Design and validate a wide-bandwidth, high-performance tunable metamaterial based on cultural-innovation shell materials and its sensing applications

The application of metamaterials in controllable thermal emission devices is an interesting field. However, most of the demonstrated thermal emitters required continuous consumption of external energy (electrical or thermal) to provide an effective thermal emissivity. Here, a metamaterial containing phase change materials Ge2Sb2Te5 (GST) and shell materials with controllable thermal emission power was proposed and measured. Based on the completely amorphous state of the GST layer, an emissivity of 0.212 at wavelength 7.11 μm was achieved by this this metamaterial, while a thermal emission band (with an average amplitude of 0.857 and a bandwidth of 6.16 μm) was excited for the crystalline state. Moreover, numerous thermal emission states were excited by this metamaterial based on the intermediate states between completely amorphous and crystalline states of the GST layer. Tunability of the thermal emission window was obtained by this metamaterial sample. The temperature sensitivity of this metamaterial thermal emitter was 341 nm °C−1. By increasing the thickness of the GST or shell layers, the thermal emission performance of the metamaterial was enhanced. Since the phase transition of GST does not require the continuous consumption of external energy, the metamaterial has the potential to be used in the development of low-power heat emitters, as well as temperature sensors.


Introduction
Electromagnetic (EM) metamaterials/metasurfaces contained artificially designed and prepared periodic micro/nanostructures [1][2][3][4][5].The coupling and interference between incident electromagnetic waves and periodic micro/nanostructures of EM metamaterials/metasurfaces excited rich and diverse resonance behaviors [6][7][8][9][10][11]. Numerous functional devices were proposed and developed using the absorption, reflection, or transmission properties of metamaterials for electromagnetic waves [12][13][14][15][16].Many thermal emitters required controllable thermal emission performance, such as thermal photovoltaics [17], thermal managers [18], radiation coolers [19], infrared camouflage [20], etc.However, the inherent thermal emission performance of these reported natural materials and metamaterials often lacked adjustability.Currently, the wavelength selectivity was achieved by most reported thermal emitters through changing structural parameters [21][22][23][24].However, the flexible controllability was not provided by these thermal emitters.Therefore, in order to develop tunable thermal emitters, many control strategies were proposed by researchers, such as, current/voltage modulation strategies: applying voltage/current sensitive materials (such as graphene) to thermal emitters to achieve tunable thermal emission performance [25].For example, artificial thermochromic materials have been applied to thermal emitters and smart materials.In the application process, flexible modulation of thermal emissivity is achieved by controlling its own temperature to obtain the required thermal emissivity and emission wavelength [26].Therefore, metamaterial devices based on artificial thermochromic materials can also be applied in the field of space exploration and solar energy collection.For example, developing tunable thermal emitters based on the resonant characteristics of phase change materials.VO 2 was a typical temperature sensitive phase change materials.When ambient temperature was below 67 °C, VO 2 was in an insulating state.When temperature exceeded 67 °C, VO 2 transitions reached an insulating state to a metallic state and was applied in tunable thermal emitters [20].The tunable thermal emission performances were achieved by these confirmed thermal emitters through changing external conditiowasns.However, these thermal emitters required continuous consumption of external energy to maintain this modularity.Therefore, developing a low energy or zero energy controllable heat emitter was of great significance.
In this work, a metamaterial thermal emitter with large bandwidth and tunable characteristics was proposed and developed.The Ge 2 Sb 2 Te 5 (GST) materials and shell materials were applied to this metamaterial structure.The GST compound was a typical temperature-dependent (or thermal) phase change material (PCM).GST materials were often used in microelectronics, such as optical storage devices, rewritable DVD-RAM, etc.On the one hand, GST materials were considered to be an important component of the next generation of non-volatile electronic memory, such as phase-change random access memory (PC-RAM) [27,28].On the other hand, GST materials had also been used to develop nanostructured PCM for short-period superlattices, which were also known as interfacial phase change materials [29,30].The researchers noted that the GST material compound system had four typical hexagonal layered structures, and the relative stability of these structures was related to heat or temperature [31].
GST materials revealed obvious tunable phase transition characteristics, while shell materials achieved large bandwidth thermal emission properties.Due to the fact that phase distortion and intermediate states do not require continuous external energy consumption to maintain, GST materials were widely used in various fields, such as optical storage [32], absorbers [33], color devices [34], and so on.On the other hand, new materials with novel properties were constantly discovered and applied.For example, Polybutylene Adipate-co-Terephthalate (PBAT) particles had been noted for their good mechanical properties and biodegradability, and had been applied to the preparation of three-dimensional structures with high compressive strength and energy absorption properties [35].For example, tunable 4D print samples could be prepared using PLA-TPU blends.The shape memory effect of the sample could be modulated by controlling the input energy and temperature to obtain the desired consolidation and stress recovery properties [36].In addition, the reuse of shell materials was a new hot spot in the field of environmental protection materials.By using processes such as, grinding or calcination, shell materials were usually prepared into nanoscale particles and widely used as adsorbent or antibacterial materials.However, shell materials were not widely used in the metamaterials development.The absorption and thermal emission properties of shell materials for electromagnetic waves were not received attention from researchers.In the proposed metamaterial thermal emitter structure, the GST layer was defined by a hexagonal hole array and served as a tunable thermal emission resonator, while the shell material served as the intermediate layer to provide the basic thermal emission performance.Due to the completely opposite thermal emission properties between the completely amorphous and crystalline states of GST arrays, the switching of thermal emission performance between two band windows was achieved by the proposed metamaterial structure.Numerous intermediate states between completely amorphous and crystalline states by the this metamaterial structure without external energy maintenance.In addition, the thermal emission performance of the metamaterial structure could be enhanced by increasing the thickness of the shell materials.

Structural design
The proposed metamaterial structure scheme was shown in figure 1.The metamaterial adopts a multi-layer stack structure, including: the bottom layer was a complete metal layer (a silver layer), the middle layer was a shell material layer, and the top layer was a GST layer defined by an array of hexagonal holes.In this paper, simulation software HFSS was used to obtain the intensity of the electric field distribution and the simulated absorption of the metamaterial unit.In the proposed metamaterial unit modeling process, all the relevant properties of the three materials (including: relative permittivity, relative permeability, dielectric loss coefficient, etc) were automatically matched by the simulation software without manual input.The scanning range in simulations was 1.0-45.0THz.An ideal electrical boundary was adopted on the x-axis boundary of the structural unit, and an ideal magnetic boundary was set to be the y-axis boundary [37], so as to eliminate the energy loss in the horizontal direction between the structural units.Perfectly matched layers were used on the top and bottom of the structural unit to eliminate dispersion loss, and the scan step size was 0.01 THz.Electromagnetic waves were incident from directly above the structural unit.The metamaterial structural parameters were shown in table 1.

Simulation and calculation
The resonant properties of the GST layer were shown below [38]: In equation (1), Q scat was set to be the ratio between the geometrical and scattering cross-sections, R was the radius of a sphere, and l was the derivative, p l = k 2 was set to be the wave-number, the a m was the contribution of electric multipole coefficient, and the b m was the contribution of magnetic multipole coefficient.Here, the resonance electric and magnetic were a m and b , m which could be revealed as follows: Here, y ( ) nkR was set to be first Riccati-Bessel function, x ( ) kR m was set to be second Riccati-Bessel function.n was set to be refractive index of GST.The resonance properties of the SU-8 layer were set up according to this reported work [39].The resonant properties of metal layers were shown below:  In equation ( 5), g = ´s 9 10 D 13 1 and w = ´s 1.37 10 p 16 1 were set to be the collision and plasma frequency, respectively [40].The parameter w was the calculated frequency.

Samples
Our samples could be prepared according to the following steps: (a) A silicon nitride layer (2 mm thick) was set to be a temporary substrate, which should be cleaned and dried.Then this temporary substrate should be fixed in a glue-dumping machine (Applying: MSC-400Bz-6 N spinner).The temporary substrate was covered by a 5 μm SU-8 layer through this MSC-400Bz-6 N spinner (All experimental parameters: the spin time was 5.5 min, and the spin speed was 1850 rpm).Finally, this temporary substrate should be placed on a hot plate (Applying: C-MAG HP10, the temperature was set to be 95 °C), and the SU-8 layer was dried and cured.(b) This substrate was fixed in the evaporation chamber of the vacuum coating machine (ZZL-U400C), and the silver layer was deposited on this SU-8 layer (All experimental parameters: the working pressure should be 55e −10 (atm), vacuuming time should be 6.5 h, and the warm-up time should be 30 min).(c) After the silver layer was cooled, the shell layer was deposited on the silver layer by the same vacuum coating machine (ZZL-U400C, all experimental parameters: the working pressure should be 55e −10 (atm), vacuuming time should be 6.5 h, and the warm-up time should be 55 min).(d) After the shell layer was cooled, the GST layer was deposited on the shell layer by the same vacuum coating machine (ZZL-U400C, all experimental parameters: the working pressure should be 55e −10 (atm), vacuuming time should be 6.5 h, and the warm-up time should be 52 min).(e) The SU-8 layer was removed with acetone, and the semi-finished product was cleaned by acetone and ultrapure water, and also was dried again with a hot plate (Applying: C-MAG HP10, the temperature was set to be 92 °C).(f) This semi-finished product was fixed in an etching machine CABL-9000C and the proposed hexagonal hole array was defined on the GST layer, as shown in figure 1(c).(g) The achieved samples were cleaned by the ultrasonic equipment.Absorbance of the samples was obtained using the device Bruker Optics Equinox 55, as shown in figure 2, the excitation port was set to be 8 cm, and the receiving port was set to be 10 cm.The device Bruker Vertex 70 FTIR was used to obtain the emissivity of the samples, as shown in figures 3 and 4. (h) Images of the samples were obtained with the device Leica DM2700 M, as shown in figure 1(c).

Results and discussion
Our suggested metamaterial sample contained multiple layers of material (metal layer, GST layer, shell material layer).In the process of penetrating the metamaterial sample, the energy of electromagnetic wave was lost due to the dielectric absorption of various materials.According to Kirchhoff's law of thermal radiation, the absorption property of the sample was directly related to the thermal emission property, and the absorption rate was approximately equal to the emissivity.In the first experiments, the absorption properties of the metamaterial samples was measured, as shown in figure 2. At room temperature, the GST layer was in an amorphous state, and a very weak absorption peak (amplitude 0.223, resonance wavelength 7.09 μm) was achieved by the metamaterial samples.Since the thickness of the underlying metal contained in the metamaterial sample reached 2.0 μm, which was much larger than the penetration depth of electromagnetic waves, the transmittance was zero.The absorption rate could be expressed according to the following equation: For the amorphous state of the GST layer, most of the energy of the electromagnetic wave was lost by the reflection of the metamaterial sample, and only a part of the energy was absorbed due to the dielectric loss, resulting in a very weak absorption peak (red curve) in figure 2. In the experiments, the annealing temperature of this metamaterial samples was increased to 150 °C, the annealing time was set to 4 min, and the GST layer was in a fully crystalline state.A strong absorption band (Mean amplitude 0.873, Central resonance wavelength 20.27 μm, Resonance bandwidth 6.22 μm) was excited by this metamaterial sample, as shown by the black curve in figure 2. Clearly, the metamaterial samples exhibited distinct absorption properties due to the switching of the GST layer between amorphous and crystalline states.
According to Kirchhoff's law of thermal radiation, the thermal emission properties of this metamaterial sample were similarly switchable.Under amorphous conditions, the thermal emissivity of the structure was measured and simulated, as shown in figure 3.At resonance wavelength 7.11 μm, a thermal emission peak 0.212 was found, while at other frequency bands, the average thermal emissivity of the metamaterial sample was 0.114.This was because the GST layer in amorphous condition is approximately transparent to electromagnetic waves.Most of the energy of electromagnetic waves was reflected by the underlying metal layer and can't be absorbed.Such resonance behavior resulted in the thermal emissivity amplitude of the metamaterial samples reaching only 0.114, as shown in figure 3.Meanwhile, the simulated emissivity peak of this structural unit was 0.168.Therefore, when the GST layer was in an amorphous state, the metamaterial sample could be defined as a  The resistance loss intensity and electric field intensity distributions of structural elements were simulated, respectively, as shown in figures 5 and 6.The intensity of resistance loss and electric field were important factors to determine the amplitude of emissivity.Under the condition of no definite shape, the resistance loss of electromagnetic wave was suppressed due to the transparent characteristics of the GST layer, as shown in figure 5(a).Most of the energy of the electromagnetic wave was reflected into the air by the bottom metal layer, resulting in the suppression of the resonance absorption behaviors, as shown in the red curve in figures 2 and 3. Synchronously, the resonant electric field intensity of GST layer was also inhibited under the condition of no definite morphology, as shown in figure 6(a).The resonant thermal emission behaviors of electromagnetic wave in this frequency band were suppressed.This was because the following relationship between the resistance loss and the resonant electric field: It should be pointed out that under the condition of no definite shape, the energy loss of the bottom metal layer for electromagnetic wave was also inhibited.The resonant electric field intensity of the bottom metal layer was approximate to that of the GST layer, as shown in figure 6(c).The energy of electromagnetic waves was reflected into the air instead of being lost by the metal layer.When the GST layer was in the crystalline state, the resistive loss of electromagnetic waves was enhanced, as shown in figure 5(b).At the same time, the resonant electric field of the GST layer was also enhanced synchronously, as shown in figure 6(b).The thermal emission peaks in figure 4 were excited by these resonance behaviors.It should be pointed out that under the crystalline condition, the bottom metal layer still acts as an electromagnetic wave emitter and does not show a strong energy loss behaviors, as shown in figure 6(d).

GST layers with different thicknesses
The measurement results in figures 3 and 4 indicated that the GST layer had a direct effect on the thermal emission properties of this metamaterial sample.The thermal emission spectra under different GST layer thickness conditions were measured, as shown in figures 5 and 6.In the experiments, the thickness of the GST layer was set to t1 = 4.00 μm, t1 = 6.00 μm, and t1 = 8.00 μm.Since the phase transformation of the GST layer requires an annealing treatment, we first measured the thermal emission properties of the three samples under amorphous conditions, as shown in figure 7. The amplitudes of three thermal emission peaks were: 0.212, 0.224, and 0.241, respectively, and the resonant wavelengths were: 7.11 μm, 7.62 μm, and 8.01 μm, respectively.Based on these measurement results, the thickness of the GST layer does not directly affect the emission properties of metamaterial samples (maximum difference in amplitude 0.2).Because the electromagnetic waves absorption by this GST layer (with a smaller refractive index) was suppressed in the amorphous condition (the red curve in figure 2).When the thickness of the GST layer was increased to t1 = 8.00 μm, the emission amplitude was 0.241.The increase in emissivity was mainly due to the dielectric loss of the GST layer to electromagnetic wave energy.The samples with different thicknesses of GST layers were annealed (the annealing temperature was 150 °C, the annealing time was 4 min ) separately, and the measured emission spectra were shown in figure 8. Obviously, the thermal emission properties of the metamaterial samples are enhanced.The mean amplitudes of the three  emission bands are: 0.857, 0.892, and 0.933, and the central resonance wavelengths are: 20.23 μm, 24.57μm, and 28.66 μm, respectively.Resonance bandwidths are: 6.16 μm , 6.34 μm, and 6.65 μm.Comparing the measurement results in figures 7 and 8, it can be seen that obvious switchability of emission properties were exhibited by the metamaterial samples, in the crystalline state, the thermal emission properties exhibit an 'ON' resonance behavior, while in the amorphous state, the thermal emission window is 'OFF'.

Effect of annealing temperature on emission properties
The crystallinity and phase transition of the GST layer were depended on the annealing temperature and annealing time, therefore, it is necessary to reveal the effect of temperature on the emission properties of this metamaterial sample.In the following experiments, the annealing temperatures of the metamaterial samples were set to 100 °C, 110 °C, 120 °C 130 °C, 140 °C, and 150 °C, respectively, as shown in figure 9.When the annealing temperature was increased to 110 °C, the emission amplitude was increased from 0.212 to 0.325, as shown by the red curve in figure 9.When the annealing temperature was further increased, the emission amplitude of the metamaterial sample was continuously enhanced, and the synchronous resonance wavelength was continuously shifted to long wavelengths, as shown in figure 9.These measurements indicated that the emission properties of this metamaterial sample were obvious continuous tunability.It should be pointed out that the crystallinity of the GST layer was temperature-stable.Once any one of the intermediate states of the GST layer was annealed, there was no need to continuously consume an external heat source.Therefore, any one of the emission peaks measured in figure 9 was stable at room temperature.The detailed measurement results  under different annealing conditions were shown in figure 10.According to the statistical data in figure 10, the sensing sensitivity of the metamaterial heat emitter during temperature rise was as follows: Where, l D was the central resonance wavelength shift amplitude, DT was the temperature difference, S was the sensitivity.The measurement results of sensing sensitivity were: S 1 = 280 nm/°C, S 2 = 310 nm/°C, S 3 = 322 nm/°C, S4 = 337 nm/°C, and S 5 = 341 nm/°C.These measurements indicate that the metamaterial thermal emitter was sensitive to ambient temperature and had the potential to be applied to temperature sensing.This metamaterial sample exhibited an approximately linear relationship between the emission amplitude and annealing temperature, and a similar relationship existed between the synchronous, resonant wavelength and annealing temperature.The emission properties (amplitude and resonance wavelength) of this metamaterial sample could be selectively controlled by the annealing temperature.
On the one hand, the tunability of this metamaterial sample was related to the crystallinity of the GST layer.The resonant properties (dielectric constant, refractive index) of the GST layer were directly related to the degree of crystallinity.Therefore, the equivalent medium theory was employed to explain the frequency tunability of this metamaterial sample [41]: Resonance frequency can be achieved based on these effective medium parameters: According to the equivalent medium theory, the resonant wavelengths of the structural unit under different temperature conditions were:7.12μm, 9.87 μm, 13.11 μm, 16.08 μm, 18.10 μm, 20.31 μm.According to formula (12), the refractive index of the dielectric layer had a direct influence on the resonance frequency.The refractive index of shell material was independent of temperature, Therefore, the tunability of the resonance frequency should come from the phase transition of the GST layer.To verify the relationship between the refractive index of the GST layer and the annealing temperature, the refractive index of the GST layer was extracted, as shown in figure 11.The real part of the refractive index of the GST layer was increased with annealing temperature, resulting in an increase in the resonant frequency in figure 9 .At the same time, the imaginary part of the refractive index was also increased with the increase of the annealing temperature.The coefficient loss of the electromagnetic wave was enhanced in the process of penetrating the GST layer due to the huge imaginary part of the refractive index.On the other hand, the emission amplitude of this metamaterial sample was enhanced (as shown in figure 9) because the dielectric loss of the GST layer was enhanced with annealing temperature.According to the equivalent medium theory, the dielectric properties of the GST layer could be defined using the Lorentz-Lorenz relation [42]: e l e l e l e l e l e l In the equation above, e a was set to the permittivity of amorphous, e C was set to permittivity of crystalline.According to formula (13), the dielectric constant of the GST layer was increased with the annealing temperature, and simultaneously, the imaginary part of the refractive index was also strengthened.These two factors cause the dielectric loss of the GST layer to be enhanced for electromagnetic waves.The dielectric loss intensity distributions of the GST layer under different annealing temperature conditions were shown in figure 12.With the increase of the annealing temperature, the resonant current on the surface of the GST layer was strengthened, forming a strong induced electric field.The electromagnetic wave was coupled with the induced electric field in the process of penetrating the GST layer, which excited the local resonant electric field.The energy of the electromagnetic wave was confined and absorbed by the local resonant electric field, which was transformed into the thermal emission resonance behaviors of the sample, as shown in figure 12.
In order to further illustrate the importance of shell material to the metamaterial structure, the thickness of shell layer was gradually increased in the final comparison experiment (using the same measurement conditions and equipment as in figure 2).The measurement results are shown in figure 13.The thickness of the shell layer is set to be: t2 = 8.0 μm, t2 = 10.0 μm, t2 = 12.0 μm, t2 = 14.0 μm , and t2 = 16.0 μm.When the GST layer is in a completely amorphous state, the original emission peak was gradually strengthened, and the amplitude was increased from 0.212 to 0.365.When the thickness of the shell was exceed t2 = 14.0 μm, the amplitude of the emission peak was no longer continuously increased, as shown in figure 13(a).When the GST layer was in the crystalline state (the temperature was raised in the experiment to achieve the phase transition condition).The emission band of the metamaterial was also strengthened.The mean values of emission bands measured were: 0.857, 0.873, 0.931, 0.961, and 0.964, respectively, as shown in figure 13(b).Therefore, the thermal emission properties of the metamaterial were enhanced by increasing the thickness of the shell material.It should be pointed out that when the thickness of the shell material was exceed t2 = 14.0 μm, the thermal emission performance of the metamaterial does not increase.Thickness was the core factor that determines the thermal emission capacity of shell materials.When the thickness was exceed t2 = 14.0 μm, the thermal emission properties of the shell material was tended to be saturated and no longer provide additional strengthening.By comparing the measurement results in figures 9 and 13, it can be seen that the enhancement effect of shell material on the emission performance of the metamaterial was different from that of GST layer.In figure 9, the emission peak of the metamaterial was gradually strengthened.However, the thermal emission outside the emission peak range were not increased, as shown in figure 9.This was because the thermal emission properties outside the emission peak range were mainly provided by shell materials in metamaterial structures.Therefore, in figure 13, due to the increase in thickness, the thermal emission performance of the shell material was enhanced, which resulted in the overall enhancement of the thermal emission performance of the metamaterial, as shown in figure 13.More importantly, no matter whether the GST layer was in the crystalline state or the amorphous state, the thermal emission peak and the waveform of the thermal emission band were not changed with the thickness of the shell materials, but only the amplitude was increased (the resonance frequency was kept stable), as shown in figure 13.Such measurement results were completely different from figure 9. Therefore, the basic thermal emission properties of the metamaterial was provided by the shell materials, while the the resonant thermal emission peak (amorphous state) or thermal emission band (crystalline state) was supported by the energy absorption of the GST layer to electromagnetic waves.The measurement results in figures 7 and 8 also show that the basic thermal emission performance of the metamaterial was not significantly enhanced even if the  thickness of the GST layer was increased, but only the thermal emission peak or emission band (amplitude and resonance frequency) was changed.Therefore, the shell material was important for the thermal emission properties of the metamaterial.

Conclusion
In this paper, a metamaterial with tunable dual-band thermal emission properties was proposed and confirmed.Based on two distinct phases of the GST layer, a emissivity peak (0.212 at 7.11 μm ) and a thermal emission band ( average amplitude of 0.857 and a bandwidth of 6.16 μm) were obtained separately.In addition, based on a series of stable intermediate states in the GST layer, the thermal emission window of the metamaterial sample could be effectively modulated by changing the ambient temperature.The metamaterial heat emitter was sensitive to ambient temperature, and the temperature sensitivity was 341 nm/°C.At the same time, the wavelength and amplitude of the metamaterial emitter could be widely selective by changing the thickness of the GST or shell layers.On the one hand, this metamaterial thermal emitter maybe used in many fields, such as light sources, infrared imaging, photovoltaics, temperature sensors, and radiation coolers.On the other hand, the sensitivity and thermal emissivity of this metamaterial sample need to be further enhanced to meet the challenges of industrial applications.

2 Figure 1 .
Figure 1.(a) Resonance hole array of metamaterial thermal emitter.(b) Side view of metamaterial thermal emitter.(c) Optical photos of metamaterial thermal emitters.The yellow part represents GST, the green part represents the shell layer, and the red part represents the metal layer.

Figure 2 .
Figure 2. The absorption measurement results of the suggested metamaterial sample under different conditions.The black curve corresponds to the crystalline state of GST, and the red curve corresponds to the amorphous state of GST.

Figure 3 .
Figure 3. Simulation and measurement results of emissivity (GST is in a completely amorphous state).

Figure 4 .
Figure 4. Simulation and measurement results of emissivity (GST is in a crystalline state).

Figure 5 .
Figure 5. Resistance loss (Q) distribution of the GST layer.(a) GST was in a completely amorphous state at resonance wavelength 7.11 μm.(b) GST was in a crystalline state at resonance wavelength 20.24 μm.

Figure 6 .
Figure 6.Distribution of electric field intensity in GST layer.(a) GST was in a completely amorphous state at resonance wavelength 7.11 μm.(b) GST was in a crystalline state at resonance wavelength 20.24 μm.Electric field intensity distribution of the bottom metal layer.(c) GST was in a completely amorphous state at resonance wavelength 7.11 μm.(d) GST was in a crystalline state at resonance wavelength 20.24 μm.

Figure 7 .
Figure 7. Emittance measurement results of the metamaterial under different thickness conditions (GST is in a completely amorphous state).

Figure 8 .
Figure 8. Emittance measurement results of the metamaterial at different thicknesses (GST in crystalline state).

Figure 9 .
Figure 9. Emission spectrum measurements of the metamaterial sample at different annealing temperatures.

Figure 10 .
Figure 10.Statistics of emission spectrum results of the metamaterial sample under different annealing temperatures.

Figure 11 .
Figure 11.Refractive index of GST at different annealing temperatures.(a) The real part.(b) Imaginary part.

Figure 13 .
Figure 13.(a) Measurement results of the thermal emissivity of the metamaterial under different thickness of shell layer, GST layer is in an amorphous state.(b) Measurement results of the thermal emissivity of the metamaterial under different thickness of shell layer, GST layer is in the crystalline state.

Table 1 .
All of the metamaterial structural parameters.