Switchable Dual-Function Terahertz Metamaterial Device Based on Vanadium Dioxide

Abstract

On the basis of the temperature-controlled phase change properties of vanadium dioxide (VO₂), a dual-functional terahertz metamaterial device is proposed. The device can be switched between an absorber and a polarization converter. Simulation results demonstrate that the device acts as a terahertz wave absorber when the VO₂ is in the metallic state, and the reflected extinction ratio is less than −15 dB in the frequency range between 1.07 and 1.19 THz. Moreover, the absorption performance is insensitive to polarization. When the VO₂ is in the insulated state, the device behaves as a polarization converter, which can convert line-polarized light to cross-polarized light. The polarization conversion rate of the device is over 90% in the frequency range between 1.43 and 1.51 THz. The proposed dual-functional metamaterial device with tunable and diverse functions has broad and potentially useful uses in areas such as terahertz detection, modulation, and switching.

1. Introduction

The terahertz band of electromagnetic waves demonstrates several attractive characteristics, including non-ionization, fingerprint spectrum, sensitivity to weak resonance, and strong penetration of nonpolar materials, which have promising applications in communication [1], sensing [2], and imaging [3]. Due to the lack of natural materials that can directly interact with terahertz waves, metamaterial-based multifunctional devices need to be created in order to boost the development of terahertz technology. Metamaterials are artificially invented sub-wavelength periodic composite structures that display unique physical properties not found in natural materials, such as perfect absorption [4], anomalous reflection [5], and negative refraction [6], due to their novel optical properties. Currently, metamaterials have been extensively applied in stealth technology [7], meta-lenses [8], antennas [9], and other areas. The performance of optical metamaterials is mainly determined by their structures, particularly the unit structure. For optical metamaterials with specific unit structure arrangements, their optical properties are usually stable. Metamaterials in combination with tunable materials can achieve rich and switchable multifunctional optical devices as metamaterial technology advances [10,11,12]. Vanadium dioxide (VO₂) is a metal oxide with the feature of phase change [13]. As an ideal tunable material, it displays the transition from the insulating state to the metallic state at 340 K [14,15,16,17,18]. At room temperature, VO₂ has a high resistivity and high insulation in its monoclinic crystal structure. When the temperature is higher than 340 K, the conductivity of VO₂ rises by four to five orders of magnitude in a narrow temperature range, transforming it into a cubic structure with metal properties. More importantly, the VO₂ phase change can be reversed, and at a temperature lower than 340 K, the conductivity of VO₂ will completely recover to its original state. In addition to temperature control, the phase change of VO₂ can also be realized through electrical stimulation, pressure, chemical doping, and other methods [19,20]. Therefore, the appearance of VO₂ material lays the foundation for realizing tunable multifunctional metamaterial devices.

Since the classic three-layer metamaterial absorber was put forward by Landy et al. in 2008 [21], numerous metamaterial absorbers have been proposed. In the early stages of research, most absorbers were narrowband and non-tunable. In 2017, a metamaterial functional device was proposed that could switch from a narrow absorption band to a wide absorption band by applying the phase change of VO₂ [22]. In the year 2018, a metamaterial device that could switch between broadband absorption at different frequency bands using VO₂ was reported [23]. In recent years, absorbers have gradually been combined with other functions by utilizing the tunable properties of different materials, such as perfect reflection [24], electromagnetic induction transparency [25], and polarization conversion. Based on the VO₂ structure, researchers have achieved switching between narrowband perfect and broadband absorption and polarization conversion [26,27]. However, improving performance during the functional switching process of metamaterials remains a significant challenge.

In the work, a dual-functional terahertz metamaterial device is proposed based on the temperature-controlled phase change properties of VO₂ material, which achieves the best performance known so far on the basis of a simple structure. When VO₂ is in the metallic state, the device exhibits a high extinction ratio of THz waves, with a reflection extinction ratio less than −15 dB in the range of 1.07–1.19 THz, and functions as a THz wave absorber with insensitive polarization dependence. When VO₂ is insulating, the device achieves a high polarization conversion efficiency of more than 90% in the range of 1.43–1.51 THz for incident linearly polarized light to cross-polarization conversion and serves as an efficient polarization converter. Currently, only a fixed spectrum or a single function can be achieved by the majority of metamaterial devices. The metamaterial proposed in this work has a simpler structure and better performance in both functions, with greater potential for applications in fields such as optoelectronic detection, thermal imaging, thermal emitters, and solar cells.

2. Device Design

The schematic diagram of the crystal cell structure for the VO₂-based switchable dual-functional terahertz metamaterial device is shown in Figure 1a. The metasurface structure is composed of five layers from the top down: the VO₂ square ring, the upper layer polyimide, the gold square aperture ring structure, the lower layer polyimide, and the gold reflection layer. The metal-dielectric-metal structure is commonly applied in the design of an absorber. The uppermost metal state of VO₂ with a symmetric structure can generate an electromagnetic reaction with the incident electromagnetic wave to realize perfect absorption, and the symmetric square ring structure also aids in the elimination of anisotropy. The dielectric loss material, the polyimide layer, acts as an intermediate layer to attenuate the incident terahertz waves entering the material to a certain extent. The gold aperture ring with an asymmetric structure is used to achieve polarization rotation, and the bottom layer employs a gold reflection layer with a thickness greater than the skin depth to prevent from transmitting electromagnetic waves. Moreover, the design of a fully covered metal reflection layer can effectively reduce processing difficulty. Through the optimization of the geometric parameters, the period of the crystal cell is P = 90 μm. Figure 1b shows the top view of the VO₂ square ring with its geometric parameters l₁ = 37 μm, l₂ = 5 μm, and d = 5 μm. Figure 1c shows the top view of the gold square aperture ring structure with its geometric parameters l₃ = 50 μm and w = 7 μm. Figure 1d shows the lateral view of the crystal cell, in which the thickness of the VO₂ square ring, polyimide layer, and gold layer are respectively t₁ = 0.2 μm, h = 7.5 μm, and t₂ = 0.2 μm.

The designed device was numerically simulated using the full-wave electromagnetic field simulation software CST with a frequency-domain solver. The crystal cell was set with a periodic boundary condition on the x- and y-axes and an open boundary condition on the z-axis. A y-polarized TE mode THz wave was normally incident on the surface of the unit structure along the -z direction. The phase change of VO₂ causes a big transmission in its conductivity, corresponding to different phases with different conductivities. When VO₂ is in the metallic state, its dielectric constant is usually described by the Drude model [28,29,30,31]. When in the insulating state, its dielectric constant is ε_d = 9. During simulation, the electrical conductivity of gold was set to 4.56 × 10⁷ S/m [32], and the dielectric constant of polyimide was set to 3.5 with a loss tangent of 0.0027. In practice, a Janis cryostat whose temperature can be regulated by a resistance heater can be utilized to manage the device’s overall temperature [33]. The device is fixed on the cold finger of the low-temperature thermostat, and a thermistor is mounted on the cold finger about 10 cm away from the device to monitor its temperature.

3. Results Analysis

3.1. Terahertz Wave Absorber

When VO₂ is in the metallic state, the operating state of the metasurface device is a terahertz absorber, mainly composed of the top VO₂ structure, polyimide layer, and gold reflection layer. The absorption rate is a vital parameter that indicates the performance of the absorber, and the calculation formula for the absorption rate is A(ω) = 1−|S₁₁|²−|S₂₁|², where A(ω) is the absorption rate, S₁₁ is the reflection coefficient, and S₂₁ is the transmission coefficient. Based on using a gold thin film with a thickness greater than the skin depth in the bottom layer of the structure in this work, the transmission coefficient of the absorber is S₂₁ = 0, so the formula for the absorption rate can be simplified to A(ω) = 1−|S₁₁|². Figure 2a demonstrates the reflection coefficient of the designed absorber, which shows that the designed absorber has a reflection extinction ratio of less than -15 dB in the range of 1.07–1.19 THz, exhibiting excellent absorbing performance. There is a sharp absorption peak at f = 1.13 THz in the figure, with an absorption rate of 100%. To better understand the sensitivity of the designed terahertz absorber to polarization, we studied the changes in the device’s reflection coefficient when TE and TM polarized incidence were applied. As shown in Figure 2a, with the symmetry of the structure, the designed absorber shows superb polarization insensitivity. As a classical phase transition material, when VO₂ transits from an insulating state to a metallic state, the relationship between its conductivity and temperature can be expressed as σ = -iε₀ω(ε_c−1) [4,33,34], where σ is the conductivity, ε₀ is the vacuum permittivity, and ε_c is the dielectric constant of VO₂, which is temperature-dependent. Figure 2b shows the absorption spectra of the absorber corresponding to different conductivities of VO₂. When the conductivity of VO₂ transits from 200 S/m to 2 × 10⁵ S/m, the absorption rate of the absorber rises from 5.9% to 100%.

To further illustrate the physical mechanism of the designed high-performance absorber, Figure 3 shows the electric field distribution at different frequencies above the VO₂ square ring structure in the absorber’s xy plane. It can be observed that, at the resonant frequency of f = 1.13 THz, the energy is mainly concentrated in the gap between adjacent rings, whereas at frequencies outside the resonant frequency, such as f = 0.8 THz and f = 1.4 THz, the energy in the gap between adjacent rings is not concentrated. This indicates that the absorption effect of the absorber is mainly due to the strong coupling between adjacent unit cells. For frequencies f = 1.13 THz, since the concentrated energy is in the medium between the adjacent square rings, the coupling of the adjacent square rings caused the absorption. What’s more, as for resonant frequencies, energy is concentrated in a parallel pattern to the electric field, which demonstrates that the electric dipole resonance is excited. According to the power distribution at the absorption peak, the author concluded that the absorption is caused by electric dipole resonance.

In Figure 4, to investigate geometric parameters’ influence on the structural metamaterial on the absorption rate, we calculated the impact of the VO₂ square ring width l₁ and the hole width l₂ on the absorber’s absorption performance. Figure 4a indicates that with the increase of l₁, the absorption peak shifts towards lower frequencies, while the absorption rate changes and reaches a maximum at l₁ = 37 μm. Figure 4b indicates that as l₂ increases, the central frequency remains unchanged and the absorption rate increases, reaching a maximum at l₂ = 5 μm.

3.2. Terahertz Wave Polarization Converter

When VO₂ is in the insulating state, the metasurface device, mainly achieved through the gold square aperture ring structure, polyimide layer, and gold reflection layer, works as a terahertz wave polarization converter. As shown in Figure 5a, due to the anisotropic characteristics of the gold square aperture ring structure, the incident y- and x-polarized wave can be resolved into two orthogonal components on the u- and v-axis, and it will stir up a resonance in the u- and v-axis directions with a phase difference of ±π/2 or ±π between them, thereby realizing the cross-linear polarization conversion. Generally, assuming that the normal incident wave is y-polarized, the reflected electromagnetic wave can be expressed as: $\overrightarrow{E_r}=r_{xy}{A}_y\cos \left({\phi }_{xy}\right)\overrightarrow{e_x}+r_{yy}{A}_y\cos \left({\phi }_{yy}\right)\overrightarrow{e_y}$ where r_xy and r_yy are the reflection coefficients for y-to-x and y-to-y polarization conversions, respectively. To characterize the polarization conversion capability of the proposed device, the efficiency of the polarization converter can be expressed as $PCR={\left|r_{xy}\right|}^2/\left({\left|r_{xy}\right|}^2+{\left|r_{yy}\right|}^2\right)$. To precisely determine the polarization of the reflected wave, Figure 5a demonstrates the change of the reflection coefficients r_xy and r_yy with frequency. At 1.47 THz, r_xy and r_yy are approximately 1 and 0, respectively, indicating that almost only x-polarized terahertz waves are near this frequency. Figure 5b shows the conversion efficiency of the polarization converter. It can be seen that the conversion efficiency is over 90% within the limit of 1.43–1.51 THz. The working broadband of polarization conversion is determined by the threshold range where the conversion efficiency is greater than 90%, indicating that almost all incident waves within this limit can be transformed into cross-polarized waves.

With the aim of further investigating the cross-polarization conversion mechanism of the converter, we ran the simulation that showed the surface current distribution of the polarization converter at different frequencies on the xy plane, as shown in Figure 6. It can be observed that the current distribution on the metal surface is not concentrated outside the resonant frequency (f = 1.0 THz or 2.0 THz). However, near the resonant frequency f = 1.47 THz, the concentration of the surface current mainly occurs on the gold square aperture ring structure, and the current vector direction on the gold square aperture ring structure (shown as the blue arrow in Figure 6b) is parallel and opposite to that on the surface of the gold substrate (shown as the black arrow in Figure 6b), forming a magnetic resonance. Efficient cross-polarization conversion can be achieved through the strong electromagnetic coupling effect.

In addition, we also analyzed the effects of the gold square aperture ring structure parameters, l₃ and w, on the device’s polarization conversion performance. Figure 7a shows that as l₃ rises gradually, the center frequency of PCR redshifts, while the maximum conversion efficiency remains constant. As shown in Figure 7b, as w increases, the operating bandwidth increases, the center frequency remains constant, and the maximum conversion efficiency remains constant.

4. Conclusions

In general, based on the temperature-controlled phase change properties of VO₂ material, a dual-functional terahertz metamaterial device has been proposed in this paper that can switch between absorber and polarization converter functions. Simulation results show that when VO₂ is in its metallic state, the device functions as a terahertz wave absorber with a reflectivity extinction ratio of less than −15 dB in the range of 1.07–1.19 THz, demonstrating good absorption performance and insensitivity to polarization. When VO₂ is in its insulating state, the device behaves as a polarization converter, with a cross-linear polarization conversion efficiency of over 90% in the range of 1.43–1.51 THz. The designed dual-functional metamaterial device is characterized by tunability and diversity, providing creative ways to create a fast-response active terahertz wave modulator.