer, which is widely used as the substrate of fabricating micro-nano structures, is subjected to a 50-s cleaning process at 100 W, using a microwave plasma asher (Alphaplasma, AL-18, Germany). Afterward, Au is deposited (thickness: 200 nm, which is thick enough for the perfect conductor and reflector by considering the mean free path of Au51) on the Si substrate using an E-beam evaporator (ULVAC). The dielectric material i.e., ZnS, is added (thickness: 200 nm) using an E-beam evaporator (ULTEC). A positive photoresist (GXR-601), which is softened by the UV irradiation, is then coated at 3000 rpm for 30 s onto the specimen with the deposited Au and ZnS films. A pattern is obtained using MA6 (KarlSuss) as a mask aligner, which generates the UV beam on the coated surface of positive photoresist, with a photomask. After developing the photoresist within 30 s using an AZ MIF 300 developer, an Au thin film with a thin adhesion layer (Ti) is deposited onto the specimen (thickness: 200 nm) using an E-beam evaporator (ULVAC, ei-5k). Subsequently, the lift-off process is conducted in acetone and the fabricated surface is realized (see Figure 1c).

2.3. Simulation The simulation is conducted with a commercial code of COMSOL Multiphysics 5.2a using a finite element method with a wave optics module52. The unit cell of MSAE (see Figure 1b) is modeled, except for the substrate, because the metal ground is a perfect reflector. For the simulation of the electromagnetic behavior, the excitation electromagnetic wave propagates along the -z axis as a plane wave. The optical properties of Au are extracted from the Drude-Lorentz dispersion model53, which describes the collective behavior of electrons applied by the electromagnetic field in the metal, where a plasma frequency and damping constant of 2.06 PHz and 13.34 THz (based on the literature), respectively, are employed39. The optical properties of ZnS, referred to as the complex number of refractive indexes, are taken as a reference54. The maximum element size of free space is set as 300 nm, based on the mesh independence test. From the simulation results, we obtain the S-parameters S11 (ratio between electric field of incident wave and reflected wave) and S21 (ratio between electric field of incident wave and transmitted wave)55. Using these S-parameters, we convert the reflectivity (R) of |𝑆11|2 and transmittivity (T) of

|𝑆21|2. Based on the energy balance17 we calculate the absorptivity (A) as follows: A = 1 – R – T = 1 - |𝑆11|2 -


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|𝑆21|2 55. According to Kirchhoff’s law, the absorptivity is the same as the emissivity17. We can therefore determine the emissivity from the S-parameters of the simulation results.

2.4. Optical Characterization The spectral emissivity of the fabricated surface is measured by via Fourier transform infrared spectroscopy (FTIR, Bruker, spectral range: 2.5-25 μm) with a KBr beam splitter based on the Michelson interferometer56. Using the emission accessory with temperature adjustment (Bruker A540), we measure the emissive power from the sample. We compare the measured emissive power of sample with that comparison of reference blackbody source (SR-200, CI systems, Israel). The spectral emissivity of a 1.3 cm x 1.3 cm specimen is calculated from 𝑆𝑠𝑎𝑚𝑝𝑙𝑒(𝜆,𝑇) 𝑆 𝑏𝑙𝑎𝑐𝑘𝑏𝑜𝑑𝑦(𝜆,𝑇) = 𝜀𝑠𝑎𝑚𝑝𝑙𝑒(𝜆,𝑇)/𝜀𝑏𝑙𝑎𝑐𝑘𝑏𝑜𝑑𝑦(𝜆,𝑇), where S is the magnitude of the signal.

2.5. Thermographic measurements Infrared wave (IR) camouflage is demonstrated by capturing time-dependent IR images on the heating plate. The specimen is place on the heating plate (MSH-20D, WiseStir, Korea) to replicate the heating condition from the external condition such as viscous heating on the aircraft. To reduce the additional radiative energy from the heating plate, we cover the aluminum foil around the plate. The brass hollow plate is placed on the MSAE specimen. Above the heating plate, the IR camera (FLIR A655sc, 8-14 μm) is positioned and we measure the thermal image using an IR camera (FLIR A655sc, 8-14 μm). As the temperature of the heating plate increases, we capture intermittent IR images with 360 s.



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3. Results and Discussion 3.1. Optical Characteristics of MSAE for IR Camouflage Figure 2a shows the spectral emissivity resulted from the experiment, which is measured by the FT-IR (Fourier Transform InfraRed spectroscopy), and simulation where the diameter is varied for a fixed pitch of 4.0 μm. As shown in Figure 2a, we achieve selective absorption/emission phenomena at the specified wavelength using MSAE. For IR camouflage, as previously mentioned, a camouflage surface with (i) low emissivity of the detected band (3-5 μm, 814 μm) for IR signature reduction and (ii) high emissivity of the undetected band (5-8 μm) for dissipation of the reduced emitting energy at the detected band are desired. The results show that the optical behaviors of MSAE satisfy the requirements of the IR camouflage surface indicating that MSAE represents a suitable surface for IR camouflage. Additionally, the spectral emissivity (see Figure 2a) extracted from the simulation results corresponds closely to the experimental data. Maximum values of spectral emissivity value of 0.991 at 7.82 μm and 0.992 at 8.61 μm (simulation results) and 0.952 at 7.95 μm and 0.951 at 8.55 μm (experimental data) are obtained for metal disc diameters of 1.7 and 1.9 μm, respectively. In addition, several peaks (e.g., blue line at 3.02 μm and green line at 3.24 μm) can be estimated by comparing the simulation and experimental data. Although the difference between the experimental and simulation results increases at the smallest diameter of 0.9 μm because of the difficulty of fabrication via photolithography, we judge the study of optical behavior characterizing MSAE for IR camouflage through simulations (where the diameter is varied) reliable. Based on the experimental and simulation data, we demonstrate the IR camouflage surface through an IR camera with a band wavelength of 8-14 μm. We choose the case of d = 0.9 μm and 1.9 μm for evaluation of the camouflage, as these diameters reveal the effect of different spectral emissivity at 8-14 μm on the camouflage. Figure 2b shows the images obtained in the visible and IR regime. The two specimens with d = 0.9 μm and 1.9 μm are placed on the heating plate (MSH-20D, WiseStir, Korea) to increase the temperature of the MSAE. In the visible regime, d = 0.9 μm and 1.9 μm are indistinguishable because the size of the MSAE structure can only scatter the incident EM wave in the visible band. However, as the increase in temperature on the heating surface from t = 0 sec to t = 360 sec, the IR camouflage associated with d = 0.9 μm having a low spectral emissivity at 8-14 μm appears contrary to that associated with d = 1.9 μm having high spectral emissivity. This indicates that the reduced near-zero IR signature from MSAE with d = 0.9 μm owing to the low spectral emissivity deceives the IR detector for the heated plate.


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We examine the relation between the resonance wavelength and the geometrical factors such as the metal disc diameter and pitch, which is related to the analysis of the localized (diameter) and delocalized behavior (pitch) regarding the dependence for the resonant wavelength, by performing simulations where the metal disc diameter (d = 0.5-2.0 μm) is varied for a fixed pitch (p = 4.0 μm), and the pitch (p = 3.0-4.5 μm) is varied for a fixed diameter (d = 1.25 μm). Figures 3a and b show the optical behavior associated with the diameter and pitch dependence and two peaks (R1 and R2) characterizing this behavior are observed. R1 is dependent on the pitch but is independent of the diameter, whereas, R2 is dependent on the diameter, but is independent of the pitch. These optical behaviors are closely correlated with the localized and delocalized plasmon behavior57. The delocalized plasmon behavior usually occurs around MDM structure and relates to the relation between the external electromagnetic field and material’s property. The localized plasmon behavior is usually indicated by the confinement of the electromagnetic field in the dielectric material or medium. It is generated by the induced current caused by the external electromagnetic field. The resonant wavelength is caused by the interaction between the induced current and external electromagnetic field because the resonant behavior occurs at the n-fold’s frequency (related to wavelength) between the induced current in the metal disc and external electromagnetic field. Based on these characteristics, although the plasmonic behavior is only partly explained by the change in geometrical parameters, the delocalized plasmon behavior (such as surface plasmonic resonance) is dependent on the pitch. In contrast, the localized plasmon behavior (such as magnetic resonance) is dependent on the diameter57. This suggests that the resonance of R1 and the resonance of R2 are correlated with the surface plasmonic effect and the magnetic resonance, respectively. To determine the magnetic resonance at R2, we extract the normalized magnetic field at the intersection of the y-z plane in Figure 2d. As shown in Figure 3b, the magnetic resonance occurs at 6.02 μm (owing to confinement in the dielectric material). This indicates that, as expected, R2 exhibits localized plasmon behavior and the diameter of the metal disc plays a key role in achieving IR camouflage via selective absorption and emission of MSAE. Based on the optical behavior of MSAE, we connect the spectral emissivity of MSAE with the IR camouflage considering the energy balance. As previously mentioned, the IR camouflage with low emissivity of the detected band for signal reduction and prevention of thermal instability via high emissivity of the undetected band for dissipation of reduced emitting energy are desired. We achieve these by controlling the resonance wavelength of R2 through the undetected band (5-8 μm). Figure 2f shows the dependence of the resonant wavelength (R2) on the metal



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disc diameter. As shown in the Figure 3c, the wavelength changes linearly with the diameter, except for a wavelength of around 4 μm, which may correspond to an interaction between delocalized and localized plasmon behavior. Based on the results shown in Figure 3d, we set the target of the metal disc diameter for matching the resonant wavelength with an undetected band. The shortest wavelength of undetected band (i.e., 5 μm) and the longest wavelength (i.e., 8 μm) are associated with diameter values of 0.97 μm and 1.75 μm, respectively. Thus, for IR camouflage, the target diameter of MSAE ranges from 0.97 μm to 1.75 μm.


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3.2. Analysis of Radiative Energy from MSAE Surface Based on Planck’s law, we calculate the spectral blackbody emissive power representing the radiative energy from the surface as follows17: 𝐸𝑏𝜆(𝜆,𝑇) =

2𝜋ℎ𝑐20

[

𝜆5 exp [

ℎ𝑐0 𝑘 𝜆𝑇

― 1]

]

(1)

where Ebλ, λ, h, c0, k and T are the spectral blackbody emissive power, the wavelength, Planck’s constant, the speed of light in a vacuum, Boltzmann constant and the temperature, respectively. In this study, to evaluate separately the dissipation and reduction emissive energy of the surface considering the band, we discretize the equation as a function of wavelength as follows: 𝐸𝜆1 ― 𝜆2(𝑇) =

∫

𝜆2

𝜀(𝜆,𝑇) 𝐸𝑏𝜆(𝜆,𝑇) 𝑑𝜆≅

𝜆1

∑𝜀(𝜆,𝑇) 𝐸

𝑏𝜆(𝜆,𝑇)∆𝜆

(2)

where λ1, λ2 and ε are the shortest wavelength of the band, the longest wavelength of the band and spectral emissivity, respectively. Using this equation, we evaluate the emissive energy of each band. To analyze the radiative behavior of MSAE for IR camouflage, we should evaluate the emissive power of detected and undetected band, respectively. Figure 4a shows the emissive power through the detected and undetected band from the surface (temperature: 450 K). The emissive power is calculated from Eq. (2). As shown in Figure 4a, the emissive power varies with the metal disc diameter. This is especially true for diameters ranging from 0.97 μm to 1.75 μm, where the emissive power through the undetected band is concentrated with low emissive power through the detected band. This shows that MSAE can suppress the emissive power, which is proportional to the IR signal on the IR detector, through the detected band (3-5 μm, 8-14 μm) and dissipate the reduced emitting energy through the undetected band (5-8 μm). Interestingly, there is a range having concentrated emissive power through undetected band maintaining the low emissive power through detected band. This indicates that, during the design of MSAE for IR camouflage, an optimal metal disc diameter that yields maximum IR camouflage performance can be expected. Furthermore, we should consider the surface temperature variation because the emissive power peak of blackbody has changed. Figure 4b shows the temperature dependence of the emissive power for wavelengths covering 5-8 μm. In accordance with the Stefan-Boltzmann law: 𝐸 = 𝜀𝜎𝑇4 (where ε, σ and T are emissivity, Boltzmann constant and surface temperature, respectively), the emissive power increases with the temperature. The change in surface



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temperature affects the distribution of this power. As Figure 4b, for surface temperature 300 K and 600 K, the maximum emissive power occurs at 1.6 μm and 1.47 μm, respectively. The blackbody emissive power distribution and peak value obey Planck’s and Wien’s displacement law and, hence, we should consider the temperature effect, which affects changes in the emissive power through the undetected and detected band. For this reason, in the evaluation of the IR camouflage, we simultaneously consider the factors such as the radiative energy and temperature.


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3.3. Evaluation of Camouflage Performance of MSAE To integrate the effect of IR signature reduction and dissipation of reduced emitting power through detected and undetected band respectively with temperature effect, we suggest a factor of camouflage performance to evaluate the magnitude of IR camouflage including thermal dissipation. In the first step of this evaluation, we determine the emissive power ratio of the designed surface and the reference surface as follows: 𝜙𝜆1 ― 𝜆2 =

𝐸𝜆1 ― 𝜆2, 𝑑𝑒𝑠𝑖𝑔𝑛𝑒𝑑 𝑠𝑢𝑟𝑓𝑎𝑐𝑒

𝐸𝜆1 ― 𝜆2, 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒

(3)

This ratio (ϕ) corresponds to the emissive power ratio at each band from λ1 to λ2 between designed and reference surface. We think that the camouflage performance should be considered with respect to an existing surface owing to recognition of signature control and consideration of energy dissipation effect. Therefore, when we deduce the IR camouflage performance, we should compare the magnitude of reduction and dissipation energy of the designed surface with that of the reference surface. In addition, when we assume that the spectral emissivity is independent of the temperature49, the emissive power can only be related to the temperature via the blackbody emissive power described by Planck’s law as E = σεT4. The method for determining the energy ratio between surfaces is therefore reasonable for treatment of the radiative energy through each band. Since the IR detector usually has a single detected band (3-5 μm or 8-14 μm)47, we compare η of the reduction of emitting energy through a single detected band with η of the dissipation energy through a single undetected band. The camouflage performance at a single detected band is then determined from: Camouflage performance of single band (CP) = 𝜙𝑢𝑛𝑑𝑒𝑡𝑒𝑐𝑡𝑒𝑑 𝑏𝑎𝑛𝑑 𝜙𝑑𝑒𝑡𝑒𝑐𝑡𝑒𝑑 𝑏𝑎𝑛𝑑

(4)

This corresponds to the dissipation energy ratio over the reduction energy ratio of each band. Based on Eqs. (3) and (4), when we reduce the emissive energy of the detected band, the camouflage performance increases due to the reduction in the IR signature. Similarly, when we increase the dissipation energy of the undetected band, the performance increases, owing to a reduction in the thermal instability and the same signature level through the detected band. Equation (4) indicates that the maximum camouflage performance can be achieved by the designed surface having with both maximum reduction and maximum dissipation of emissive power. Therefore, we think that the integrated factor of camouflage can be represented by the camouflage performance considering thermal dissipation as well as reduction of IR signature. Furthermore, if we consider the double-undetected band (λ1-λ2, λ3λ4) simultaneously, we can determine the total camouflage performance as follows:



Total camouflage performance (TCP) = 𝐶𝑃 (𝜆1 ― 𝜆2) ∙ 𝐶𝑃 (𝜆3 ― 𝜆4)

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(5)

Equation (5) yields the multi-spectral camouflage performance considering the energy dissipation and reduction. Therefore, we use Equation (5) to evaluate the MSAE total IR camouflage with varied diameter and temperature. Figure 5 shows the diameter and temperature dependence of the total camouflage performance (TCP) corresponding to IR camouflage. The designed surface is MSAE and the reference surface is an Au surface (IR signature control is usually achieved through metal surface or particle)58. Additionally, in this study, the dissipated emissive power band of detected 3-5 μm and 8-14 μm band is shared of undetected 5-8 μm band. Figure 5 reveals several thermophysical behaviors associated with the IR camouflage. The extreme values correspond to Au and the blackbody surface, indicating that (i) Au is only the reduced infrared signature and reference surface for camouflage, and (ii) the blackbody surface is an ideal surface for the maximum energy dissipation through the entire undetected band. As Figure 5 shows, at 600 K which is the stagnation temperature of aircraft at the altitude of 5000 m with Mach Number = 2.0, the TCP values of Au and the blackbody are 1 and 0.49, respectively. Contrary to these values, TCP of MSAE at 600 K is 9.6 and 19.6 times higher than those of extreme surfaces such as Au and the blackbody, respectively. This indicates that simultaneous IR signature reduction and dissipation of reduced emitting energy are required for increasing TCP of the designed surface. Thus, MSAE is a suitable material for IR camouflage that satisfies the requirements of signature reduction and the dissipation of reduced signature. Additionally, TCP decreases with increasing temperature, which the maximum value of TCP at 300 K and 600 K is 16.5 and 9.6, respectively, owing to the increase in the emissive power term in the denominator. The emissive power term is close relation with the temperature based on Planck’s law17, this indicates that the temperature is one of the dominant factors affecting TCP and large values of the emissive power lead to deteriorations in the camouflage performance. The thermal instability problem is also connected with this issue, i.e., if we control only the low emissivity, the surface temperature increases due to the energy balance, leading to decrease in the camouflage performance. Consequently, for IR camouflage deduced from Figure 5, we should consider the temperature for evaluating the camouflage performance. Furthermore, the target surface temperature affects the optimal design point as shown in Figure 5. The maximum point of TCP varies with temperature such as the metal disc diameter of 1.49 μm and 1.21 μm (i.e., 0.28 μm smaller) at 300 K and 600 K, respectively. This can be explained by Wien’s displacement law and Planck’s law17. According


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to Wien’s displacement law, the wavelength corresponding to the maximum of emissive power decreases with increasing temperature and affects the magnitude of the emissive power at each band. Therefore, to design a adequate MSAE for IR camouflage, the operating temperature should be determined to obtain the maximum TCP. By simultaneously considering the reduced signature at a detected band and dissipation of reduced emitting energy through an undetected band, the suggested concept of TCP can help to design the high camouflage performance of MSAE. In addition, the optical behavior of thermophysical and geometrical factors associated with MSAE has also helped to enhance the IR camouflage performance. We observe that, compared with a metal surface (Au) and a blackbody, MSAE exhibits superior IR camouflage performance. In conclusion, we suggest a design guideline for MSAE associated with IR camouflage that yield the maximum performance considering temperature variation.



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4. Conclusion We investigate a metamaterial selective absorber/emitter (MSAE) for IR camouflage by considering IR signature reduction and dissipation of reduced emitting energy. To deceive the IR signature against the IR detector, we reduce the IR signature of the target bands at the IR detector. The target bands consist of detected band (3-5 μm and 8-14 μm) regarding to the atmospheric transmittance. The problem is that the reduced emitting energy yields an increase in temperature and, consequently, a thermal instability due to the energy balance without appropriate energy dissipation. For this reason, to satisfy the purpose of IR camouflage without thermal instability, we adopt MSAE capable of selective absorption/emission. We investigate the optical behavior of MSAE via the experiments and simulations. From this investigation, we obtain the spectral emissivity of the designed MSAE and demonstrate the IR camouflage using an IR camera (8-14 μm). Varying geometrical factors such as the diameter and pitch reveal that the metal disc diameter of MSAE plays the key role in controlling the IR signature for IR camouflage. Based on Planck’s law, we calculate the emissive power by considering the spectral emissivity of MSAE through bands with wavelengths of 3-5 μm, 5-8 μm and 8-14 μm. As a result, using MSAE, we manipulate the maximum emissive power through a specified region of the undetected band. We also recognize the effect of temperature on the emissive power distribution due to Planck’s law. For this reason, we propose a camouflage performance (CP) and a total camouflage performance (TCP) that account for the (i) reduction and dissipation of emissive power through detected and undetected bands simultaneously, and (ii) temperature effect, where the temperature increases relative to that of reference surface. The results show that, at 600 K, the TCP of MSAE is 9.6 times higher than that (1) of Au and 19.6 times higher than that (0.49) of the blackbody. An optimal metal disc diameter of MSAE for IR camouflage is identified. In conclusion, we suggest that the metal disc diameter of MSAE for optimal IR camouflage should be temperature dependent, because the emissive power distribution varies with emitted surface. Through this study, we improve the understanding of metamaterial selective absorber/emitter by considering of Planck’s law for IR camouflage. In addition, we infer that the ideal MSAE should have the emissivity of zero at the detected band (3-5, 8-14 μm) and the emissivity of unity at the undetected band (5-8 μm) for maximizing the CP suggested by this study. The evaluation of IR camouflage can provide clues for the evaluation of camouflage performance associated with performance of multi-spectral bands such as acoustic, microwave, and visible bands.


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ACKNOWLEGEMENTS This work was supported by the Center for Advanced MetaMaterials (CAMM) funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CAMM-No. NRF-2014M3A6B3063716) and the Human Resources Development program (No. 20174030201720) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), grant funded by the Korea government Ministry of Trade, Industry and Energy. Also, this work was supported (in part) by the Yonsei University Research Fund (Yonsei Frontier Lab. Young Researcher Supporting Program) of 2018.



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Table of Contents



LIST OF FIGURES

Figure 1. (a) Atmospheric transmittivity (light-blue shaded area)44 with detected band (3-5 μm and 8-14 μm) and undetected band (5-8 μm) indicated, (b) Schematics of unit cell of MSAE for IR camouflage based on metal disc diameter (d), thickness of metal disc (t1), dielectric layer (t2) and metal ground (t3), (c) SEM (Scanning electron microscope) image of MSAE surface in case of d = 1.9 μm and p = 4.0 μm. Figure 2. (a) Spectral emissivity with the change of metal disc diameter including the comparison between the experiment and simulation, (b) demonstration of IR camouflage using IR camera (detecting wavelength: 8-14 μm) through the comparison of d = 0.9 and 1.9 μm on heating plate. Figure 3. Spectral emissivity of MSAE with the change of (a) diameter at p = 4 μm and (b) pitch at d = 1.25 μm with R1 of delocalized plasmon behavior and R2 of localized plasmon behavior. (c) normalized magnetic field (a.u.) in the cross section of y-z plane with induced current (white arrow) at a resonant wavelength. (d) deduction of magnetic resonant wavelength with the change of diameter including the guidance to match the undetected wavelength (5-8 μm) with metal disc diameter. Figure 4. Dependence of emissive power (a) for the metal disc diameters at a fixed surface temperature of 450 K and (b) through wavelengths of 5-8 μm (undetected band) for the metal disc diameter and surface temperature. Figure 5. Total camouflage performance at wavelengths of 3-5 and 8-14 μm with different surface temperatures. The performance of reference surface such as gold surface and blackbody surface is also shown.

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Figure 1. (a) Atmospheric transmittivity (light-blue shaded area)44 with detected band (3-5 μm and 8-14 μm) and undetected band (5-8 μm) indicated, (b) Schematics of unit cell of MSAE for IR camouflage based on metal disc diameter (d), thickness of metal disc (t1), dielectric layer (t2) and metal ground (t3), (c) SEM (Scanning electron microscope) image of MSAE surface in case of d = 1.9 μm and p = 4.0 μm.



Figure 2. (a) Spectral emissivity with the change of metal disc diameter including the comparison between the experiment and simulation, (b) demonstration of IR camouflage using IR camera (detecting wavelength: 8-14 μm) through the comparison of d = 0.9 and 1.9 μm on heating plate.


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Figure 3. Spectral emissivity of MSAE with the change of (a) diameter at p = 4 μm and (b) pitch at d = 1.25 μm with R1 of delocalized plasmon behavior and R2 of localized plasmon behavior. (c) normalized magnetic field (a.u.) in the cross section of y-z plane with induced current (white arrow) at a resonant wavelength. (d) deduction of magnetic resonant wavelength with the change of diameter including the guidance to match the undetected wavelength (5-8 μm) with metal disc diameter.



Figure 4. Dependence of emissive power (a) for the metal disc diameters at a fixed surface temperature of 450 K and (b) through wavelengths of 5-8 μm (undetected band) for the metal disc diameter and surface temperature.


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Figure 5. Total camouflage performance at wavelengths of 3-5 and 8-14 μm with different surface temperatures. The performance of reference surface such as gold surface and blackbody surface is also shown.


Metamaterial-Selective Emitter for Maximizing Infrared Camouflage

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