1. Introduction

Terahertz (THz) technology has attracted increasing interest due to its significant research and application values in the fields of wireless communications, security inspection, biomedicine, nondestructive testing and astronomy [1–5]. Due to the weak interaction between natural materials and THz waves, the development of THz functional devices such as waveguide, lens, absorber, filter, phase shifter and polarizer have encountered certain obstacles [6–10]. In order to solve this problem, artificially designed metamaterials have been widely used in THz functional devices. Metamaterials utilize resonance and localized field effect to enhance the interaction between THz wave and matter, thus effectively regulating the amplitude, phase, polarization and propagation direction of electromagnetic wave [11–13]. In addition, by combining metamaterials with tunable materials such as graphene [14–16], Dirac semi metal [17,18], photosensitive silicon [19,20] and vanadium dioxide (VO₂) [21–34], dynamic tunability of device functions can be realized. Among them, phase change material VO₂ can be transformed from insulating state to metallic state with a transition temperature about 340 K, which can be achieved by thermal, electrical or optical stimulus. It has the advantages of rapid response, large modulation depth, simple modulation method and reversible phase change, thus being widely used in THz functional devices. For example, Zhao et al. [26] proposed a metal-VO₂ grating structure accompany with two patterned metal layers, which realized the dynamic switching of broadband polarization conversion and asymmetric transmission by modulating the resistance state of VO₂; Song et al. [27] presented a six-layered structure composed of Au grating and VO₂ patches for broadband absorption and broadband polarization conversion; Jiang et al. [28] proposed a metamaterial structure involving three layers of cascaded metallic gratings with rotation angle 45° and VO₂ layer, which can achieve both reflective and transmissive cross-polarization conversion. Despite the marvelous achievements, the structures of the above-mentioned metamaterials are relatively complex and require more than one step lithography. In order to achieve the miniaturization of modern system, a multifunctional device with simple structure and easy to integrate with other devices is highly desirable.

In this paper, we propose a dynamic switchable multifunctional THz device with a relatively simple structure. It consists of VO₂ layer, polyimide (PI) dielectric layer and metal grating and requires only one step lithography. The proposed device can be switched between broadband absorber, reflective broadband cross-polarization converter (CPC), reflective linear-to-circular (LTC) polarization converter, transmissive narrowband cross-polarization converter and transmissive narrowband filter, depending on the conductivity of VO₂ and the incident direction of THz wave. By analyzing the distributions of magnetic field and current density, the physical mechanisms of broadband absorption and reflective broadband cross-polarization conversion are clarified. Furthermore, the effects of incident angle and polarization angle on the performance of the device are discussed.

2. Structure and simulation setup

The schematic of the proposed multifunctional THz device is illustrated in Fig. 1 (a) and (b), which is a four-layered structure of PI/VO₂/Au/PI from bottom to top. The Au-VO₂ alternating gratings with different thicknesses are periodically arranged along the x-axis and sandwiched between two layers of PI. The geometric dimensions of unit structure are t₁ = 40 µm, t₂ = 34 µm, t₃ = 12 µm, t₄ = 66 µm, w₁ = 35 µm and w₂ = 65 µm, respectively. The thickness of VO₂ layer is 800 nm. The proposed structure can be fabricated by micromachining which needs only one step lithography, with the process flow shown in Fig. 1(c). Firstly, PI is spin-coated on top of SiO₂/Si substrate. Then, VO₂ layer of 800 nm can be formed by chemical vapor deposition (CVD) with a layer of PI spin-coated on top of it. After that, a trench with width of w₂ and height of t₁ is created by photolithography and etching. Then the trench is filled by Au sputtering and the redundant Au is removed by lift-off. After that, the top layer PI is spin-coated on top of Au and the SiO₂/Si substrate can be removed by hydrofluoric acid (HF).

 

Fig. 1. (a) Cutaway view of the proposed multifunctional THz device. The incident electromagnetic wave vector k along the -z/+z axis is defined as forward/backward incident. (b) Cross-sectional view of the periodic unit cell. (c) Fabrication process of the proposed device.

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The performance of the designed device is numerically studied with finite element method (FEM) simulation tool. Periodic boundary conditions are applied to the x and y directions and perfectly matched layer is used in the z direction. The Floquet port is set to the z-axis direction of the unit structure. In addition, transition boundary condition is applied to the VO₂ layer in the simulation, due to the thickness of the VO₂ layer is relatively thin, which is not conducive to mesh generation. The incident electromagnetic wave vector k along the -z and + z axis are defined as forward and backward incident, respectively. The angle between the incident wave and the normal direction of the xoy plane is the incident angle θ, which is set to 0 by default. That is, the electromagnetic wave is normal incident. The amplitude of electric field of the incident wave is set as ${E_x} = 1\ast \textrm{sin}\varPhi $, ${E_y} = 1\ast \textrm{cos}\varPhi $, ${E_z} = 0$, where $\varPhi $ is the incident polarization angle. Typically, $\varPhi $ is set to 45°, namely, the angle between the incident wave electric field E and the + y-axis is 45°. The default polarization state of the incident wave is u-polarized unless otherwise stated.

The relative permittivity of VO₂ in THz regime can be described by Drude model [35]:

3. Results and discussion

By adjusting the incident direction of the electromagnetic wave and the resistance state of VO₂, the proposed device can work as broadband absorber, reflective broadband cross-polarization converter, reflective linear-to-circular polarization converter, transmissive narrowband cross-polarization converter and transmissive narrowband filter. These functional modules will be discussed in detail in the following sections.

3.1 Broadband absorber

When VO₂ is in the metallic state and the electromagnetic wave is forward incident (i.e., along the -z direction), the VO₂ layer can be used as a substrate to form a broadband absorber along with the metal grating and PI layer on the upper side of the VO₂ layer. The absorption efficiency A can be calculated by the following formula: $A = 1 - {|{{S_{11}}(\omega )} |^2} - {|{{S_{21}}(\omega )} |^2}$, where S₁₁ and S₂₁ are reflection parameter and transmission parameter, respectively. The absorption curve, $|{{S_{11}}} |$ and $|{{S_{21}}} |$ are shown in Fig. 2. It can be found that a wide absorption frequency band ranging from 3.53 THz to 9.68 THz with the corresponding absorption efficiency above 90% is obtained and the relative bandwidth 2| f_max−f_min |/| f_max+ f_min | is 93%.

 

Fig. 2. Absorption curve, $|{{S_{11}}} |$ and $|{{S_{21}}} |$. of the absorber module when VO₂ is in the metallic state and the electromagnetic wave is forward incident and u-polarized.

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Absorber can confine the incident electromagnetic wave to a limited area within the device, thus enhancing the interaction between the electromagnetic wave and the device and convert the radiation energy into other forms of energy such as thermal loss. Several methods can be used to generate local fields, such as Fabry Pérot (F-P) cavity resonance [11,38], electromagnetic resonance [23,25] and surface plasmon polariton [39], which can be used alone or jointly [14,40]. In order to clarify the physical mechanism of the absorption curve, the magnetic field and current density distributions for frequencies at which the absorption efficiencies reaching the local maximum values, namely 4.14 THz, 5.31 THz, 6.26 THz and 8.72 THz, are rigorously investigated, as depicted in Fig. 3.

 

Fig. 3. The distributions of magnetic field and current density for the broadband absorber module at four frequencies at which the absorption efficiencies reaching the local maximum. (a), (b), (c) and (d) are for frequencies of 4.14 THz, 5.31 THz, 6.26 THz and 8.72 THz, respectively.

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The magnetic field intensity is indicated by color and the direction and magnitude of current density are indicated by arrows, wherein the black arrows represent the displacement current in PI and the red arrows represent surface current of VO₂ and metal. The upper part of each image is the top view of the absorber (i.e., xoy plane) and the lower part is the front view of the absorber (i.e., xoz plane). To be noted, the device structure below the VO₂ layer is omitted for simplicity. A significantly enhanced local magnetic field can be observed in the closed current loop formed by displacement current and surface current, indicating that the main absorption mechanism is electromagnetic resonance. For frequency at 4.14 THz, the resonance mainly comes from three parts. The current loop formed by the surface current of the metal grating and the displacement current in PI layer causes the magnetic resonance, indicated by the white dotted box in Fig. 3 (a). The surface current at the center of VO₂ layer can be decomposed into current components parallel and perpendicular to the displacement current in PI, as depicted in the yellow dotted boxes in Fig. 3 (a). As the parallel components are opposite to the direction of the displacement current, magnetic resonance is induced. The surface current in the red dotted boxes can also be decomposed into components parallel and perpendicular to the displacement current in the PI layer. As the parallel components are in the same direction as the displacement current, electric resonance is generated. For frequency at 5.31 THz, the surface current at the corners where the metal grating contacts the VO₂ layer and the displacement current in PI layer form a current loop and magnetic resonance is generated, as indicated in the red dotted boxes in Fig. 3 (b). The displacement current above the central part of VO₂ layer can be divided into current components parallel and perpendicular to the surface current of VO₂ layer, as indicated by the yellow dotted box in Fig. 3 (b). Since the surface current and the parallel components of displacement current are in the same direction, electrical resonance is caused. For frequencies at 6.26 THz and 8.73 THz, the magnetic resonance mainly occurs above the metal grating, as the surface current of the grating ridge and the displacement current in the PI layer are anti-parallel, indicated by the white dotted boxes in Fig. 3 (c) and (d).

3.2 Reflective broadband CPC and LTC polarization converter

When VO₂ is in the metallic state and the electromagnetic wave is backward incident (i.e., along the + z direction), the VO₂ layer can act as a substrate to form a reflective broadband cross-polarization converter and a linear-to-circular polarization converter along with the metal grating and PI layer under the VO₂ layer. When the incident wave is u-polarized, the reflected electromagnetic wave is v-polarized for certain frequency range, thus realizing cross-polarization conversion. The cross-polarization reflection coefficient can be calculated by R_vu= E_vr/E_ui, where E_vr is the amplitude of reflected v-polarized wave and E_ui is the amplitude of incident u-polarized wave. The co-polarization reflection coefficient can be calculated by R_uu= E_ur/E_ui, where E_ur is the amplitude of reflected u-polarized wave. The phase difference of the reflected u-polarized wave and reflected v-polarized wave can be calculated by $\Delta \varphi = {\varphi _{uu}} - {\varphi _{vu}}$, where ${\varphi _{uu}}$ and ${\varphi _{vu}}$ are the phase of reflected u-polarized wave and v-polarized wave, respectively. In order to better evaluate the performance of the cross-polarization converter, polarization conversion rate (PCR) is introduced to describe the efficiency of polarization conversion. The PCR for incident u-polarized wave in reflection mode can be expressed as PCR^r=|R_vu|²/(|R_vu|²+|R_uu|²). Besides, the device can also achieve mutual conversion of left-handed circular polarization (LCP) and right-handed circular polarization (RCP). The u and v in the subscripts in the above equations can be replaced by R and L to describe the cross-polarization conversion parameters for circularly polarized incident wave. As for the function of linear-to-circular polarization conversion, ellipticity is introduced to estimate the performance of the polarization conversion, which can be calculated by $E = 2|{{R_{vu}}} ||{{R_{uu}}} |sin\Delta \varphi /({{{|{{R_{vu}}} |}^2} + {{|{{R_{uu}}} |}^2}} )$. When $E ={-} 1/{+} 1$, the linearly polarized incident wave is converted to RCP/LCP wave.

As shown in Fig. 4 (a) and (b), PCRs for linearly polarized and circularly polarized incident waves demonstrate the same characteristics and the values are more than 0.97 from 0.77 THz to 1.79 THz, with a relative bandwidth of 80%. It can be concluded that effective cross-polarization conversion is achieved in this frequency range. In addition, the R_vu and R_uu for u-polarized incident wave are the same at 0.66 THz and 1.86 THz with the phase difference $\Delta \varphi $ close to –270° and –90°, respectively. It means that the ellipticities at theses tow frequencies are +1 and –1 respectively, i.e., the linearly polarized incident wave at these two frequencies can be converted into LCP wave and RCP wave, respectively.

 

Fig. 4. (a) Simulated cross-polarization reflection coefficient, co-polarization reflection coefficient, PCR and ellipticity for linearly polarized incident wave. (b) Simulated cross-polarization reflection coefficient, co-polarization reflection coefficient and PCR for circularly polarized incident wave. (c) Phase difference of the reflected u-polarized wave and v-polarized wave.

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The distributions of magnetic field and current density corresponding to the frequencies at which PCR reaches its local maximum are studied to clarify the physical mechanism of cross-polarization conversion, as shown in Fig. 5. The color indicates the strength of the magnetic field, the red arrows correspond to surface current of metal and VO₂ layer and the black arrows indicate displacement current in PI. The left column is the front view (i.e., xoz plane) of the device and the right column is the top view (i.e., xoy plane) of the device. For frequency at 0.83 THz, it can be seen from the left column in Fig. 5 (a) that the surface current on the sidewalls of the metal grating and VO₂ layer and the displacement current in the PI form a closed current loop, generating an induced magnetic field H₁ along the + y-axis direction. This induced magnetic field H₁ can be decomposed into H_1u along the u direction and H_1v along the v direction, as depicted in the right column in Fig. 5 (a). The u component H_1u is parallel to the incident electric field E_i, while the v component H_1v is vertical to E_i. As a result, the cross-coupling between H_1u and E_i leads to cross-polarization rotation with a u-to-v polarization conversion while H_1v has no contribution to the polarization conversion [41]. Similarly, it can be seen from Fig. 5 (b-d) that at 1.15 THz, 1.44 THz and 1.72 THz, the magnetic resonance is mainly caused by the anti-parallel currents at the lower surface of the metal grating and the displacement current in PI. The u components H_2u/H_-3u/H_-4u of the induced magnetic field H₂/H₃/H₄ are parallel to the incident electric field E_i, resulting in the cross-polarization conversion.

 

Fig. 5. The distributions of magnetic field and current density for the reflective broadband cross-polarization converter module at (a) 0.83 THz, (b) 1.15 THz, (c) 1.44 THz and (d) 1.72 THz, respectively.

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3.3 Transmissive module

When VO₂ is in insulating state and the incident wave is in the forward/backward direction, the proposed metamaterial structure can work as a transmissive device with two functions of a narrowband cross-polarization converter and a narrowband filter. As can be seen from Fig. 6, in this mode the transmissive and reflective responses for forward and backward incidences are basically the same. When VO₂ is in the insulating state, it can be treated as a dielectric layer. Since electromagnetic waves in THz frequencies can penetrate dielectric with very low loss, most of the incident electromagnetic waves interact with the metal grating. For the device structure proposed here, the metal grating structure seen by the forward incidence and backward incidence is the same. Thus, in this mode the transmissive and reflective responses for forward and backward incidences are basically the same. When the incident wave is u-polarized, T_vu and T_uu represent the amplitude of v-polarized and u-polarized transmitted wave, R_vu+ R_uu represents the sum of reflected v-polarized and u-polarized wave. The PCR for incident u-polarized wave in transmission mode can be expressed as PCR^t=|T_vu|²/(|T_vu|²+|T_uu|²). For frequency at 1.32 THz, 24% of the incident wave is reflected, while most of the transmitted wave is v-polarized with a PCR^t of 92%, which means the device can act as a transmissive cross-polarization converter at this frequency. For frequency at 1.84 THz, 25% of the incident wave is reflected and the transmitted u-polarized wave is 62%, which means the device can be used as a transmissive narrowband filter at this frequency.

 

Fig. 6. Simulated results for transmissive and reflective responses for (a) forward incident wave and (b) backward incident wave when VO₂ is in insulating state.

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3.4 Performance analysis

For practical application, the influence of the polarization angle Φ under normal incidence on the performance of the broadband absorber module and the sensitivity of each functional module to the incident angle θ are further investigated. Absorption spectrum of the absorber module with different polarization angles is shown in Fig. 7 (a). As the polarization of incident wave change from y-polarized (Φ = 0°) to x-polarized (Φ = 90°), the absorption efficiencies drop slightly around 10 THz, while the rest are basically maintained. It can be concluded that the broadband absorption module can be considered as polarization-independent though the structure of the proposed device is not rotationally symmetric. However, the broadband absorption module is sensitive to the incident angle, as shown in Fig. 7 (b). When the incident angle is less than 20°, the broadband absorption remains stable with slight blue-shift. As the incident angle increases, the absorption spectrum deteriorates and splits. Fig. 7 (c) is the spectrum of PCR of the reflective broadband cross-polarization converter at different incident angles θ. As the incident angle increases from 0° to 60°, the working frequency band is gradually blue-shifted, while the PCR remains above 90%. The T_vu spectrum of the transmissive narrowband cross-polarization converter and T_uu spectrum of the transmissive narrowband filter at different incident angles θ are depicted in Fig. 7 (d) and (e), respectively. For transmissive narrowband cross-polarization converter, the working frequency is slightly blue-shifted with the value of T_vu gradually decreases with increasing θ. For transmissive narrowband filter, the working frequency is gradually blue-shifted and the value of T_uu first increases and then decreases with increasing θ.

 

Fig. 7. Absorption spectra of the absorber module (a) with different polarization angles Φ under normal incidence and (b) with different incident angles θ for u-polarized incidence. (c) The PCR spectrum of the reflective broadband cross-polarization converter with different incident angles θ. (d) The T_vu spectrum of the transmissive narrowband cross-polarization converter with different incident angles θ. (e) The T_uu spectrum of the transmissive narrowband filter with different incident angles θ.

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Finally, the comparison between the proposed multifunctional device and some recently reported devices is listed in Table 1 to address the novelty and performance of the device designed here. As lithography is the most complex and expensive process in micromachining, we use number of lithography required to represent the complexity of the devices’ structure. It can be found that by introducing one functional material, the proposed device is simple in structure and can achieve broadband absorber, reflective broadband cross-polarization converter, reflective linear-to-circular polarization converter, transmissive narrowband cross-polarization converter and transmissive narrowband filter with fairly good performance, which may pave the way for practical multifunctional THz devices.

Table 1. Comparison between references and our work.

View Table

4. Conclusions

We have proposed and demonstrated a VO₂-based multifunctional THz device which can achieve broadband absorption, reflective broadband cross-polarization conversion, reflective linear-to-circular polarization conversion, transmissive narrowband cross-polarization conversion and transmissive narrowband filtering under different working conditions. The device consists of a simple four-layered structure which needs only one step lithography. When VO₂ is in the metallic state and the electromagnetic wave is forward incident, the device serves as a broadband absorber, with absorption efficiency higher than 90% from 3.53 THz to 9.68 THz. When VO₂ is in the metallic state and the electromagnetic wave is backward incident, the device acts as a reflective broadband cross-polarization converter with PCR over 97% from 0.77 THz to 1.79 THz and a reflective linear-to-circular polarization converter at 0.66 THz and 1.86 THz. The magnetic field and current density distributions indicate that the broadband absorption originates from electromagnetic resonance and the reflective broadband cross-polarization conversion is mainly caused by magnetic resonance. When VO₂ is in the insulating state, the proposed device can work as a transmissive narrowband cross-polarization converter at 1.32 THz and a transmissive narrowband filter at 1.84 THz. In addition, the broadband absorber module is insensitive to polarization angle and remains stable when the incident angle is less than 20°. As the incident angle increases, the working frequency band of the reflective broadband cross-polarization converter module gradually shifts blue, while the PCR remains above 90%. The working frequencies of transmissive narrowband cross-polarization converter and filter modules start to blue shift and the transmissive coefficients vary with increased incident angle.