Self-Adaptive Multistage Infrared Radiative Thermo-Optic Modulators Based on Phase-Change Materials

Abstract

Phase-Change Materials (PCMs) are widely applied in dynamic optical modulation due to the dramatic changes in their complex refractive index caused by temperature variation. As the functionality varies, the application of a single PCM cannot meet the compact, efficient and broadband needs of optical modulators. In this work, we combine vanadium dioxide (VO₂) and a chalcogenide (Ge₂Sb₂Te₅ (GST) or In₃SbTe₂ (IST)) to obtain a VO₂–GST/IST multiple-stack film that is optimized by a genetic algorithm. This film has a wide spectrum and high modulation properties with three self-switchable modes varied by temperature, including transmission, absorption and reflection. The optimal results are an average normal transmittance, absorbance, and reflectance of 0.76, 0.91, 0.86 in 3–5 μm and 0.72, 0.90, 0.90 in 8–14 μm under different temperature ranges. The film enhances the transmission and absorption properties due to the formation of anti-reflective coating and Fabry–Perot resonance. Compared with GST, the film maintains high reflectance due to the metal-like interface reflection of crystalline IST, which exhibits metallic properties. For different polarization states, the film demonstrates great directional insensitivity when the incidence angles vary from 0° to 60°. The designed self-adaptive multistage infrared radiative thermo-optic modulator has promising implications for optical fuse, fiber-optic communication and energy storage fields.

1. Introduction

Thermo-optic modulators are widely used in optical communication, photonic circuits, data encoding, and optical metrology and spectroscopy, and their performance is closely related to the efficiency of optical devices [1,2]. With the development of science and technology, compact, efficient, and broadband thermo-optic modulators urgently need to be developed. Total transmittance, absorbance and reflectance are dependent on each other based on the conservation of energy. Hence, the modulation of the three properties can be applied to optical fuses [3], smart windows [4,5], optical absorbers [6], Anti-Reflective Coatings (ARCs) [7,8] and so on. Specifically, as an example, optical fuses in the optical system can block an excessive quantity of heat transferring into the devices by modulating the transmittance, absorbance and reflectance of the film [3]. Given the above, researchers can exploit many advanced materials such as electrically induced phase-change materials [9,10,11], thermally induced phase-change materials [12,13,14,15,16,17,18,19,20,21,22], grapheme [2,23,24,25,26], and liquid crystals [27,28,29] to achieve these performances. Phase-Change Materials (PCMs) are extensively used in thermo-optic regulation fields due to their phase transition properties. Therefore, their micro-nanostructures are often designed to achieve the self-adaptive adjustment of radiative properties [30], and even multistage adjustment.

Among PCMs, vanadium dioxide (VO₂) has attracted the wide attention of researchers for its phase transition temperature t_c, which is closer to room temperature. It undergoes the transition from dielectric state to metallic state to accomplish optical tuning when the temperature is above t_c. Due to its properties of dielectric to metallic phase transition, VO₂ has been widely applied to achieve intelligent thermal and optical regulation [12,17,19]. On the other hand, Germanium telluride antimony (GST) and indium telluride antimony (IST) do not require external electric or thermal energy to maintain the desired radiative properties [31]. Therefore, they have more important significance in the fields of multispectral regulation, thermal camouflage, radiative cooling and so on [13,14,15,16,18,20,22]. When the temperature is above their phase transition temperature t_s, GST/IST switches from an amorphous to crystalline state, accompanied by significant changes in the complex refractive index. Meanwhile, most current thermo-optic modulators are two-phase-type modulated by a single PCM [32,33,34,35,36,37,38,39], such as hybrid and orthogonal junction modulators with VO₂, plasmonic modulators with GST and reflection modulators with Sb₂S₃.

At present, the simulation of the optical adjustment of the optical properties is particularly required for active photonics. With only one type of PCM, it is highly challenging to accomplish a gradual process of heat change across a constrained temperature range [40]. Fully utilizing the advantages of different PCMs can meet the multistage modulation requirements of various optical components. For example, Kasali et al. [41] used two PCMs, GST and VO₂, to make radiative heat diodes and analyzed the rectification coefficients of three shapes; they found that the rectification factors of radiative heat diodes made using two PCMs were higher than when using a single via theoretical derivation. Lyu et al. [3] combined GST/IST and VO₂ to research the thermal modulation switching of the transmission, absorption, and reflection mode in 1.5–4.5 μm, and explored the effects of the annealing time and thickness on the final results to achieve the switch of working wavelengths. However, the modulator only exhibited the great single-wavelength switching of the transmission, absorption and reflection mode near 2.8 μm. Liu et al. [42] proposed a near-field multistage radiative thermal rectifier with VO₂ and GST, and explored four types of active terminal structures for multistage thermal rectification, including multi-film and composite nanograting structures. The results confirmed that having two PCMs at the active end plays an important role in the heat flux multistage modulation of a radiative thermal rectifier. Meng et al. [40] designed structures using GST and VO₂ to maximize the optical contrast between four different reflective states. Nevertheless, they only optimized the single VO₂ and GST layer thicknesses, which lacks flexibility, and the switch of the VO₂/GST/Al film can only operate in the reflectance mode in a narrow spectral range. Hence, there have been relatively few studies on the combination of two PCMs in recent years. Most of the proposed PCM-based structures are composed of only a single-stack film or a patterned structure with a limited modulation of the optical properties. Research on the multistage regulation of different wavelength ranges in thermo-optic fields is lacking. Therefore, there is still much room for optimization in terms of thermo-optic properties.

Herein, it is promising to consider combining two or more PCMs to take advantage of their phase-change performance. Moreover, the phase transitions of VO₂ and GST/IST do not affect each other, which makes the hybrid structures more widely and flexibly applied [40]. In this work, we combine VO₂ with Ge₂Sb₂Te₅ (GST) or In₃SbTe₂ (IST) to propose a self-adaptive multistage infrared modulator that can be switched between the transmission mode, absorption mode and reflection mode. The VO₂–GST/IST multiple-stack film is selected to be designed and optimized due to its ease of manufacture. Then, the optimal thicknesses of 3–5 μm and 8–14 μm are obtained using the genetic algorithm and employed to calculate the radiative characteristics of the VO₂–GST/IST multiple-stack film. Finally, the self-adaptive multistage infrared modulator is operated in the wide spectrum with a high modulation. The proposed modulator can provide a fundamental platform for smart and switchable optical components to achieve multistage modulation under different temperature conditions.

2. Design and Optimization of Multiple-Stack Film

The multiple-stack film is composed of GST/IST layers and VO₂ layers arranged alternately, as shown in Figure 1a. The VO₂ layers undergo a reversible insulator-to-metal phase transition at 68 °C [43,44,45,46], while the GST/IST layers undergo a crystallization process at 160 °C/300 °C for an amorphous-to-crystalline phase transition [15,47,48,49,50]. Therefore, the multiple-stack film undergoes three different states to achieve the switch of the transmission, absorption, and reflection mode, as the temperature gradually increases to 68 °C and 160 °C/300 °C. As shown in Figure 1b, when the temperature is below 68 °C (the phase transition temperature of VO₂, t_c = 68 °C), neither dielectric VO₂ (dVO₂) nor amorphous GST/IST (aGST/aIST) undergoes phase transition. The multiple-stack film is in the dielectric or amorphous state and the penetration capacity of the incident wave is strong. When the temperature is between 68–160 °C for GST and between 68–300 °C for IST (the phase transition temperature of GST/IST, t_s = 160 °C/300 °C), only VO₂ undergoes phase transition from the dielectric state to metallic state (mVO₂). At this time, GST/IST appears in the amorphous state and VO₂ appears in the metallic state. When the VO₂ is thin enough, the incident wave can penetrate the VO₂ layer. The incident wave energy is gradually absorbed due to the increase in the propagation distance, which makes the film demonstrate absorption properties. Therefore, the VO₂–GST/IST multiple-stack film exhibits an absorption mode in this temperature range. When the temperature is above 160 °C/300 °C, both VO₂ and GST/IST undergo phase transition. VO₂ behaves in the metallic state and GST/IST behaves in the crystalline state (cGST/cIST), resulting in an increased proportion of reflected waves.

At present, there are many methods used for the fabrication of multiple-stack films, such as Magnetron Sputtering (MS), Sol–gel, Chemical Vapor Deposition (CVD) and Pulsed Laser Deposition (PLD) [51]. During the fabrication, considering the inevitable need to grow the VO₂ layer on the GST/IST layer under high-temperature conditions, the higher temperature will cause the GST/IST to transform into the crystalline state, and the change in crystallization is non-volatile. At this time, a reamorphization operation is required to transform the GST/IST into the amorphous state [15,16], and finally the VO₂ is inverted into the dielectric state via an annealing process. Thus, the high transmission mode of the film at a low temperature is obtained.

At present, the Rigorous Coupled Wave Analysis (RCWA) method is efficient and precise in its ability to solve Maxwell’s equations, so the reflectance and transmittance of the multiple-stack film are calculated using it [12,52,53]. Considering the differences in the complex refractive indices under various preparation methods, the complex refractive indices are taken from refs. [3,54] for VO₂, ref. [14] for GST, and ref. [3] for IST, as shown in Figure 2. The absorbance is equal to unity minus the reflectance and transmittance (A = 1 − R − T), according to the concepts of absorption (A), reflection (R), and transmission (T). Then, using the genetic algorithm can optimize the radiative characteristics of the multiple-stack film, and the optimization parameters are thickness and the number of layers. It is noteworthy that the objective function in the genetic algorithm is the crucial information, since it determines the search direction of the optimization process. Due to the relatively short research spectral range, the integral average value is directly taken within the spectral range to increase the universality of the multiple-stack film. The objective function in this work is set as follows:

$$F\left(h_1,h_2,\mathrm{...},h_n\right)=\left(1-{\overline{T}}_d\right)+\left(1-{\overline{A}}_m\right)+\left(1-{\overline{R}}_c\right)$$

(1)

$${\overline{T}}_d=\frac{{\displaystyle {\int }_{{\lambda }_1}^{{\lambda }_2}{T}_d(\lambda )d\lambda }}{{\lambda }_1-{\lambda }_2}, {\overline{A}}_m=\frac{{\displaystyle {\int }_{{\lambda }_1}^{{\lambda }_2}{A}_m(\lambda )d\lambda }}{{\lambda }_1-{\lambda }_2}, {\overline{R}}_c=\frac{{\displaystyle {\int }_{{\lambda }_1}^{{\lambda }_2}{R}_c(\lambda )d\lambda }}{{\lambda }_1-{\lambda }_2}$$

(2)

where h₁, h₂, …, h_n is the thickness of the nth layer, individually. ${\overline{T}}_d$, ${\overline{A}}_m$ and ${\overline{R}}_c$ are, respectively, the integral average value of normal transmittance for the dVO₂–aGST/aIST state, normal absorbance for the mVO₂–aGST/aIST state and normal reflectance for the mVO₂–cGST/cIST state, while $T_d\left(\lambda \right)$, $A_m\left(\lambda \right)$ and $R_c\left(\lambda \right)$ are the spectral normal transmittance, absorbance, and reflectance when the wavelength is between λ₁ and λ₂. In addition, the transmission, absorption and reflection modes mentioned in the work are all characterized by the average normal transmittance, absorbance and reflectance.

3. Calculation Results and Discussion

In this section, a detailed study is carried out for VO₂ made using the sol–gel method, and the optimized results of the VO₂–GST/IST multiple-stack film in the 3–5 μm and 8–14 μm spectral wavelength are shown in

Table 1. Results of optimized thicknesses and corresponding optical parameters for the VO₂–GST/IST multiple-stack film for different numbers of layers in 3–5 μm and 8–14 μm.

Structure	Waveband (μm)	Optimized Thicknesses (μm)						${\overline{\mathit{T}}}_{\mathit{d}}$	${\overline{\mathit{A}}}_{\mathit{m}}$	${\overline{\mathit{R}}}_{\mathit{c}}$
		h1	h2	h3	h4	h5	h6
VO2–GST multiple-stack film	3–5	0.163	0.448	-	-	-	-	0.63	0.84	0.63
		0.165	0.174	0.049	0.214	-	-	0.67	0.84	0.62
		0.172	0.142	0.193	0.213	0.390	0.311	0.67	0.86	0.60
	8–14	0.563	1.138	-	-	-	-	0.59	0.79	0.61
		0.558	0.140	0.515	0.956	-	-	0.65	0.86	0.54
		0.566	0.144	0.522	1.864	0.673	1.019	0.72	0.86	0.55
VO2–IST multiple-stack film	3–5	0.157	0.480	-	-	-	-	0.71	0.88	0.86
		0.158	0.111	0.127	0.273	-	-	0.74	0.90	0.86
		0.165	0.121	0.201	0.677	0.853	0.366	0.76	0.91	0.86
	8–14	0.574	1.213	-	-	-	-	0.67	0.80	0.90
		0.580	0.106	0.511	0.761	-	-	0.72	0.90	0.90
		0.580	0.106	0.056	0	0.454	0.762	0.72	0.90	0.90

. From Table 1, it can be seen that the increase in the number of film layers leads to better modulation for the VO₂–GST/IST multiple-stack film, which is in the spectral wavelength of 3–5 μm and 8–14 μm. Moreover, further increasing the number of optimized layers has no significant enhancement effect. In detail, for the VO₂–GST multiple-stack film, the modulation effect is optimal when the number of layers is increased to four in 3–5 μm and to six in 8–14 μm. For the VO₂–IST multiple-stack film, the modulation effect is optimal when the number of layers is increased to six in 3–5 μm and to four in 8–14 μm. Next, based on these optimized thicknesses and number of layers, the spectral normal transmittance, absorbance, and reflectance of the VO₂–GST/IST multiple-stack film are calculated, respectively, at different temperature ranges (Figure 3). As can be seen from Table 1 and Figure 3, the VO₂–GST/IST modulator can be dynamically switched between the high transmission, absorption and reflection mode via increasing the temperature from the room temperature to the phase-change temperature (t_c = 68 °C and t_s = 160 °C/300 °C). The optimal properties are calculated as = 0.76, ${\overline{A}}_m$ = 0.91, and ${\overline{R}}_c$ = 0.86 in 3–5 μm when there are six optimized layers in Figure 3b, and ${\overline{T}}_d$ = 0.72, ${\overline{A}}_m$ = 0.90, and ${\overline{R}}_c$ = 0.90 in 8–14 μm when there are four optimized layers in Figure 3d for the VO₂–IST multiple-stack film.

Based on the above results, it can be concluded that, on the one hand, compared with other structures containing single PCMs [12,13,14,15,16,17,18,19,20,21,22], the multiple-stack film can achieve multistage modulation via a two-stage phase transition to obtain better performance. On the other hand, compared with other structures containing two PCMs [40,41,42], the multiple-stack film tends to obtain a large modulation. Through the optimization of the VO₂–IST multiple-stack film, the film exhibits large normal transmittance, absorbance and reflectance over a wider range of the near-infrared wavelength band (3–5 μm) compared with the single-stack structure of Lyu et al. [3].

To clearly demonstrate the mechanisms of the enhanced radiative properties caused by the genetic algorithm optimization, the electric fields of the VO₂–IST multiple-stack film are illustrated at different states in Figure 4, for example. When VO₂ is in the dielectric state and GST/IST is in the amorphous state, both GST and IST are highly transparent media with the small refractive index n and attenuation coefficient κ. It can be seen that the electric field intensity changes slightly after passing through the multiple-stack film from Figure 4a,b, exhibiting high normal transmission properties. Note that in Figure 3a,c, the attenuation coefficient of crystalline GST increases as the wavelength increases. This leads to an increase in absorbance so that the normal reflectance decreases for the VO₂ layers in the metallic state and the GST layers in the crystalline state. Figure 3b shows that there are multiple peaks and valleys in the transmission mode, and the same phenomenon also appears in Figure 3c due to the formation of ARC [7,8]; the enhancement or weakening of interference is displayed by the strength of the electric field in the incident and propagation region. According to Figure 5a, the appearance of multiple normal transmission peaks owing to the reduced reflectance is also the reason for the increased average normal transmittance of the multiple-stack films. When VO₂ is in the metallic state and GST/IST is in the amorphous state, the electric field intensity along the +z direction roughly decreases in Figure 4c,d, but interference effects also appear in the partial diagrams. This indicates that the multiple-stack film exhibits a high normal absorption performance at this time, and the normal absorbance is enhanced due to the formation of the Fabry–Perot resonance cavity as the number of layers is increased (Figure 5b). It is likely that the reflection of the plane wave at the metal-like planar interface of IST [55,56,57] means that the incident wave rarely penetrates the top IST layer. The electric field inside the multiple-stack film is very small, but the incident area is large owing to the strong interference between the incident and reflected wave, as shown in Figure 4e,f. Therefore, the average normal reflectance is high when VO₂ is in the metallic state and IST is in the crystalline state. In addition, the average normal reflectivity of the VO₂–GST multiple-stack film is not high, possibly due to the increase in the normal transmittance caused by cancel interference [7].

In summary, it can be clearly seen that the effects of the VO₂–IST multiple-stack film are better than those of the VO₂–GST multiple-stack film. The most obvious behavior is a high normal reflection mode, as shown in Figure 3, mainly because IST presents the metallic features with strong normal reflection properties in the crystalline state at both 3–5 μm and 8–14 μm. In this condition, the large refractive index and attenuation coefficient are displayed for IST, resulting in a large normal reflection to the incident wave (Figure 4e,f). We subsequently will discuss the relevant analysis using the VO₂–IST multiple-stack film as an example. Meanwhile, the VO₂–GST multiple-stack film will still be used for the transition of the high transmission and absorption mode.

The incident angle of the incident wave is not always normal in various application scenarios, so it is necessary to calculate and analyze the situation of oblique incidence. Figure 6 shows the spectral direction transmittance/absorbance/reflectance contours of the VO₂–IST multiple-stack film in 3–5 μm for different states with the TM and TE wave. When the incident angle is less than 50° for both the TE and TM wave, the multiple interference effect results in an angle-dependent transmission [58], but the direction transmittance changes little throughout 3–5 μm and the value of the direction transmittance stays above 0.6 with the dVO₂–aIST state multiple-stack film (Figure 6a,b). In addition, the direction transmittance is close to unity in the ranges of 3–3.25 μm, 3.7–4 μm, and 4.5–5 μm for the whole oblique incidence angle. Interestingly, possibly due to the decrease in the spectral reflectance, the spectral selectivity of the multiple-stack film decreases at 50°–70° under the TM wave, showing a high spectral direction transmission performance at any wavelength ranging from 3 to 5 μm. For the multiple-stack film with VO₂ layers expressing the metallic state, the spectral direction absorbance seems to be independent of the incident angle when it is less than 60° and stays above 0.8 in 3–5 μm, as shown in Figure 6c,d. Similar phenomenon can be seen in Figure 6e,f for the reflection mode. The spectral direction reflectance changes little when the incident angle is less than 60° and stays above 0.8 for the TM wave, and the spectral direction reflectance increases to almost unity for the TE wave.

Figure 7 illustrates the spectral direction transmittance/absorbance/reflectance contours of the VO₂–IST multiple-stack film in 8–14 μm for different states with the TM and TE wave. As can be seen in Figure 7a,b, for the VO₂–IST multiple-stack film in the dVO₂–aIST state at 10.5–13 μm, the spectral direction transmittance for both the TE and TM wave almost stays above 0.8 when the incident angle is less than 60°. In addition, the TM wave also appears to experience the same phenomenon as that in Figure 6a. When VO₂ layers change from the dielectric state to metallic state, the spectral direction absorbance stays above 0.7 in 8–14 μm. It is insensitive to the incident angle when the incident angle is less than 60°, as shown in Figure 7c,d. Moreover, for the IST layers in the crystalline state, the spectral direction reflectance stays above 0.8 in the incident angle ranging from 0° to 70°, as is shown in Figure 7e,f. The reflectance is always close to unity within a wide spectral range for the TE mode in Figure 6 and Figure 7. This is because the dielectric constant of IST in the crystalline state is similar to that of metal, which has a large refractive index n and attenuation coefficient κ over a wide bandwidth. Additionally, the reflectance also has a relatively wide angular bandwidth, which is similar to the phenomenon observed in refs. [3,59]. This phenomenon can be explained by the fact that the multiple-stack film with a thickness much smaller than the incident wavelength is not sensitive to changes in the incident angle, as discussed in refs. [3,60]. All these data prove that the spectral transmittance, absorbance, and reflectance of the multiple-stack film perform greatly against both the incident angle and wave polarization.

It also should be mentioned that there are certain differences in the complex refractive index of VO₂ layers measured in the literature [3,54], which is possibly caused by the preparation methods and temperatures [12]. However, they slightly impact the radiative characteristics and physical mechanism of the multiple-stack film through calculation (

Table 2. The performances and the optimal parameters of the VO₂–GST/IST multiple-stack film for VO₂ layers synthesized using typical growth techniques.

Growth Technique	Waveband (μm)	Material	Multiple-Stack Film Parameters (μm)						${\overline{\mathit{T}}}_{\mathit{d}}$	${\overline{\mathit{A}}}_{\mathit{m}}$	${\overline{\mathit{R}}}_{\mathit{c}}$
			h1	h2	h3	h4	h5	h6
Sol-gel [3]	3–5	GST	0.165	0.174	0.049	0.214	-	-	0.67	0.84	0.62
		IST	0.165	0.121	0.201	0.677	0.853	0.366	0.76	0.90	0.86
	8–14	GST	0.566	0.144	0.522	1.864	0.673	1.019	0.72	0.86	0.55
		IST	0.580	0.106	0.511	0.761	-	-	0.72	0.90	0.90
Magnetron Sputtering [54]	3–5	GST	0.186	0.114	0.230	0.102	0.394	0.262	0.54	0.85	0.62
		IST	0.171	0.079	0.126	0.143	0.056	0	0.67	0.90	0.86
	8–14	GST	0.555	0.203	0.237	0.103	0.202	-	0.54	0.79	0.59
		IST	0.574	0.132	0.332	0.142	0.292	0	0.59	0.86	0.90

), especially for the absorption and reflection mode. The complex refractive indices of GST and IST can also be adjusted by changing the proportion of elements and the anneal temperature [14,61,62,63,64]. In particular, the GST/IST layer can be crystallized using the laser writing method in the specific regions to obtain a different performance [60]. Meanwhile, the optimal value will be obtained correspondingly through optimization. Additionally, the phase transition temperature of VO₂ can be changed using doping elements such as tungsten (W) [65,66]. In conclusion, the designed structures will be widely applicable to various temperature scenarios.

4. Conclusions

In this work, we propose a multiple-stack film structure combining two phase-change materials (PCMs) alternately, namely VO₂ and Ge₂Sb₂Te₅/In₃SbTe₂ (GST/IST). The multiple-stack film is optimized with the genetic algorithm to obtain a wide-spectrum, high-modulation and self-adaptive multistage infrared radiative thermo-optic modulator. The multiple-film is able to be switched between the transmission, absorption, and reflection mode with three states, respectively, in 3–5 μm and 8–14 μm. The effects of the VO₂–IST multiple-stack film are better than those of VO₂–GST, and the film performs better as the layer number increases. The optimal results are ${\overline{T}}_d$ = 0.76, ${\overline{A}}_m$ = 0.91, and ${\overline{R}}_c$ = 0.86 in 3–5 μm, and ${\overline{T}}_d$ = 0.72, ${\overline{A}}_m$ = 0.90 and ${\overline{R}}_c$ = 0.90 in 8–14 μm for the VO₂–IST multiple-stack film, respectively. Moreover, continuing to increase the number of optimized layers has no significant optimizing effect. The normal transmission and absorption properties of the VO₂–GST/IST multiple-stack film are enhanced mainly due to the formation of Anti-Reflective Coatings (ARC) and Fabry–Perot resonance. Meanwhile, crystalline IST makes the multiple-stack film stay in the high normal reflection mode due to the metal-like interface reflection. Then, the multiple-stack film exhibits good directional insensitivity within the incident angle range of 0°–60° for both the TE and TM wave in three modes. For the typical growth techniques of the VO₂ layers considered here, the radiation characteristics of the film are not significantly different. All in all, the combination of PCMs is a promising research topic, and will promote the design of multistage and multispectral thermal–optic modulation elements.