# Switchable Terahertz Band-Pass/Band-Stop Filter Enabled by Hybrid Vanadium Dioxide Metamaterial

Authors: Ying Chen; Jianwei Cheng; Chaowu Liang
Journal: Advances in Condensed Matter Physics
 (2020)

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## Abstract

To date, little research has been carried out on the integration of switchable and diversified functionalities into a single metamaterial in the terahertz (THz) range. Here, a hybrid vanadium dioxide (VO2) metamaterial was designed with switchable properties of band-pass filter and band-stop filter in the frequency range of 0.3–1.6 THz. Simulations demonstrated that under TE polarization, the proposed system acted as band-stop filter with the center frequency of 0.95 THz when VO2 is in the insulating state. Upon the transformation of VO2 into the metallic state, the proposed system behaved as a band-pass filter with a transmittance of >80%. The physical mechanism of the band-pass/band-stop conversion was examined by analyzing the surface current distribution of the designed device. The switchable characteristics of this structure can enable its wide application in tunable THz functional components such as amplitude modulators, polarization control, and intelligent switches.


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## Body

## 1. Introduction

Metamaterials are artificial composite materials consisting of periodically arranged units that show some extraordinary electromagnetic properties including a negative refractive index [1], electromagnetically induced transparency [2], and perfect absorption [3]. Recently, metamaterial absorbers have shown great potential for application in stealth technology, detection, and communication. In 2008, Landy et al. proposed the concept of a metamaterial-based perfect absorber [3]. Due to the presence of a gap in the terahertz range, various metamaterial absorber models with different terahertz frequencies (e.g., single band [4], multiband [5, 6], and broadband absorbers [7–9]) have been proposed. However, early terahertz absorbers exhibit a single resonance mode and their absorption performance is nontunable, limiting their potential for use in practical applications. Therefore, there is an urgent need for the development of a tunable absorber with absorption performance that can be tuned by optical, thermal, or electric excitations [10, 11].VO2 is an ideal thermally controlled material that undergoes a phase transition from the insulating state to the metal state at 67°C [12]. Due to this transition, transmittances of terahertz waves vary significantly before and after the phase transition so that the amplitude of terahertz waves can be controlled to a certain extent by the phase transition. Additionally, VO2-based metamaterials that exhibit advantages such as a high modulation depth, low insertion loss, multiple driving methods, and good repeatability are currently some of the main candidates for the development of devices to achieve control of terahertz waves. Under the influence of external light and thermal excitation, a VO2 reconfigurable metamaterial has been demonstrated to achieve various functions such as a tunable narrowband filter [13], tunable broadband absorber [14], and high-speed amplitude modulator [15]. Nevertheless, the inclusion of the additional laser and heater components may have significant effects on the integration, anti-interference ability, and production cost of such VO2-based devices. Therefore, electric excitation is the preferred method for realizing the dynamic reconfiguration of metamaterials. Unfortunately, the electrically excited VO2-based reconfigurable metamaterials reported to date have exhibited various limitations and disadvantages. For example, in 2011, Jeong et al. reported an electrically excited VO2 reconfigurable metamaterial. However, the voltage required for reconfiguration was as high as 500 V [16]. In subsequent work based on this approach, Liu et al. designed a multilayer composite structure to achieve high-speed modulation for mid-infrared signals with a trigger time of only 0.25 s. However, the current required for the operation of this device was as high as 4 A [17]. In 2017, Zhou et al. shaped VO2 into a grid form that was then inserted in the middle of the metal comb, obtaining the modulation depth of 87% in a wide frequency range of 0.4–1.2 THz and reducing the required current to 0.5 A. However, this device must be preheated to approximately 68°C for operation [18]. Recently, Park et al. fabricated a narrow-band filter in the terahertz frequency band by ingeniously designing the upper metal structure of a VO2-reconfigurable metamaterial and using the strong coupling characteristics of metal structure so that a driving voltage of only 5.5 V was required. However, the fabricated device showed a high insertion loss, and the maximum transmission rate was only 43% [19].In summary, investigations of the electrically excited VO2 metamaterials have attracted intense attention in the research and development of terahertz functional devices. Due to the need to ensure a low insertion loss, the current research effort has focused on enabling more flexible functions, higher modulation depth, faster response rate, and lower driving power of these devices. In the development of terahertz filters, Chen et al. proposed an adjustable terahertz absorber with a multidefect combination-embedded VO2 film structure that can control the phase change of VO2 by changing the external environment temperature and enable the variation of the absorption rate at two frequencies from 99.8% to 1.0% [20]. Ma et al. introduced the conceptual design of the fractal Koch curve and fabricated a novel terahertz band-pass filter. They showed that the resonant frequency of the filter was 0.715 THz, the transmission coefficient reached 0.92, and the −3 dB bandwidth was 21.9 GHz [21]. Li et al. proposed a switchable dual-control terahertz broadband absorber based on a hybrid graphene-VO2 metamaterial. Numerical simulations showed that the amplitude of the broadband absorption band (1.69–3.21 THz) can be dynamically adjusted from 25.4% to 99.2% by adjusting the electronic conductivity of VO2 [22]. Zhang et al. proposed a vanadium dioxide-based multilayer metamaterial with bifunctional properties of absorption and polarization conversion. It was found that the designed system behaves as a single-band absorber when vanadium dioxide is in the metallic state and behaves as a cross-polarization converter when vanadium dioxide is in the insulating state [23]. Chen et al. achieved the dual functionalities of perfect absorption and high transmission in a single metamaterial by introducing vanadium dioxide film into a multilayer structure. Their device acts as a narrowband absorber when vanadium dioxide is in the conducting state and acts as a transparent conducting metal when vanadium dioxide is in the insulating state [24, 25].In this work, a metamaterial filter with a metal structure unit composed of two horizontally placed riveted metal oscillators and a vertical metal strip was designed and examined by electromagnetic simulations. The rectangular VO2 film is located underneath the metal structure. Simulation results show that in the TE mode, when VO2 was in the insulating state, this device exhibits the characteristics of a band-stop filter with the band-stop central frequency of 0.95 THz. When VO2 changed to the metal state, the band-stop characteristics of the device disappeared gradually and transmission at the central frequency increased from 1% to 80%, while transmission in other frequency bands gradually decreased and the device exhibited band-pass characteristics.


## 2. Design of Device Structure

In this work, a type of commercial full wave CST 2019 microwave studio simulation software (3DS, USA) is used to model the device. Figure1(a) schematically illustrates the three-dimensional unit structure of the proposed terahertz filter, while Figure 1(b) shows the side view of the unit structure. Here, the electric field of the terahertz wave is along the x-direction (TE mode), the magnetic field of the terahertz wave is along the y-direction, and the incident direction of terahertz wave is normal incidence.Figure 1
(a) Three-dimensional schematic diagram of the unit structure of the device. (b) Side view of the unit structure. White: silicon; blue: silicon nitride; red: VO2; yellow: aluminum.
(a)(b)The entire device is a multilayer structure, and the side view of its unit structure is shown in Figure1(b). From bottom to top, the device consists of a silicon substrate (dielectric constant ε = 11.9), silicon nitride (ε = 7, loss tangent = 0.05), and a metal layer (aluminum, electronic conductivity σ = 3.56 × 107 S/m) with a rectangular VO2 film embedded in the center of the silicon nitride layer. Figure 1 shows the top view of the metal structure and the VO2 film. The metal unit structure is composed of two horizontal riveted metal oscillators and a vertical metal strip, and the rectangular VO2 layer is located directly below the metal structure, where L1 and L2 are the unit side lengths, W is the line width of all metals, L is the width of VO2 (the VO2 length is L2), and s denotes the interval between the riveted oscillator and the metal strip. All of the units are arranged periodically, and the upper side is connected to the positive electrode, while the lower side is connected to the negative electrode, so that the device structure is formed as shown in Figure 1. The filter dimensions are approximately 1 cm × 1 cm, and the width of left and right electrodes is approximately 60 μm. The thickness of the silicon layer used in the simulation is 35 μm, the thickness of the silicon nitride layer is 500 nm, and the thickness of the VO2 layer in the middle is 120 nm. The final optimized structure parameters are set to L1 = L2 = 85 μm, s = 7 μm, L = 55 μm, and W = 5 μm.


## 3. Simulation of the Device Performance

The simulation of the device used a frequency-domain solver that was set to the unit cell bound in both thex- and y-directions, and in the z-direction, the boundary condition of incident surface was set to open addspace, while the boundary condition of the detection surface was set to open to avoid the occurrence of Fabry–Pérot oscillation. In the simulation, we simulated the electric excitation of the device by changing the electronic conductivity of VO2.First, our numerical simulations studied the effect of the insertion of the VO2 film in the insulating state (with electronic conductivity of 0.5 Ω−1 cm−1) on the transmission spectrum of this device. The results of the simulations are shown in Figure 2 and suggest that the inserted VO2 film has little effect on the transmission spectrum in the range of 0.3–1.6 THz for both TE and TM modes.Figure 2
Influence of VO2 on the transmission spectrum of the device under (a) TE mode and (b) TM mode.
(a)(b)Then, we changed the electronic conductivity of VO2 to simulate the dynamic changes of its transmission spectrum when the device is electrically excited. Figure 3(a) shows the evolution of the transmission spectrum when VO2 changed from the insulating state to the metal state with the electronic conductivity of 30,000 Ω−1 cm−1. It is observed that under the TE mode, the transmission at the central frequency of 0.95 THz gradually increased with increasing electronic conductivity of VO2. For the electronic conductivity of VO2 of 30,000 Ω−1 cm−1, the transmission at 0.95 THz increased from 1% to approximately 81%. This implies that at this frequency, the device gradually changes from the band-stop filter to the band-pass filter. Meanwhile, the behavior of the material for the frequency bands on the left (such as 0.5 THz) and right (such as 1.4 THz) sides of central frequency changed from band-pass to band-stop filter, and transmission was reduced from 80% to 8%. Thus, under electric field excitations, the proposed filter achieved transition from band-pass to band-stop filter in the frequency range of 0.3–1.6 THz due to phase transition of the VO2 layer. Additionally, the central frequencies of the band-pass and band-stop filter are almost the same.Figure 3
Transmission spectra of the device for different electronic conductivities under (a) TE mode and (b) TM mode.
(a)(b)Figure3(b) describes the influence of the electronic conductivity of VO2 on the transmission spectrum of device under the TM mode. It is observed that when VO2 was in the insulating state, the transmission of the device was higher than 50% in the range of 0.3–0.8 THz. As VO2 gradually changed to the metal state, its transmission decreased in a wide frequency range and eventually was lower than 5%. Thus, due to the lack of symmetry of the unit structure, the proposed device exhibited completely different characteristics under TE and TM modes.


## 4. Physical Mechanism of Band-Pass to Band-Stop Transition of the Device

To investigate the physical mechanism of the conversion from a band-pass filter to a band-stop filter under the TE mode, the surface currents in the device before and after the VO2 phase transition were simulated. When VO2 was in the insulating state, the device exhibited band-stop characteristics and its surface current is shown in Figure 4. For the terahertz wave frequencies of 0.6 THz and 1.4 THz, the surface current in the device was mainly concentrated on the riveting-type oscillator and was approximately 2300 A/m. For the terahertz wave frequency of 0.95 THz, the current reached 4800 A/m, demonstrating that the band-stop behavior in this device at 0.95 THz is caused by the strong surface current on the riveted oscillator and that the transmission in the left and right frequency bands is higher because the oscillation current is small and its influence on the transmittance is low, giving rise to the band-pass characteristics.Figure 4
Surface current distribution for the device when VO2 is in the insulating state for the frequencies of (a) 0.6 THz, (b) 0.95 THz, and (c) 1.4 THz.
(a)(b)(c)When VO2 was in the metal state, the device displayed band-pass behavior and its surface current is shown in Figure 5. Obviously, a strong surface current was observed only when VO2 was in the metal state. In Figure 5, the VO2 layer in the metal state is marked in yellow. As shown in Figure 5(a), both the top riveted oscillator and the VO2 layer underneath the oscillator produced surface currents at the incident terahertz wave frequency of 0.6 THz. Meanwhile, the direction of the strong current oscillation on the sides of VO2 was opposite to that of the metal structure. Therefore, the oscillation effects of these two strong currents canceled each other out. It can be concluded that the band-stop characteristics in the low-frequency region are mainly caused by the weak current that oscillates on the VO2 surface. Figure 5(b) shows the surface currents on the metal layer and the VO2 layer with for the incident terahertz wave frequency of 1.4 THz. In this case, the current was mainly concentrated on both sides of the VO2 layer for dipole oscillation and its dipole moment was small, giving rise to the band-stop characteristics in the high-frequency region of the spectrum. Figure 5(c) shows the surface currents on the metal layer and the VO2 layer for the incident terahertz wave frequency of 0.95 THz. It is observed that strong oscillation currents on these two layers propagated in opposite directions and canceled each other out. In this case, the surface current on VO2 was extremely weak. Therefore, the device showed band-pass characteristics at 0.95 THz.Figure 5
Surface current distribution of the device when VO2 is in the metal state for the frequencies of (a) 0.6 THz, (b) 0.95 THz, and (c) 1.4 THz.
(a)(b)(c)


## 5. Conclusions

A VO2-based terahertz filter has been proposed in this work, and numerical simulations revealed that the proposed device can achieve conversion from band-stop to band-pass filter under the TE mode. When VO2 is in the insulating state (electronic conductivity = 0.5 Ω−1 cm−1), the device exhibited the characteristics of a band-stop filter with the central frequency of 0.95 THz. When VO2 transitioned to the metal state (electronic conductivity = 30,000 Ω−1 cm−1), the transmission of the device at the central frequency increased from 1% to 80%, while its transmission at the other frequency bands decreased and the device transformed from the band-stop to the band-pass filter. Under the TM mode, the proposed device displayed the characteristics of a broadband terahertz amplitude modulator. Due to the phase transition of VO2, the spectrum transmission varied between 50% and 5% in the frequency range of 0.3–0.8 THz. Because of its simple structure, facile fabrication, function tunability, and low cost, the proposed filter is highly promising for use in future terahertz systems.



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*Source: 3902835-2020-12-07.xml*