Any object with a temperature above absolute zero emits infrared thermal radiation, and the thermal radiation spectrum can be described by Planck’s law.1^,2 The mid-infrared (MIR) band of 3 to 7μm is frequently used in both military and civilian applications, as most thermal emissions are concentrated in this range, making it highly significant.3^–6 In addition, polarization-dependent thermal emission has wide applications in fields such as information encryption, target detection, and optical communication.7^–12 However, most thermal emitters in nature produce nonpolarized thermal emission, exhibiting near-zero spin angular momentum.13 Traditionally, generating linearly polarized (LP) thermal emission in the MIR can be achieved using a linear polarizer to filter broadband nonpolarized light emitted by a blackbody.14 Circularly polarized MIR thermal emission is typically produced using an infrared polarization optical system composed of a lens, a filter, a linear polarizer, and a quarter-wave plate for the desired incident wavelength.15 However, these traditional methods lead to low-energy efficiency, low compactness, and bulky devices. Therefore, it is necessary to explore new methods to generate infrared linearly or circularly polarized thermal emission.

In recent years, the use of nanophotonic structures, such as gratings,16^–18 photonic crystals,19 optical antennae,20^,21 and metasurfaces,22^–26 has enabled the generation of spectrally and angularly tailored linearly or circularly polarized thermal emission. The study of generating and manipulating MIR linearly and circularly polarized thermal emission is significant for high-contrast infrared beacons, target detection, and infrared polarization stealth.27^–30 Typically, nanostructured antennas, such as dipole antennas, can exhibit different optical responses to different linear polarizations. These structures can couple and enhance radiation in specific polarization directions, thus generating LP thermal emission. For example, Schuller et al.20 designed an MIR thermal emitter based on SiC nanowires that can produce strong TE and weak TM-polarized thermal emission through the interaction of Mie resonances and surface phonon polaritons. On the one hand, the study showed that by changing the radius of the nanowires, nanostructured antenna-based thermal emitters could achieve control over LP thermal emission. On the other hand, optical structures that simultaneously break mirror and inversion symmetries can produce chiral optical responses and have different optical responses to left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light.31^–34 For instance, Nguyen et al.35 proposed a chiral metasurface that can generate circularly polarized thermal emission with a degree of polarization (DoP) greater than 0.5 over a broad spectral range of 5 to 7μm. Recently, Wang et al.36 proposed a symmetry-broken metasurface with significant circular polarization dependence in the MIR region. The circularly polarized thermal emission of the chiral metasurface can also be controlled by altering the shape of the unit cells. However, these nanophotonic structures can only individually control linearly or circularly polarized thermal emission. In addition, they can only achieve dynamic modulation of polarized thermal emission by altering the structural parameters of the nanophotonic structures, which is not conducive to practical applications. Achieving dynamic control of both linearly and circularly polarized thermal emission simultaneously remains a challenge. Furthermore, infrared polarization imaging systems can easily detect and identify the differences in thermal emission polarization between stealth targets and the natural environment.37^,38 Therefore, there is an urgent need to design a structure that can simultaneously control linearly and circularly polarized thermal emission properties to achieve infrared polarization stealth.

In this paper, we propose a chiral plasmonic metasurface emitter (CPME) composed of asymmetric L-shaped and I-shaped antennas. The emitter consists of In3SbTe2 (IST) antennas, an Al2O3 dielectric layer, and an Au substrate. The CPME can achieve full Stokes parameter control of MIR thermal emission. Using the phase-change material (PCM) IST, this emitter can achieve dynamic modulation of polarized thermal emission without changing the structural parameters. In the amorphous state, the emissivity of various polarized lights of the CPME is generally low, with the DoP generally below 0.2. At this time, the CPME exhibits polarization-independent characteristics. In the crystalline state, the CPME can selectively emit polarized light with different polarization states. Specifically, the total DoP is greater than 0.5 in the range of 3.4 to 5.3μm, the degree of linear polarization (DoLP) is greater than 0.4 in the range of 3.0 to 5.1μm, and the degree of circular polarization (DoCP) is greater than 0.4 in the range of 4.5 to 5.6μm. The physical mechanisms of polarized thermal emission are further explored based on Kirchhoff’s law by calculating the near-field distribution and power loss distribution. The simulated DoP, DoLP, and DoCP are then provided to demonstrate full Stokes parameter control of the thermal emission. Finally, numerical calculations indicate that the proposed CPME has potential applications in infrared polarization detection and antidetection.

To achieve dynamic control of the full Stokes vector in the MIR band, we designed a CPME based on the PCM IST. As shown in Fig. 1(a), the metasurface consists of a bottom Au substrate, a middle Al2O3 dielectric spacer layer, and a top IST antenna array. The top antenna array is composed of asymmetric L-shaped and I-shaped PCM IST antennas.39^,40 Compared with traditional PCMs (such as VO2 and GST), IST exhibits stronger metallic characteristics in its crystalline state.41^–45 For example, the conductivity of crystalline IST is 10 to 1000 times greater than that of crystalline GST, earning it the nickname “bad metal.” This property enhances the response of the metal–insulator–metal (MIM) structure to incident light, enabling polarization-dependent thermal emission. The material properties of commonly used PCMs are detailed in Note S1 in the Supplementary Material. When the PCM IST is in the amorphous state, the metasurface shows a weak response to various polarized light, rendering it polarization-independent. In the crystalline state, IST exhibits strong metallic properties, and the purpose of the dielectric spacer layer is to form a chirality-dependent Fabry–Perot cavity between the asymmetric metal antennas and the opaque metal substrate, thereby enhancing or suppressing the absorption of differently polarized light.46^–48Figure 1(b) shows the schematic of a unit cell of the CPME. From top to bottom, the thicknesses of the three layers are t1=220nm, t2=430nm, and t3=200nm. Figure 1(c) shows a top view of the unit cell. The metasurface has different periodicities in the x and y directions. The structural parameters of the unit cell are Px=1.1μm, Py=1.32μm, antenna lengths l1=1.06μm, l2=0.55μm, antenna width w=0.15μm, gap g=0.05μm, s1=0.175μm, and s2=0.13μm. We can demonstrate the manufacturing feasibility of the proposed CPME through the fabrication methods described in Refs. 11 and 49. First, 200 nm of Au and 430 nm of Al2O3 are sequentially deposited on a silicon substrate using magnetron sputtering. Next, inductively coupled plasma etching was used to obtain the final nanostructures. A limitation of this study is that because the phase transition temperature of IST is 300°C and because its transition rate is relatively low, the rapid switching of IST between amorphous and crystalline states presents a potential challenge for practical applications. In future research, we need to find a new method to facilitate and expedite the phase transition process of IST.

In this work, we performed numerical simulations of the proposed metasurface using the finite element method (FEM).50 In the simulation model, a perfectly matched layer (PML) was used as the open boundary along the z axis. Floquet periodic boundary conditions were applied to the boundaries in the x and y directions. The type of incident source was polarized light incident along the z axis. The incident light wavelength range was set to 3 to 7μm. To ensure simulation accuracy and save time, the mesh size was optimized. The model parameters used in the simulation are shown in Table 1. The refractive index parameters of Au and Al2O3 were sourced from Palik’s book,51 whereas the dielectric constant of IST was sourced from Ref. 52. For the proposed chiral metasurface, the presence of the bottom reflective layer results in zero transmission for the incident light. Due to zero transmission, the absorptivity of incident light can be obtained by calculating the reflectivity Rij. Here, Rij is defined as the ratio of the reflected i-polarized light when j-polarized light is incident, where the subscripts i and j represent LCP, RCP, and LP lights, respectively.53 For example, the absorption rates for LCP, RCP, and x-polarized lights are AL=1−RLL−RRL, AR=1−RRR−RLR, and Ax=1−Rxx−Ryx, respectively. According to Kirchhoff’s law of thermal radiation, ε(λ,θ,p)=α(λ,θ,p), where the emissivity (ε) of a reciprocal system is equal to the absorptivity (α) at thermal equilibrium.54 Here, λ, θ, and p represent wavelength, direction, and polarization state, respectively.

Figures 2(a)–2(f) show the simulated emissivity/absorptivity spectra of the chiral metasurface in the MIR region when IST is in crystalline and amorphous states. It can be observed that when the phase change material IST is in the amorphous state, the metasurface exhibits consistent and very low absorption for various types of polarized light. This is because, in the amorphous state, IST behaves as a dielectric, leading to a weak response to incident light. In addition, the presence of the bottom reflective layer causes most of the incident light to be reflected. However, when the phase change material IST is in the crystalline state, the metasurface shows significant differences in absorption for different polarized light. As shown in Figs. 2(a)–2(f), at the resonant wavelength of 4.7μm, the chiral metasurface’s absorption rates for 0 deg, 45 deg, 90 deg, and 135 deg LP lights, as well as LCP and RCP lights, are 0.88, 0.66, 0.37, 0.57, 0.86, and 0.36, respectively. Corresponding to the coordinate system, the polarization direction of 0 deg polarized light is along the x axis, the polarization direction of 45 deg polarized light is along the symmetry axis of the x and the y axes, the polarization direction of 90 deg polarized light is along the y axis, and so on. When IST transitions from the amorphous to the crystalline state, the absorption differences for orthogonally polarized lights (0 , 90 , 45 , and 135 deg, LCP and RCP) increase from 0 to 0.51, 0.09, and 0.50, respectively. Therefore, the metasurface can utilize the state of IST to control the polarization characteristics of MIR thermal emission.

To elucidate the physical mechanism behind the differential absorption/emission of different polarized light by the CPME proposed in this paper, we simulated the normalized electric-field distribution on the metasurface at the resonant wavelength of 4.7μm under the normal incidence of CP and LP lights. Figure 3 shows the electric-field distribution on the chiral metasurface under different polarized light incidences when IST is in its crystalline state. The electric field distribution in the amorphous state can be found in Note S3 in the Supplementary Material. As shown in Figs. 3(a) and 3(b), when CP light is incident on the metasurface, the local electric field at the gaps of the L-shaped and I-shaped antennas and at the ends of the antennas is significantly enhanced and suppressed, respectively. When LCP light is incident, the electric field is highly localized at the gaps and ends of the two metal antennas, whereas when RCP light is incident, the localized electric-field distribution is almost invisible. Theoretically, the localized electric-field distribution for LCP light results from the excitation of surface plasmon resonance.53 Different field distributions can selectively enhance the interaction between light and matter, leading to spin-selective absorption. According to Kirchhoff’s law of thermal radiation, the CPME exhibits strong circular polarization-dependent emission. For LP light incidence, as shown in Figs. 3(c)–3(f), when 0 deg, 45 deg, and 135 deg LP lights are incident on the metasurface, the local electric field at the antenna endpoints is appropriately enhanced, characteristic of surface plasmon resonance. When 90 deg LP light is incident, the electric field at the antenna ends is weaker, with no apparent resonance mode. Similarly, the CPME exhibits a significant LP-dependent emission.

To further explain CPME’s selective absorption of incident polarized light, we present the three-dimensional power loss distribution on the metasurface under different polarized light incidences, as shown in Fig. 4. The power loss distribution in the amorphous state can be found in Note S4 in the Supplementary Material. The following figure simulates the power loss distribution at the resonant wavelength of 4.7μm. Because metals are the main source of loss, the absorbed incident light by CPME is primarily dissipated through the metal antennas and metal substrate. As shown in Figs. 4(a) and 4(b), when LCP light is incident perpendicularly, the power loss in the top metal antennas is significant. However, when RCP light is incident, the power loss in the metal antennas is relatively small. This further demonstrates the metasurface’s selective absorption of circularly polarized light. As shown in Figs. 4(c) and 4(e), when 0 deg LP light is incident perpendicularly on the metasurface, the power loss in the metal antennas is significant, whereas for 90 deg LP light, the power loss is lower. Figures 4(d) and 4(f) show that when 45 deg and 135 deg LP lights are incident, significant power loss occurs in the I-shaped and L-shaped antennas, respectively.

The Stokes parameters are typically used to describe the polarization state of light. These parameters are determined by measuring different polarization components of light and are widely used in optics and electromagnetics to analyze the characteristics of polarized light. Earlier, we introduced the selective absorption/emission of different polarized lights by the proposed CPME. Here, we use the Stokes parameters to more intuitively and effectively describe the polarization characteristics of CPME under specific conditions, such as whether it tends to absorb/emit light in a particular polarization direction. The spectral Stokes parameters of CPME can be obtained using the following equations55: S(λ)=[S0(λ)S1(λ)S2(λ)S3(λ)]=[ε0deg(λ)+ε90deg(λ)ε0deg(λ)−ε90deg(λ)ε45deg(λ)−ε135deg(λ)εLCP(λ)−εRCP(λ)],(1)where ε0deg, ε45deg, ε90deg, and ε135deg represent the spectral emissivity measured at polarization directions of 0 deg, 45 deg, 90 deg, and 135 deg, respectively. εLCP and εRCP represent the spectral emissivity of LCP and RCP lights, respectively. S0 represents the total intensity of light, which includes both polarized and unpolarized lights. S1 represents the intensity difference between the horizontal and vertical polarization components. S2 represents the intensity difference between the 45 deg and 135 deg LP components. S3 represents the intensity difference between the LCP and RCP components.

As shown in Fig. 5(a), in the wavelength range of 3 to 7μm, the spectral S1, S2, and S3 parameters are almost zero. This indicates that when IST is in the amorphous state, CPME maintains consistent absorption for various polarized lights. In other words, the thermal emission of the metasurface is completely unpolarized. The value of the spectral S0 parameter is also low, indicating that CPME has a low absorption ratio for the incident light, verifying the previously mentioned weak response of CPME to the incident light. As shown in Fig. 5(b), in the wavelength range of 3 to 7μm, the spectral S1 and S3 parameters can reach 0.61 and 0.49, respectively, whereas the spectral S2 parameter is generally low. This indicates that when IST is in the crystalline state, CPME exhibits significant differences in absorption for 0 deg and 90 deg LP lights, as well as LCP and RCP lights, whereas the absorption for 45 deg and 135 deg LP lights are similar. This further demonstrates that in the crystalline state, the MIR thermal emission of CPME is partially polarized, including both LP and CP light emission. In summary, by changing the state of IST, CPME can achieve full Stokes parameter control of MIR thermal emission.

We also calculated typical polarization parameters, including the spectral DoP, DoLP, and DoCP of CPME in both amorphous and crystalline states. According to Eq. (1), the spectral DoP(λ), DoLP(λ), and DoCP(λ) can be calculated using the following expressions, respectively35: $$\begin{gathered} DoP=s12+s22+s32/s0DoLP=s12+s22/s0 \\ DoCP=|s3/s0|. \end{gathered}$$(2)

The calculation results are shown in Fig. 6. As illustrated in Fig. 6(a), the spectral DoP, DoLP, and DoCP are almost zero in the MIR region, indicating that the MIR thermal emission of CPME is almost completely unpolarized in the amorphous state. As shown in Fig. 6(b), when IST transitions to the crystalline state, the spectral DoP, DoLP, and DoCP increase significantly, indicating that CPME produces polarization-dependent thermal emission. In the wavelength range of 3 to 7μm, the maximum values of DoP, DoLP, and DoCP can reach 0.69, 0.55, and 0.47, respectively. These results demonstrate that the polarization characteristics of MIR thermal emission can be controlled by changing the state of CPME.

Radiation intensity can quantitatively describe the thermal radiation capability of an object at a certain temperature. To investigate the polarization characteristics of CPME thermal emission in the 3 to 7μm band, numerical calculations were performed on the radiation intensity, full Stokes parameters, DoP, DoLP, and DoCP at T=100°C and different crystalline states, as shown in Fig. 7. The radiation intensity of the object in the 3 to 7μm MIR band for different polarization states can be calculated using the following equation:56Ip(θ,T)=∫3μm7μmεp(λ,θ)×Eb(λ,T)cosθdλ,(3)Eb(λ,T)=2πc02λ−5/[exp(hc0/kλT)−1],(4)where I represents the radiation intensity, the subscript p indicates the linear or circular polarization state, θ is the emission angle, T is the temperature of the object, and ε is the spectral emissivity. Eb represents the spectral radiation intensity of a blackbody, where h is Planck’s constant, c0 is the speed of light in a vacuum, and k is Boltzmann’s constant. As shown in Figs. 7(a) and 7(b), when the temperature of CPME is 100°C and it is in an amorphous state, the radiation intensity for different polarization states is the same, and the Stokes parameters S1, S2, and S3 are almost zero. Therefore, DoP, DoLP, and DoCP are very low (less than 0.1), indicating that CPME’s MIR thermal emission is completely unpolarized. When the temperature of CPME is 100°C and it is in a crystalline state, the radiation intensity for different polarization states is not equal, resulting in nonzero Stokes parameters S1, S2, and S3. In this case, the average DoP of CPME in the 3 to 7μm range is 0.51, the average DoLP is 0.39, and the average DoCP is 0.29, indicating that the MIR thermal emission of CPME is partially polarized in the crystalline state, including both linear and circular polarization components.

To study the effect of the emission angle on the polarization characteristics of CPME MIR thermal emission, numerical calculations were performed for DoP, DoLP, and DoCP at different emission angles, as shown in Fig. 7(c). For the amorphous state, as the emission angle increases from 0 deg to 60 deg, DoP, DoLP, and DoCP slightly increase but remain very small (below 0.2), indicating that the polarization characteristics of CPME’s MIR thermal emission are weak in the amorphous state. In addition, in the crystalline state, DoP, DoLP, and DoCP increase significantly, demonstrating that CPME exhibits strong polarization characteristics in MIR thermal emission. Moreover, as the emission angle increases from 0 deg to 60 deg, the changes in the various polarization components are limited. Overall, the polarization characteristics of CPME proposed in this study are almost unaffected by the emission angle.

The linear and circular polarization characteristics of CPME are directly related to the state of the PCM IST, which holds significant practical application potential. To demonstrate this, a practical example of remote detection and counter-detection using CPME is provided. Figure 8(a) illustrates the infrared polarization imaging of the target, where the target is an Al plate coated with an airplane pattern. The entire structure consists of a 100×100-unit cell array. The airplane pattern is covered with CPME, whereas the remaining area is exposed as a smooth Al plate. The infrared polarization camera operates in the 3 to 7μm MIR region. When the target Al plate is at a temperature of 373 K, the DoP, DoLP, and DoCP of CPME in different crystalline states within the 3 to 7μm wavelength range at normal incidence were numerically calculated.

Subsequently, Figs. 8(b) and 8(c) respectively display the infrared polarization, linear polarization, and circular polarization images of the target at normal incidence in its crystalline and amorphous states. As shown in Fig. 8(b), when IST is in its crystalline state, CPME exhibits highly polarized MIR thermal emission, whereas the exposed smooth Al plate produces almost no polarized thermal emission. This allows the infrared polarization camera to easily identify the airplane pattern on the Al plate. In practical applications, because natural backgrounds typically do not exhibit polarization-related thermal emission characteristics, CPME could potentially be used as a high-contrast infrared beacon for remote detection. As illustrated in Fig. 8(c), when IST is in its amorphous state, the polarization degree of CPME’s thermal emission is very low (less than 0.05), causing the airplane pattern to blend into the background plate. Consequently, detecting the airplane pattern on the Al plate with an infrared polarization camera becomes increasingly difficult. In this scenario, CPME can serve as a low-contrast infrared stealth structure, effectively countering infrared polarization-based remote detection. In summary, the CPME proposed in this paper shows great potential for applications in infrared polarization detection and counter-detection.

In summary, we proposed a CPME composed of an array of asymmetric L-shaped and I-shaped resonators. The CPME consists of an IST antenna array, an Al2O3 dielectric layer, and an Au substrate. By leveraging the significant differences in material properties before and after the IST phase transition, we skillfully controlled the full Stokes parameters of MIR thermal emission without altering the overall structure of the metasurface. In the amorphous state, the CPME’s thermal emission exhibits polarization-independent characteristics. In the crystalline state, the CPME can selectively emit polarized lights of different polarization states. Within the MIR range, the DoP, DoLP, and DoCP can be adjusted over a wide range at normal incidence. The physical mechanism of full-vector polarization control was investigated through the near-field distribution and power loss distribution of the structural unit. Finally, we briefly discussed the potential applications of the CPME as a contrast-controllable MIR beacon and infrared polarization stealth structure by numerically calculating the DoP of the CPME at different emission angles.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 61775050).

Biographies of the authors are not available.