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Abstract
In the paper, a type of phase change metamaterial for tunable infrared stealth and camouflage is proposed and numerically studied. The metamaterial combines high temperature resistant metal Mo with phase-changing material GST and can be switched between the infrared “stealthy” and “non-stealthy” states through the phase change process of the GST. At the amorphous state of GST, there is a high absorption peak at the atmospheric absorption spectral range, which can achieve infrared stealth in the atmospheric window together with good radiative heat dissipation in the non-atmospheric window. While at the crystalline state of GST, the absorption peak becomes broader and exhibits high absorption in the long-wave infrared atmospheric window, leading to a “non-stealthy” state. The relationship between the infrared stealth performance of the structure with the polarization and incident angle of the incident light is also studied in detail. The proposed infrared stealth metamaterial employs a simple multilayer structure and could be fabricated in large scale. Our work will promote the research of dynamically tunable, large scale phase change metamaterials for infrared stealth as well as energy and other applications.
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1. Introduction
With the rapid development of military science and technology, more and more infrared detection equipment has been applied to the battlefield, which poses a greater threat to the survivability of weapons and equipment [1–5]. How to deal with the increasing detection accuracy has gradually become a difficult problem for the military of each country, and the infrared stealth technology has gradually become a research hotspot of scientists [6–8]. The key to realize infrared stealth is to control the infrared radiation of the target. At present, the wavebands detected by the widely used infrared detectors are all in $3{\; }\sim {\; }5\; \mu m$ and $8\; \sim \; 14\; \mu m$, which are also called the atmospheric windows and correspond to the high transmittance bands in the atmospheric transmittance line [9–13]. To achieve better infrared stealth performance, the emissivity of the target in these two atmospheric windows should be reduced, so as to decrease the probability of being detected by the detector, while the emissivity in $5\; \sim \; 8\; \mu m$ should be as high as possible to maintain the attenuation and absorption of electromagnetic wave in the atmosphere, and obtain better radiation heat dissipation. According to Kirchhoff's law [14], for an object in thermal equilibrium, its emissivity is equal to the absorptivity, so regulating emissivity of the target is also equivalent to regulating the absorptivity.
In recent years, stealth metamaterials have become a research hotspot in the field of stealth technology [15–22], by taking advantage of the perfect properties of metamaterials, various metamaterial absorbers have been designed to achieve stealth in radar [23–25], laser [26–29], infrared [30–33] and other wavebands [34–36]. But most stealth metamaterials are achieved by specific materials and structures, which are usually fixed, so that the target can only be thermally camouflaged at a fixed background temperature. However, in practical applications, the target may constantly move and the background temperature will change. Once this happens, the static thermal camouflage performance will be significantly reduced due to the inconsistent infrared characteristics of the target to the background, thus, the camouflaged target will be discovered. Compared with static camouflage, dynamic thermal camouflage has stronger practicability and broader military application prospects, which can be realized with tunable metamaterials [37]. The tuning strategies generally used can be classified into two types: One is mechanical reconfiguration based on the dependence of the electromagnetic response on structural variations [38,39]. The other is incorporation with active materials, either as surrounding media or constituent materials, for the sensitivity of their optical properties to external stimuli enables the flexible control of metamaterials [40,41]. A variety of active photonic metamaterials responding to the application of heat, light, current, voltage, or electric/magnetic field have been proposed [42], but most response of the metamaterial is volatile, for the “switched state” can be maintained only when the stimulus is present.
Unlike other materials, the optical properties of the amorphous and crystalline states of the phase-change materials often differ distinctly due to diverse bonding mechanisms. More importantly, the two phases both are perfectly stable, and can be well reserved without the external stimulus, which is called as non-volatile. Among them Ge2Sb2Te5 (GST) is one of the commonly used materials with great potential in tunable metamaterials and nanophotonics [43,44]. The alloy of GST can realize the transition between amorphous and crystalline states by annealing [45]. When the temperature reaches about 160℃, the transition from amorphous state (aGST) to crystalline state (cGST) can be realized, and when the temperature rises close to 600℃, it can change back to the amorphous state through rapid temperature decrease [46]. Due to the different atomic configurations, it shows completely different optical properties for the two states of GST. When it is in amorphous state, the imaginary part of relative permittivity is close to zero and can be regarded as negligible, while for the crystalline GST, it shows enormous difference, for the extinction coefficient increases with wavelength and the refractive index of it is twice as much as that of aGST [47]. Therefore, aGST is transparent in the mid-infrared band, while cGST is highly absorptive in the mid-infrared band, resulting in a clear difference in the emissivity spectrum.
Till now, many kinds of tunable infrared thermal emitters have been proposed based on the difference of permittivity of GST in different phase transition states together with the noble metal Au [48–50]. Through the design of nano-structure, it can realize the switch between the spectrally selective infrared stealth matching the atmospheric window and the non-stealth state, so as to realize the tunable infrared stealth function. But as a precious metal, the cost of Au is relatively high. Moreover, local overheating may occur at the interface of layers between the phase change material and Au during switching. This will lead to the damage of the metal layer and affect the stealth performance of the whole emitter. Therefore, in this study, we use the high melting point metal Mo [51], to replace the traditional low melting point metal, and design a tunable thermal emitter based on Mo and GST. Through the structure design, the infrared stealth function is realized when the structure is in the amorphous state, with the non-stealth function of the crystal state. The whole structure has high thermal stability, which has a very broad application prospect.
2. Results and discussion
We combine phase change material GST with metal Mo and design a multilayer structure, the schematic diagram is as shown in Fig. 1. The whole structure is within the subwavelength scale. Figure 1(a) is the three-dimensional diagram of the proposed tunable thermal camouflage emitter, the structure extends indefinitely in both x and y directions. Figure 1(b) is the cross section of the structure in the $\; x - z\; $ direction. The whole structure is composed of four parts: At the bottom is the Mo base layer, whose thickness is greater than the skin depth of the incident plane wave, which ensures that the transmittance to the incident light of the structure is almost 0. The parameters of each layer of the structure are as follows: The bottom and middle Mo layers are ${H_1} = 50\; nm$ and ${H_2} = 10\; nm$, respectively; The top and middle GST layers respectively are ${D_1} = 540\; nm$ and ${D_2} = 250\; nm$.
Fig. 1. A phased change metamaterial based on Mo and GST. (a) Structure diagram of the multilayer metamaterial; (b) Cross section of the multilayer structure.
Then, we conduct full-wave numerical simulation by using COMSOL Multiphysics, in which the refractive index of Mo is get from Palik’s handbook [52] and the relative permittivity of GST in different states is obtained from the experiment [47]. As a multilayer structure, the structure can be assumed to be infinite in x and y directions. We take the cross section in $x - z$ direction and set a two-dimensional model in the simulation, in which the period is set as $880\; nm$, and the $ x$ and $y\; $ directions are set as periodic boundary conditions. In the simulation, a plane electromagnetic wave having a wavelength range of $\; 3\; \sim \; 12.4$ $\mu m$ is used. Through certain structural design, we adjust the impedance of the whole structure to match the free space when the GST is in the non-crystalline state, so as to reduce the reflectivity of the structure and achieve high absorption. The relationship between the absorptivity of the structure and the wavelength of the incident light of two different polarizations at normal incidence is obtained, along with the atmospheric absorption spectrum, as shown in Fig. 2.
Fig. 2. Absorption spectra of the phase change metamaterial at different polarizations of the incident light with the two states of the GST.
It can be seen clearly that: Due to the high symmetry of the structure, it is polarization independent for the normal incident light and the absorption of the structure is the same for these two polarization states. What’ more, the absorber shows certain wavelength selectivity with both amorphous and crystalline states of the GST. Specially, when the GST is in the amorphous state, the wavelength selectivity of the structure matches with the atmospheric window. In the atmospheric window band, the absorption rate is relatively low, which can well achieve the purpose of infrared stealth; In the non-atmospheric window band, the absorption rate is relatively high, and there is a high absorption peak at $6.0\; \mu m$ with an absorptivity of 99.681%, which can achieve better radiation heat dissipation; While for the crystalline GST, it has higher and broader absorptivity in the infrared window band with two absorption peaks and the absorptivity respectively are 99.8% and 76.7%. Thus, it is switched to the “non-stealthy” state. This makes sense for the distinctly differ optical properties of the two states of GST. As shown in Fig. 2, the metamaterial shows two absorption peaks at around $4.55\; \mu m$ and $6.0\; \mu m$ when GST is at the amorphous state. For the small absorption peak at $4.55\; \mu m$, the maximum absorptivity is only about 27.1%. This is because most of the electromagnetic field is distributed in the amorphous GST (see Fig. 3) and the electric field in the Mo is weak. When GST is in amorphous state, its imaginary part of relative permittivity is close to zero and its absorption is small. Thus the total absorption is low. For the strong absorption peak at $6.0\; \mu m$, the electric field in the Mo becomes much stronger and almost all the energy is absorbed in Mo due to its large imaginary part of relative permittivity (see Fig. 4). So the maximum absorptivity reaches about 99%. For the crystalline state of GST, it shows enormous difference. The extinction coefficient of cGST increases with wavelength and its refractive index is nearly twice as much as that of aGST. So, these two absorption peaks move towards the far infrared region when GST is switched to the crystalline state (to about $6.71\; \mu m$ and $8.71\; \mu m$, respectively). What’s more, due to the high extinction coefficient of cGST, part of the incident light will be absorbed in the GST layers. The absorption at $6.7\; \mu m$ becomes very strong even though the field distribution (not shown here) is similar to that of the small absorption peak at $4.55\; \mu m$ for the aGST. Thus, there are two absorption peaks and the absorption spectrum becomes much broader when the GST is in crystalline state. Through the annealing process, we can switch between the two states of the GST alloy, so that the target can be transformed between the “stealthy” state and the “non-stealthy” state.
Fig. 3. (a) The distribution of electric field at the resonant absorption peak at $4.55\; \mu m$ under the amorphous state of GST; (b) The distribution of the power loss at the resonant peak at $4.55\; \mu m$ under the amorphous state of GST.
Fig. 4. (a) The distribution of electric field at the resonant absorption peak of $6.0\; \mu m$ with the amorphous state of GST; (b) The distribution of the power loss at the resonant peak of $6.0\; \mu m$ with the amorphous state of GST.
In order to better investigate the optical characteristics of the multilayer structure, we study the relationship between the absorptivity of the tunable thermal camouflage emitter with different incident angles and wavelengths under different polarization states when the GST is in different states. Figure 5(a) and Fig. 5(b) respectively show the spectral absorptivity for different incident angles at TE and TM modes with the amorphous state of GST; While Fig. 4(c) and Fig. 4(d) respectively show the spectral absorptivity at TE and TM modes with the crystalline state of GST. We can intuitively find that for the incident light under the two polarization states, the absorptivity basically does not change in a range of the incident angles for the two states of GST, which proves that the performance of infrared stealth of the structure with two states of GST is not sensitive for different incident angles.
Fig. 5. (a) Spectral absorptivity for different incident angles at TE mode with the amorphous state of GST; (b) Spectral absorptivity for different incident angles at TM mode with the amorphous state of GST; (c) Spectral absorptivity for different incident angles at TE mode with the crystalline state of GST; (d) Spectral absorptivity for different incident angles at TM mode with the crystalline state of GST.
The research results above fully demonstrate the capabilities of the proposed phase change metamaterial for tunable infrared stealth and camouflage applications. The GST layers used in our structure are $540\; nm$ and $250\; nm$, which are quite thick and it may be difficult to realize phase change from crystalline state to amorphous state. Thus, we studied another structure with an insulator layer Al2O3 between the Mo and GST layer, and reduce the thickness of each GST alloy layer. By stacking multiple groups of medium and GST layer, we can also achieve wavelength selective absorption that matches the atmospheric window.
Figure 6(a) and Fig. 6(b) respectively show the three-dimensional diagram and the cross section in the $\; x - z\; $ direction of the improved tunable thermal camouflage emitter. The thickness of the bottom and middle Mo layers of the structure respectively are $50\; nm$ and $10\; nm$. Other parameters of structure are as follows: ${H_1} = {H_2} = {H_3} = {H_4} = {H_5} = {H_6} = {H_7} = 90\; nm$, ${D_1} = {D_2} = {D_3} = {D_4} = {D_5} = {D_6} = {D_7} = 70\; nm$.
Fig. 6. An improved phase change metamaterial based on Mo and GST. (a) Structure diagram of the multilayer metamaterial; (b) Cross section of the multilayer structure. Here the thickness of GST layers is reduced to 70 nm.
Similarly, we numerically calculate the absorption spectrum of the structure through simulation software. Here, we define the refractive index of Al2O3 as 1.76 and other simulation conditions are consistent with previous studies. In the previous study, we have discussed that the absorption of our proposed multilayer structure in the two polarization modes of incident light is completely degenerate under normal incidence condition, so we only study the incident light in TM mode here. Through calculation, the relationship between the absorptivity and wavelength of the incident light of the multilayer thermal emitters with different states of GST under TM mode is obtained, as shown in Fig. 7.
Fig. 7. Absorption spectra of the improved phase change metamaterial at TM mode with the two states of the GST.
As shown in Fig. 7, the wavelength selective absorption matches the atmospheric window with low absorptivity in the atmospheric window band together with the high emissivity in the non-atmospheric window band when GST is in amorphous state. There is a high absorption peak at $5.85\; \mu m$, with the absorptivity is about 99.205%, which can achieve favorable radiation heat dissipation. When the GST alloy is in crystalline state, the emissivity of the structure becomes broader with relatively high emissivity in the atmospheric window band, leading to the “non-stealthy” state. Thus, we can realize the transformation between the “stealthy” state and the “non-stealthy” state through the phase change of GST.
However, there are too many stacked layers of the structure, resulting in a tedious and complex fabrication process. Therefore, we replace the relatively low refractive index medium Al2O3 with the high refractive index medium Ge, so as to reduce the thickness and number of layers of the structure and simplify the fabrication process. Figure 8(a) and Fig. 8(b) show the three-dimensional diagram and the cross section in the $\; x - z\; $ direction of the new metamaterial design, respectively. The thickness of the bottom and middle Mo layers of the structure remains unchanged, is $50\; nm$ and $10\; nm$, respectively. Other parameters of structure are as follows: ${H_1} = {H_4} = {H_5} = 100\; nm$, ${H_2} = {H_3} = 150\; nm$, ${D_1} = {D_2} = {D_3} = {D_4} = {D_5} = 60\; nm$.
Fig. 8. An improved phased change metamaterial based on Mo and GST. (a) Structure diagram of the multilayer metamaterial; (b) Cross section of the multilayer structure. Here the thickness of GST layers is reduced to $60\; nm$ and the use of Ge as spacer layer reduces the number of total layers.
We set the refractive index of Ge as 4 and other simulation conditions are consistent with previous studies as well. The absorption spectra of the emitter with different states of GST under TM mode are obtained along with the atmospheric absorption spectrum, as shown in Fig. 9. When the GST alloy is in the amorphous state, there are two absorption peaks at $5.3\; \mu m$ and $6.6\; \mu m$, and the absorptivity respectively is 62.318% and 91.771%, with a wide absorption bandwidth approximately is $\; 3\; \mu m$, which can match with the atmospheric window and achieve infrared stealth together with well radiation heat dissipation; When the GST alloy is in crystal state, the absorption of the structure is also relatively high in the atmospheric window band, and the metamaterial switches to the none-stealthy state. Through the phase change process, we can realize the switch between the two states of the GST alloy, so as to realize the transformation between the “stealthy” state and the “non-stealthy” state.
Fig. 9. The absorption spectra of the phase change metamaterial with different states of GST under TM mode.
3. Conclusions
In this paper, we employ the phase change material GST and propose a multilayer tunable infrared stealth metamaterial. The metamaterial is based on high temperature resistant material Mo rather than the precious metal Au which is widely used in traditional metamaterial structures. Through the annealing process, the GST can be switched between the amorphous and crystalline state, thus changing the infrared performance of the overall structure and achieving the switch between the infrared “stealthy” state and the “non-stealthy” state, which greatly improves the flexibility of the target. Then, we employ the relatively low refractive index medium Al2O3 and the high refractive index medium Ge, and design two types of multi-layer metamaterials with thinner GST films down to $70\; nm$ and $\; 60\; nm$, respectively. According to the simulation results, both of the two structures can achieve the tunable infrared stealth function. Particularly they have high emissivity in the non-atmospheric window, and can achieve fantastic radiation heat dissipation when GST is in the amorphous state. Among them, the multilayer tunable thermal emitter with high refractive index medium Ge as the dielectric layer has a relatively simpler structure with less layers. The proposed phase change metamaterial employs a simple multilayer structure and could be fabricated in large scale. It is expected to be able to realize dynamic, bidirectional switch for infrared stealth and camouflage applications. In our design, we may use the high temperature resistant metal Mo as the heater or use other transparent conductive films such as graphene as the electrode for heating (in the multilayer structure) due to its high thermal and electrical conductivity, integration versatility, and superior stability [53,54]. By controlling the pulse voltage, we can achieve temperatures up to 900 K with single-layer graphene, and the annealing process of GST can be realized. Our work will promote the research of dynamically tunable, large scale phase change metamaterials for infrared stealth as well as energy and other applications.
Funding
Hunan Provincial Science and Technology Department (2017RS3039, 2018JJ1033); National Natural Science Foundation of China (11674396); National University of Defense Technology (ZDJC19-03).
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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