A Non-volatile Switchable Infrared Stealth Metafilm with GST

Infrared stealth technology is defined by the reduction of an object's infrared emissions or the alteration of its infrared signature, thereby reducing the possibility of being detected by infrared detection systems^1–6. With the rapid development of nanophotonics, infrared stealth technology has made significant breakthroughs in recent years^7–9. At the beginning, the infrared stealth was generally realized by reducing the infrared radiation and using low-emissivity coatings. According to the Stephen-Boltzmann’s law¹⁰, the infrared radiation of the target is proportional to the emissivity and the fourth power of the absolute temperature. Consequently, by reducing emissivity to achieve infrared stealth, the heat will continue to accumulate, together with the rise in temperature, which ultimately compromises the stealth capabilities. To solve this problem, more and more research focuses on infrared stealth technology that can achieve spectrally selective control of the infrared radiation characteristics^11–16. By reducing the infrared radiation in the atmospheric window band of 3-5 μm and 8-14 μm that the current infrared detection system works on, the infrared stealth can be realized^17–21. At the same time, the radiation in the non-atmospheric window band of 5-8 μm is maintained or even increased to realize radiative heat dissipation^22–26.

To date, spectrally selective infrared stealth technology has achieved great sucess^27–29. One of the most representative methods is the structural design of metamaterials^30–36. Metamaterials are artificial materials with novel physical properties that traditional materials cannot achieve, which are generally realized by the design and manipulation of micro- or nano-structures in the sub-wavelength scale^37–46. According to Kirchhoff’s law of thermal radiation⁴⁷, the spectral emissivity and absorptivity of an object in thermal equilibrium are equal. Therefore, the regulation of absorptivity of the object is equivalent to that of the emissivity. By constructing metamaterials with different structures, the optical absorptivity can be controlled, so that the spectrally selective infrared stealth function can be well realized. However, these methods are only aimed at the stealth in specific background environment. As the background environment is not static in practical applications, this stealth state will be broken once the external environment changes. Therefore, tunable infrared stealth technology has become the center of many recent researches.

Generally speaking, one of the commonly used tuning strategies is combing stealth structures with active materials, which can change the optical properties through external stimulation, so as to regulate the infrared radiation characteristics of the entire structure^30,32. However, the response of most active materials to external stimuli is volatile, i.e. the external stimulus conditions need to be maintained to stabilize its optical properties in a certain state. To realize non-volatile switch, phase change materials are widely used^48–52. Among them, one of the most common materials is Ge₂Sb₂Te₅(GST), which has two different states: in the amorphous state, the internal atomic distribution is very chaotic, and the imaginary part of the relative permittivity in the infrared band is almost 0; When it turns to the crystal state, the relative permittivity will be significantly increased, and the absorption is higher than that in the amorphous state^53–58.

Therefore, in this paper, by using GST with the characteristics of phase change material, and the high temperature resistant metal Mo (for high temperature may be generated in the annealing process), we propose a switchable infrared stealth bilayer metafilm. When the GST is in the amorphous state, the absorptivity of the structure in the atmospheric window band is relatively low, together with the high absorptivity in the non-atmospheric window band, which can realize the infrared stealth while taking into account the radiation heat dissipation ability in the non-atmospheric window; When the GST switches to the crystal state, the absorptivity of the film in the atmospheric window band prominently increases, and the stealth function cannot be realized. By adjusting the GST in different states, the target can switch between the infrared stealth and the non-stealth states, while the infrared radiation characteristics can be flexibly controlled to adapt to the complex and changeable external environment.

Fig. 1a shows the detection mechanism of the infrared detectors. The infrared radiation emitted by the target is transmitted through the atmosphere and captured by the infrared detector. Fig. 1b depicts the schematic diagram of the designed tunable infrared stealth bilayer metafilm based on the bottom metal Mo layer and the top phase change material GST layer, together with the schematic diagram of the transition mechanism of GST between different states. The structure is optimized and the thicknesses of the two layers are $ {H}_{1}=100\;{\rm{nm}} $ and $ {H}_{2}=360\;{\rm{nm}} $, respectively.

Fig. 1  The structure diagram and the absorption spectra obtained by simulation under normal incidence. a The detection mechanism of the infrared detectors. The infrared radiation of the target is transmitted through the atmosphere and captured by the infrared detector. b Schematic diagram of the bilayer metafilm and the transition mechanism of GST between different states. When the temperature exceeds the crystallization temperature $ {T}_{c} $, the GST will gradually change from amorphous state to the crystalline state, and once the temperature exceeds the melting temperature $ {T}_{m} $, after rapid annealing, the GST can change back to the amorphous state. c The relative permittivity of GST at different states in infrared band measured by the spectroscopic ellipsometer. d The absorption spectra of the metafilm with two phase states for normally incident light and the atmospheric transmittance spectrum.

The metafilm consists of a simple bilayer structure and the absorption spectra can be calculated by numerical simulations as well as analytical methods (Supplement Figure S1). Here, in order to study the field distributions of structure, we conduct numerical simulations by using the commercial solver of COMSOL Multiphysics. As the structure extends indefinitely in both $ x $ and $ y $ directions, we take the cross section in $ x-{\textit z} $ direction in the simulation and set periodic boundary conditions on each side with the port conditions on the upper and lower boundaries. Firstly, we consider the case of normal incidence. Due to the high symmetry of the structure, it is polarization-independent for the normal incident light and the spectra are the same for the two polarization states, so here we only consider the incident light at TM mode (magnetic field vector is perpendicular to the incident plane). The relative permittivity of GST and Mo in infrared band is measured by a spectroscopic ellipsometer as shown in Fig. 1c and Supplement Fig. S2a. The relative permittivity significantly increases when it turns to the crystal state. As the substrate material Mo is optically thick, the transmission of the whole structure is close to zero. Thus, the near-unity absorption can be implemented if the reflection coefficient decreases to zero by selecting the appropriate thickness of the layers to match the impedance of the film with the air. The simulated absorption spectra of the film along the atmospheric transmittance spectrum are shown in Fig. 1d. In order to better compare the infrared radiation characteristics of different bands, we calculate the average emissivity in each band, as shown in the formula (1-2). Among them, $ {M}_{b}\left(T,\lambda \right) $ is the spectral radiance of the blackbody calculated from Planck’s blackbody radiation law⁵⁹, $ h $, $ c $, $ k $, $ T $ is Planck’s constant, the velocity of light in vacuum, the Boltzmann’s constant and temperature; $ {\varepsilon }_{f,{\lambda }_{1}\sim {\lambda }_{2}} $is the average emissivity of the film in the wavelength band $ {\lambda }_{1}\sim{\lambda }_{2} $; $ {\varepsilon }_{f}(T,\lambda ) $ is the spectral emissivity. When the GST is in the amorphous state, the infrared radiation of the film in the 3-5 μm and 8-14 μm atmospheric window band is relatively low, which can realize infrared stealth. In the non-atmospheric window band, the infrared radiation significantly improves, and a high absorption peak at 6.08 μm is observed with an absorptivity of 94%. At the same time, the average emissivity is over 0.7, which demonstrates great radiative heat dissipation ability. While for the crystal state of the GST, the average emissivity in the band of 8-14 μm significantly increases to over 0.7, and the structure turns to the non-stealth state. Therefore, by changing the GST into different states, the switch between the infrared stealth and the non-stealth state of the target can be achieved.

To interpret the absorption mechanism of the metafilm, we plot the electric field distribution and power loss diagram at the absorption peaks in Fig. 1d, which corresponds to three resonance wavelengths of $ {\lambda }_{1}=6.08\;{\text {μm}} $, $ {\lambda }_{2}=3.26\;{\text {μm}} $ and $ {\lambda }_{3}=9.78\;{\text {μm}}$ for the different states of GST. As shown in Fig. 2, we can find that the electric field is mainly concentrated in the upper GST layer with its maximum near the top surface. From the relative permittivity curve of the material (Supplement Fig. S2), we can find that compared with Mo, the imaginary part of the relative permittivity of the GST is much lower, whether in the amorphous state or in the crystalline state, so the absorption of the structure is mainly concentrated in the lower Mo layer, which can also be visually found from the power loss diagram. Moreover, due to the significant increase in the real part of the relative permittivity after the transformation from the amorphous state to the crystalline state of GST, the absorption spectrum of the structure has been redshifted, and the wavelength of the resonance absorption peak has moved from 6.08 μm to 9.78 μm. Besides, there is also an additional absorption peak at 3.26 μm that is previously located in the near infrared band (not shown here). Meanwhile, from Fig. 2c, e, it can be found that due to the stronger electric field in the Mo layer at 9.78 μm, the absorption of the structure is also improved compared with that at 3.26 μm, which is consistent with the results shown in the absorption spectra in Fig. 1d.

Fig. 2  The absorption mechanism of the metafilm. a The electric field distribution and b power loss diagram at the resonance absorption peak of 6.08 μm when GST is in the amorphous state. c The electric field distribution and d power loss diagram at the resonance absorption peak of 3.26 μm when GST is in the crystalline state. e The electric field distribution and f power loss diagram at the resonance absorption peak of 9.78 μm when GST is in the crystalline state.

Then, the angular dependence of optical absorption with TE and TM polarizations for different phase states of GST is studied, as shown in Fig. 3. It can be seen that in the investigated spectral range, the absorptivity barely changes with the increase of the incidence angle within 40 degrees, which can maintain more than 90%. The simulated results demonstrate that the proposed metafilm maintains good performance of infrared stealth with polarization-independent and wide-angle responses.

Fig. 3  The influence of the polarization state and incident angle to the absorption of the metafilm. Calculated absorptivity of different incident angles at a TE and c TM mode oblique incidences when GST is in the amorphous state. Calculated absorptivity of different incident angles at b TE and d TM mode oblique incidences when GST is in the crystalline state.

We further experimentally demonstrate the excellent switchable characteristics of the infrared stealth metalfilm. Following the optimized structural parameters in simulations, the corresponding GST-Mo bilayer samples are prepared. Initially, a Mo film with a thickness of 100 nm is deposited on a meticulously cleaned silicon substrate with a 285 nm oxide layer by high vacuum magnetron sputtering under room temperature. Subsequently a 360 nm GST layer is deposited on top of it. The deposited GST in the prepared samples is at a phase state that close to the amorphous state, and by heating the sample on a heating stage at 200°C (slightly above the crystallization temperature) for 10 minutes, we successfully complete the phase transition to the crystal stage. The samples are shown in Fig. 4a for the different states of GST, and the size of them are roughly 1 cm × 1 cm. Additionally, due to the good ductility of the two materials we used, it can be fabricated on flexible substrates like PET, as shown in Fig. 4b. Since there is a certain absorption in the visible band (Supplement Figure S4), these samples appear dark. The cross-section pictures of the prepared bilayer film at the two stages of GST under the scanning electron microscope (SEM) are shown in Fig. 4c, d. It can be seen that after phase transition of GST from amorphous state to crystalline state, its internal structure becomes more ordered, for a large number of microcrystalline structures appear. The infrared absorption spectra of the samples at different phase conditions are obtained by a Fourier transform infrared spectrometer (FTIR) as shown in Fig. 5a. Consistent with simulation results, it is evident that when GST switches from amorphous state to crystalline state, there is significant alteration in the infrared absorption/emission. Specifically, when the GST is in the amorphous state, the film can realize the infrared stealth function, while capable of good radiative heat dissipation capability. But once the GST is transformed into a crystalline state, due to the significant increase in the relative permittivity of the GST, the absorption spectrum of the entire structure is significantly redshifted, resulting in the huge increase of the infrared radiation of the structure in the 8-14 μm atmospheric window, so that the stealth function cannot be realized. Therefore, by switching between different states of GST, the film can be switched between the infrared stealth and the non-stealth states.

Fig. 4  Fabricated metafilms based on the phase change material GST. a The images of prepared bilayer film on the silicon substrate at the different stages of GST. b The image of prepared bilayer film on the flexible substrate PET. The cross-section images of the prepared bilayer metafilm under scanning electron microscope (SEM) when GST is in the c amorphous d crystalline states.

Fig. 5  The infrared radiation characteristics of the sample for the two states of GST. a The infrared absorption spectra of the fabricated bilayer metafilm obtained by the Fourier transform infrared spectrometer (FTIR). b Spectral radiant exitance of blackbody and the fabricated bilayer metafilm at 300K when the GST is in the two states.

However, upon comparing the absorption spectra between simulation and experiment, we observe a slight deviation in the experimental measurements when the GST transitions to its crystalline state. This is evident from an apparent blue shift in the absorption spectra (Supplement Figure S5a). Further investigations reveal a reduction in the thickness of the upper GST layer of the bilayer films during the transition from amorphous to crystalline phase, which is only about 330 nm compared with the 360 nm before. Therefore, we recalibrate the thickness of GST after the phase transition in the simulation and obtain a new absorption spectrum (Supplement Figure S5b). Notably, following these adjustments, it becomes evident that our sample’s measured infrared spectra agree well with our simulation results. When the GST is in the amorphous state, the average emissivity of the film in the 3-5 μm and 8-14 μm atmospheric window band are 0.31 and 0.24, respectively, enabling infrared stealth. However, within non-atmospheric window band of 5-8 μm, it demonstrates an average emissivity of 0.67 which facilitates radiative heat dissipation. When the state of GST changes to crystalline, the average emissivity of the structure in 8-14 μm exceeds 0.67, which is highly consistent with the results obtained by our simulations.

Then, we study the infrared radiation properties of the bilayer sample for the two phase states of GST by comparing it with the blackbody under 300K, as shown in Fig. 5b. It can be intuitively found that for the amorphous state of GST, the infrared radiation of the bilayer sample is mainly concentrated in the band of 5-8 μm non-atmospheric window, and the intensity of infrared radiation in the wavelength range of 3-5 μm and 8-14 μm respectively decrease by 68.86% and 75.58% compared with blackbody by calculation, which can achieve infrared stealth together with radiative heat dissipation. Meanwhile, for the crystalline state of GST, the infrared radiation in the 8-14 μm atmospheric window band is sharply increased to 67.34% of the blackbody, which means that the infrared stealth function cannot be realized.

Finally, we demonstrate the different infrared emissivity of the samples before and after the phase transition. The temperature of the samples with both amorphous and crystalline states of GST are measured under the background temperature of 30°C, 50°C, 75°C and 100°C, respectively. Simultaneously, a silicon substrate of comparable size is placed as a reference, as shown in Fig. 6. Among them, sample A is at amorphous state, sample B is the comparably sized silicon substrate reference, and the sample C is at crystalline state. We can intuitively find that compared with the sample in the crystal state, when GST is in the amorphous state, the temperature of the sample is significantly lower than the background temperature. Furthermore, as the background temperature increases, there is a continuous increase in temperature difference between samples in the two states. When the background temperature reaches 100°C, the temperature difference between the two samples is as high as 28°C, which is a very intuitive demonstration of the huge change in the infrared radiation characteristics of the sample after the phase transition. What’s more, it further proves the tunable performance of our proposed switchable infrared stealth film and the possibility of practical application.

Fig. 6  Demonstration of the different infrared emissivity of the samples before and after the phase transition. a−d Measured temperature of the three samples (sample A is at amorphous state, sample B is the comparably sized silicon substrate reference, and the sample C is at crystalline state) under the background temperature of 30°C, 50°C, 75°C and 100°C. e Measured temperature curves of the three samples under the background temperature.

To better demonstrate the excellent properties of our proposed tunable infrared stealth metafilm, we compare it with the previous work of infrared stealth materials which are also based on the phase change material GST. The comparison is made from the aspects of constituent structure and the average emissivity in the three wavelength bands studied for the two states of GST (see Table 1). It shows the huge difference in the infrared emissivity of our proposed metafilm between the two states of GST. Furthermore, compared with the tunable infrared stealth materials proposed by other works, the metafilm has a simpler structure with two layers, which is easier to fabricate and shows broad application prospects.

In this paper, a non-volatile switchable infrared stealth bilayer metafilm is proposed and experimentally demonstrated based on the phase change material GST and the high temperature resistant metal Mo. The results indicate that the infrared radiation characteristics of the metafilm will change significantly when the GST is in different states due to the great difference of the relative permittivities. Specifically, when the GST is in the amorphous state, the average emissivity of the metafilm in the 3-5 μm and 8-14 μm atmospheric window band is merely 0.31 and 0.24, respectively, enabling effective infrared stealth functionality. Additionally, in the 5-8 μm non-atmospheric window band, it exhibits an average emissivity exceeding 0.67 with a high absorptivity of 94% at 6.08 μm, indicating excellent radiative heat dissipation capabilities as well. However, when GST transitions into its crystalline state, there are substantial alterations in the infrared radiation characteristics of the structure. Particularly, there is a significant increase in average emissivity from 0.24 to 0.67 within the 8-14 μm band which tunes the metafilm to a non-stealth state. Therefore, by switching the state of GST, the target can be switched between the infrared stealth and non-stealth state, so as to achieve the purpose of dynamic control of infrared radiation. Meanwhile, the infrared radiation characteristics of the metafilm are not sensitive to incident angles and polarizations.

We investigated the infrared radiation characteristics of the bilayer sample for the two states of GST by it with the blackbody under 300K. For the amorphous state of GST, the infrared radiation of the bilayer sample is mainly concentrated in the band of 5-8 μm non-atmospheric window, while for the crystalline state of GST, the infrared radiation in the 8-14 μm atmospheric window band accounts for the majority. We demonstrate the different infrared emissivity of the samples before and after the phase transition by measuring the temperature of the samples under the background temperature of 30°C, 50°C, 75°C and 100°C, respectively. Compared with the sample in crystalline state, it shows much lower temperature when GST is in the amorphous state. The apparent temperature of the metafilm with amorphous GST is 50.4°C at the temperature of 100°C, and is nearly 28°C lower than the crystalline sample under the same conditions, which fully demonstrates the great change of infrared emissivity of samples in two states.

For the annealing process in our experiment, it is realized by heating and it can only realize the amorphous-to-crystalline transition. For bi-directional switch, the crystalline-to-amorphous transition process needs a short, high-intensity power to raise the local temperature momentarily above the melting temperature. This process can be realized by pulsed laser⁶² or electric switch. In the past few years, there have been some remarkable progresses in electrical switch of chalcogenide phase change materials for tunable nanophotonic devices⁶³. However, at present, almost all the research on electrical modulation is focused on small size structures, since the electrical switch of the phase change materials at a large scale is still challenging. For our research, in order to further realize the bi-directional phase transition of GST by means of electrical switch, we need to further optimize the structure and reduce the thickness of GST layer. It is also helpful to employ the high-temperature resistant metals for the design of the devices. Besides Mo, other high temperature resistant metals such as W (Tungsten) and Ti (Titanium) are also potential choices. Our work represents an important progress in exploring phase change materials for dynamic thermal radiation control and will promote the research towards practical applications of intelligent infrared stealth technology.

The silicon substrate with a 285 nm oxide layer was carefully cleaned and a Mo film with the thickness of 100 nm was deposited by high vacuum magnetron sputtering under room temperature and the deposition power was 100W with the speed of 0.15 nm/s. After that, a 360 nm GST layer was deposited at the deposition power of 60W with the speed of 0.31 nm/s. The chamber was ceaselessly pumped to the starting pressure of $ 4\times {10}^{-4}\;{\rm{Pa}} $ and the working pressure was maintained at 0.5 Pa by filling the argon gas. The samples were placed on a heating stage at 200°C (slightly above the crystallization temperature) for 10 minutes to transform into the crystal stage.

The relative dielectric permittivity of the two materials were measured by an infrared ellipsometer. The infrared absorption spectra of the sample were measured by a Fourier transform infrared spectrometer (FTIR). The cross-section of the sample was observed by Scanning Electron Microscopy (SEM). The temperature of the sample was measured by a handheld infrared thermal imager (FLIR E95).

We performed the numerical simulations by using the commercial solver of COMSOL Multiphysics. We took the cross section in $ x-{\textit z} $ direction in the simulation and set periodic boundary conditions on each side together with the port conditions on the upper and lower boundaries. The light source was a normal incident light at TM mode (magnetic field vector is perpendicular to the incident plane). The relative permittivity of GST and Mo in infrared band were measured using a spectroscopic ellipsometer.

This research was supported by Hunan Provincial Science and Technology Department (2017RS3039, 2018JJ1033), the National Natural Science Foundation of China (11674396) and National University of Defense Technology (ZDJC19-03).