1. Introduction

In recent years, metamaterials by leveraging multiple innovative techniques, have unlocked exciting possibilities in the realm of terahertz (THz) technology [1,2]. By incorporating a high-conductivity metal thin film etching aperture into the metasurface design, it becomes possible to achieve extraordinary terahertz transmission [3–7]. Furthermore, metasurfaces take advantage of the unique electromagnetic wave absorption characteristics of the metallic resonant ring, enabling perfect absorption of terahertz waves [8,9]. Another remarkable capability of metasurfaces lies in their ability to exploit the material dispersion characteristics, enabling polarization conversion of terahertz waves [10]. In contrast to conventional optical devices, the device parameters also can be manipulated by artificially controlling the properties of metamaterials [11–17]. By harnessing this property, optical devices no longer exist as isolated entities. Instead, there is a growing emphasis on the integration of multiple functions onto a single coordinated metasurface. VO₂, a phase-change metal oxide, exhibits a reversible transition from an insulator to a metal in response to stimuli such as temperature, light, and electric fields [18–22], making it highly valuable for the development of multifunctional THz devices [23–29]. Niu et al. successfully achieved broadband absorption and polarization conversion in the THz band by controlling the properties of VO₂ [25]. Wang et al. created asymmetric transmission devices that enable transmittance and absorption conversion by incorporating Dirac metals [27]. Wang et al. designed a dual-controlled switchable THz device by combining graphene with VO₂, and their proposed structure can switch among absorption, transmission, and reflection [8]. By introducing VO₂ into the grating structure, Zhang et al. investigated a hybrid waveguide grating capable of switching between narrow-band transmission filtering and reflection filtering [28]. This year, Chen et al. made full use of the phase transition characteristics of VO₂ to achieve additional polarization conversion functions on the basis of dual-function devices with switching absorption filtering [29]. These advancements in tunable THz device using VO₂ have expanded the potential applications of metamaterials in various fields.

In the THz band, the integration of additional fields in dual-function devices holds tremendous potential for achieving a significant enhancement factor in the local electric field. However, the conventional approach of creating localized electromagnetic hotspots using sharp tips [30–32] has a limitation, as very small normalized-tip areas often result in notable reductions in absorption and transmission efficiency. Consequently, developing optical devices that can achieve substantial enhancement factors while maintaining high absorption and transmission efficiency poses a significant challenge [33,34].

In this work, we study a VO₂-based THz metamaterial multifunction device capable of switching between extraordinary terahertz efficient transmission and bidirectional absorption by regulating VO₂ conductivity. When the device functions as a transmission device, VO₂ is utilized as an insulator, ensuring that it does not interfere with the transmission performance. While operating as an absorption device, the arrangement of metallic VO₂ and the metasurface functions as two mirrors, with the PP layer serving as the spacer layer. This configuration creates an optical channel similar to a Fabry-Perot (FP) cavity, which induces the excitation of the quasi-FP resonance mode and enhances the absorption performance [35]. The design concept is illustrated by analyzing the equivalent circuit of the metasurface and its effective impedance, while validating the impact of surface plasmon resonance mode and Fabry-Perot resonance mode on the transmission and absorption spectra through investigation of charge distribution and electric field distribution. The proposed device exhibits perfect wide-angle transmission and absorption performance. Notably, it possesses structural integrity even after undergoing a 90-degree clockwise or counterclockwise rotation. As a result, the device exhibits C4 rotational symmetry, providing inherent insensitivity to deflection at small incidence angles [36]. Moreover, the introduction of two bow-tie apertures to the metal and vanadium dioxide mixture films results in a significant local enhancement factor without affecting the absorption and transmission spectra, as confirmed by monitoring the electric field, providing a novel approach for creating adjustable, multifunctional THz metasurface devices with potential applications in sensing, cloaking, filtering, and single-molecule detection.

2. Methodology

The structure of the proposed THz multifunctional device is shown in Fig. 1. It comprises several layers arranged from top to bottom: a VO₂ film, a PP layer, a hybrid-patterned metasurface, a SiO₂ layer, and a VO₂ cutoff layer. The thicknesses are expressed by h_A, h_B, h_C, h_D, and h_E respectively, and the device is strictly symmetrical. The top view of the hybrid-patterned metasurface is shown in Fig. 1(a), consisting of silver groove film embedded with VO₂ in the center region. According to the Ref. [37,38], the permittivity of PP is defined as ${\varepsilon _{pp}} = 2.25 \times (1 - j{10^{ - 3}})$ and the nondestructive dielectric layers of SiO₂ with a permittivity of 1.45 are selected as the substrate in the structure.

 

Fig. 1. Schematic of the VO₂-based switchable multifunctional THz metamaterial device: (a). hybrid-patterned metasurfaces top, (b). side, and (c). perspective views of the structure.

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The dispersion characteristics of metallic silver materials are determined by the frequency-dependent dielectric constant, for which the Drude model is commonly employed for its definition [39]. The permittivity of VO₂ is critical for the structure's absorption performance. When VO₂ is in the metallic phase, it behaves like a “poor metal”. In the THz band, the permittivity of VO₂ can be effectively described by the Drude mode, ${\varepsilon _{V{O_2}}} = {\varepsilon _\infty } - {({\omega _p}({\sigma _{V{O_2}}}))^2}/({\omega ^2} - i\gamma \omega )$, where the permittivity at an infinite frequency is ${\varepsilon _\infty } = 12$ and the damping frequency is $\gamma = 5.75 \times {10^{13}}{S^{ - 1}}$ [40]. Moreover, ${\omega _p}({\sigma _{V{O_2}}})$ is the conductivity-dependent plasma frequency at ${\sigma _{V{O_2}}}$ which can be described as $\omega _p^2({\sigma _{V{O_2}}}) = ({\sigma _{V{O_2}}}/{\sigma _0})\omega _p^2({\sigma _0})$ (${\sigma _0} = 3 \times {10^5}S \cdot {m^{ - 1}}$ and ${\omega _p}({\sigma _0}) = 1.4 \times {10^{15}}rad \cdot {S^{ - 1}}$). When the conductivity increased from 10 to 100 000 S/m, it was assumed that VO₂ underwent a transition from an insulator to a fully metallic state. This transition can be induced by various stimuli such as thermal, electrical, or optical means.

In the simulation setup, a perfect matching layer was placed in the z-direction, while periodic boundary conditions were applied in the x and y directions. Based on the skin effect, 5µm VO₂ intermediate layer at the THz band would effectively prevent the transmission light when VO₂ is in its metallic state. After optimization, the aperture parameters in hybrid-patterned metasurfaces are determined as follows: a = 65µm, b = 40µm, and c = 15µm. The thickness and period parameters of the THz multifunctional device of each layer are shown in Table 1.

Table 1. The thickness and period parameters of the THz multifunctional device.

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2.1 Application of PP layer and VO₂ layer

The design of the metasurface device incorporates a PP layer to enhance its transmission capabilities. This improvement can be attributed to the symmetric arrangement of the upper and lower layers, which forms an optical channel resembling a FP cavity. This design induces a quasi-FP resonance that couples with the plasma resonance in the metallic film, thereby significantly enhancing the overall transmission characteristics of the device. With a conductivity of 10 S/m, VO₂ is in the dielectric phase. When the PP layer is not included in the device, as depicted in the side view subplot of Fig. 2(a), the transmission in the frequency band between the two transmission peaks is inefficient and does not meet the requirements of modern optical devices. However, a noteworthy improvement is observed when the PP layer is introduced into the device. The performance of extraordinary terahertz transmission is significantly optimized, and the transmission bandwidth is noticeably expanded while still maintaining a transmission peak value above 90%.

 

Fig. 2. After setting the parameters according to the Table 1, (a) the transmission efficiency achieved by the device is compared with that without adding PP layer, (b) the absorption efficiency achieved by the device is compared with that without adding VO₂ film.

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When the device switches to functioning as an absorption device, VO₂ in its metallic state with a conductance of 10,000 S/m. The VO₂ mesosphere could behave as a cutoff layer to hinder the transmission of incident light by the skin effect. Without the outer VO₂ films, as depicted in the side view subplot of Fig. 2(b), two separate PP layer are lossy medium, which results in significant energy loss of the incident light within the layer. The absorption efficiency is unsatisfactory due to the lack of resonance mode within the structure and the loss in PP layer. With the VO₂ films as a “metal” layer outside of PP layer, a new quasi-FP resonance between the VO₂ film and the metasurface is excited, which significantly improves the absorption efficiency. This phenomenon can be attributed to the structure of the quasi-FP cavity, which typically consists of two mirrors separated by a dielectric spacer with a thickness of at least a quarter wavelength. When the outer VO₂ film is in a metallic state, it, along with the metasurface, behaves as two reflective mirrors in the vertical direction. The two reflecting mirrors are separated by a 30µm thick PP layer. Consequently, an optical channel similar to the FP cavity is formed, allowing for the excitation of quasi-FP resonance for incident waves in the range of 0.1-2.5THz [41–44]. After incorporating the VO₂ film, the device's absorption spectrum displays two distinct peaks with absorption efficiencies surpassing 99%.

2.2 Equivalent circuit model

The entire device can be analyzed and decomposed using circuit theory [45]. Taking the metasurface layer for example, the electric field component of incident light drives and displaces charges, resulting in the formation of dipoles that can be considered as micro/nano-scale capacitors. As the external field oscillates, currents flow within the metasurface, necessitating the consideration of inductance. Additionally, Ohmic loss in metals and dielectric absorption in the surrounding medium can be represented as resistors in an electronic circuit. The metasurface can be naturally modeled using an RLC circuit. When VO₂ is in metallic state, we treat it as a whole with silver (inside the black box in Fig. 3(a)), as shown in the top view of the surface and the equivalent circuit in Figs. 3(a) and (b). After this transformation, Fig. 3(b) can be represented as an equivalent impedance Z_M, which can be obtained using the following formula: ${Z_M} = {R_1} + j2\pi f{L_1} + 1/j2\pi f{c_1}$ [46].

 

Fig. 3. (a) metasurface top view, (b) metasurface equivalent circuit, (c) structure side view, (d) structurally equivalent circuit, (e) absorption spectrum and effective impedance.

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Indeed, the absorber as a whole can be represented as an equivalent circuit. Figure 3(c) shows the side view of the structure, and Fig. 3(d) illustrates the specific equivalent circuit. Each layer in the absorber corresponds to a specific electrical component. The PP layer is represented as the impedance Z₂, the metasurface is represented by the impedance Z_M, the SiO₂ layer is represented as the capacitance C₂, and the VO₂ cutoff layer is represented as the impedance Z₃. To maintain a fixed equivalent impedance Z_M for the metasurface, we adjust the overall impedance of the entire circuit by adding the VO₂ film. This corresponds to the impedance Z₁ in Fig. 3(d). In the absence of a VO₂ film (impedance Z₁), the overall impedance of the device Z, becomes excessively large. However, when a VO₂ film is added in parallel (impedance Z₁), the overall impedance diminishes. As a result, with varying incident light, the impedance Z fluctuates in close proximity to the equivalent impedance, facilitating the emergence of two distinct absorption peaks. The introduction of the VO₂ film (Z₁) effectively tunes the overall impedance of the absorber, allowing it to match the incident light conditions and exhibit the desired absorption behavior with two distinct absorption peaks.

Furthermore, the effective impedance and absorption of the structure are shown in Fig. 3(e). Indeed, achieving 100% absorption in the device is theoretically possible when the effective impedance perfectly matches the free-space impedance. This condition implies that the real part of the effective impedance should be one, while the imaginary part should be zero. When the effective impedance at the frequency corresponding to the absorption peak closely aligns with the free-space impedance, it indicates efficient absorption of the incident terahertz waves. However, at the frequency corresponding to the lowest absorption performance between two absorption peaks, the absorption experiences damping due to the substantially low real part and excessively high imaginary part of the effective impedance. This mismatch leads to less efficient absorption at that particular frequency. Moreover, the phenomenon of the equivalent impedance continuously fluctuating around the free-space impedance to produce two matching points aligns with the observations extracted from the equivalent circuit in Fig. 3(e). This continuous fluctuation is a crucial factor in creating the two distinct absorption peaks, indicating the device's ability to selectively absorb terahertz waves at specific frequencies.

3. Result and discussion

3.1 Bi-functional optical characteristics

Through analyzing the electric field and charge distribution of the device, we were able to determine its transmission and absorption characteristics. When the multi-layer array is used as a transmission device, the electric field distribution (log scale) and charge distribution (log scale) under various incident light frequencies is illustrated in Figs. 4(a1-c1), where the electric field distribution is denoted by the colormap and the charge distribution is denoted by the arrow. In this context, the electric field distribution near the aperture is referred to as the eigenmode. The eigenmode represents the local surface plasmon resonance (LSPR), which corresponds to the movement of charges occurring within the aperture of the device. Conversely, the electric field that propagates within the metal plate towards other lattices is known as the lattice mode. The lattice mode signifies the surface plasmon polaritons (SPPs), which corresponds to the movement of charges propagating towards the surrounding lattice. In this low-frequency regime, at an incident light frequency of 0.85 THz, the energy of the incident light is not efficiently coupled into the eigenmode or the lattice mode. As a result, the transmission of light through the structure is reduced, leading to lower transmission efficiency. When the incident light’ frequency increases to 1.14THz, the excitation state of both lattice mode and eigenmode is improved, corresponding to the transmission peak in the low band of transmission spectrum. At the high-frequency range, specifically at 1.63THz, the intensity of the eigenmode remains consistent, while the lattice mode continues to increase. This leads to an improvement in the extraordinary terahertz transmission performance.

 

Fig. 4. The electric field distribution (log) and charge distribution (log) of the hybrid-patterned metasurface when the structure works as transmission device, (a1), (b1), (c1) is the x-y cross-section of the metasurface, (a2), (b2), (c2) is the y-z cross-section of the device. The electric field distribution is denoted by the colormap and the charge distribution is denoted by the arrow.

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Additionally, we provide the electric field and charge distribution of the y-z cross-section of the device at the incident THz wave frequencies of 0.85THz, 1.14THz, and 1.63THz, as illustrated in Figs. 4(a2-c2) correspondingly. When f = 0.85 THz, it is evident that the electric field and charge are not present in the lower PP layer, indicating that the quasi-FP resonance mode is difficult to trigger between the upper and lower PP layers. Therefore, the transmission efficiency of the device is relatively low at this frequency. However, when f increases to 1.14THz and 1.63THz, the electric field and charge distribution in the upper and lower PP layers become more prominent. Meanwhile, the electric field and charge distribution around the metasurface are also enhanced significantly. These findings suggest an effective coupling between the quasi-FP resonance mode and the eigenmode at these frequencies, thereby promoting high transmission efficiency.

When the structure is used as an absorption device, the electric field and charge distribution (log scale) at various incident light frequencies are depicted in Figs. 5. Figs5. (a1-c1) illustrate the electric field and charge distribution for the device in the x-y cross-section at incident light frequencies of 1.15THz, 1.61THz, and 1.97THz. As observed at these three different frequencies, the electric field primarily resides within the aperture, while only a minimal field is present on the metal film and connects to the surrounding lattice. This phenomenon can be attributed to the interaction between the THz incident wave and the metasurface array structure. Consequently, most charges exhibit oscillatory motion based on the aperture within one lattice, while the remaining charges flow towards the surrounding lattice, which aligns with the distribution of the electric field. The confined electric field within the aperture indicates the presence of dipole resonance, which plays a critical role in enhancing the absorptions of the device.

 

Fig. 5. The electric field distribution (log) and charge distribution (log) of the hybrid-patterned metasurface when the structure works as absorption device, (a1), (b1), (c1) is the x-y cross-section of the metasurface, (a2), (b2), (c2) is the y-z cross-section of the device. The electric field distribution is denoted by the colormap and the charge distribution is denoted by the arrow.

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Figures 5(a2-c2) display the electric field and charge distribution in the y-z cross section. When f = 1.15THz and 1.97THz, as shown in Figs. 5(a2) and (c2), the charge and electric field are distributed between the metasurface and the outer VO₂ film, indicating a strong coupling effect between the dipole resonance and the quasi-FP resonance. When f = 1.61 THz, as shown in Fig. 5(b2), the electric field and charges are fully concentrated around the metasurface. As a result, the quasi-FP resonance cannot be effectively excited, rendering the added outer VO₂ layer ineffective. The attenuated electric field around the outer VO₂ film suggest the decoupling between the dipole resonance and the quasi-FP resonance, resulting in a decrease in absorption efficiency, as the absorption valley shown in the blue line in Fig. 2(b). In addition to its transmission and absorption capabilities, the hybrid-patterned metasurface exhibits regions on both sides in the x-direction where electric fields and charges are absent. Utilizing these areas by incorporating an aperture structure can provide additional functionality without affecting the device's transmission and absorption properties.

3.2 Effect of aperture, structure and thickness parameters on device performance

We expand our research to examine the relationship between structural parameters and the transmission and absorption spectrum in light of exploring the physical mechanisms of multi-coupling between different resonance modes. Parametric studies have revealed that the transmission and absorption properties of the device are not significantly affected by the lattice constants in the x and y directions. Figures 6(a1) and (a2) show the impact of the P_x on the transmission and absorption spectra. As the period decreases in the x direction, the metal film area decreases, resulting in a gradual increase in transmittance within the frequency range between the two transmission peaks while maintaining the stability of the resonance mode. Notably, the absorption spectrum remains almost unaffected by these changes. Similar observations can be made regarding the influence of the P_y on device performance, as illustrated in Figs. 6(b1) and (b2). With a gradual increase in the y-direction period, the transmission spectrum experiences a certain blue shift. Similarly, to Fig. 6(a2), the y direction period almost does not affect the absorption performance of the device.

 

Fig. 6. Set P_x to increase from 90µm to 110µm, (a1) transmission, (a2) absorption, set P_y to increase from 90µm to 110µm, (c) transmission, (d) absorption.

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The effects of aperture parameters a and b on transmission and absorption spectra are demonstrated in Fig. 7. As the aperture parameter a is increased, the transmission intensity of the device gradually increases, accompanied by the expansion of the transmission bandwidth on both sides, as shown in Fig. 7(a1). This phenomenon shares the same origin as that observed in P_x. However, the scanning of the aperture parameter a demonstrate that the metal plate outside the aperture has little effect on the absorption spectrum, as shown in Fig. 7(a2). As shown in Fig. 7(b1), by increasing the aperture parameter b, the transmission bandwidth decreases while improving the quality of the passband. Simultaneously, the transmission peak experiences a noticeable blue shift as a result of the aperture duty cycle variation, which influences the matching range between the resonance mode and the incident light frequency. This effect is similar to the influence of the y-direction period on the transmission spectrum, suggesting the effective excitation of the resonance mode between the metal region and the aperture when the structure is utilized as a transmission device. The variation in the aperture parameter b induces a similar effect on the absorption spectrum as observed in the transmission spectrum, as shown in Fig. 7(b2). As the aperture parameter b gradually increases, the device maintains perfect absorption while experiencing a redshift in the high-frequency absorption peak. Additionally, the two absorption peaks gradually merge, resulting in a trend that absorption performance within the frequency band between the absorption peaks is improved as the bandwidth shrink.

 

Fig. 7. Set the aperture parameter a increase from 55µm to 75µm, (a1) transmission, (a2) absorption, set the aperture parameter b increase from 90µm to 120µm, (b1) transmission, (b2) absorption.

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When designing metasurface devices, one of the key objectives is to promote the coupling of multiple resonance modes. This coupling is achieved by introducing additional PP layers. However, it is essential to strike a careful balance in setting the thickness of these PP layers due to the loss of incident light caused by the imaginary part of the refractive index in the lossy medium. Figure 8(a1) illustrates the variation in transmission efficiency as the PP layer thickness gradually decreases from 30µm to 40µm. Increasing the PP layer thickness from 30µm to 35µm results in a significant improvement in transmission efficiency. However, further increasing the thickness of the PP layer leads to a deterioration in transmission efficiency. Figure 8(a2) reveals the change of the absorption spectrum as the PP layer thickness increase from 30µm to 40µm. With the increase of PP layer thickness, the entire absorption curve gradually tends to redshift, while the absorption bandwidth of the high frequency band decline. Decreasing the thickness of the PP layer results in asymmetric intensity of two transmission peaks, while also causing a noticeable blue shift in the absorption peak. As mentioned above, the introduction of PP layer enriches the resonance mode in the device. However, when the added lossy medium layer fails to excite the resonance mode, as demonstrated in Fig. 2(b), or unrealistic parameters are established, resulting in ineffective coupling between the resonance modes in Fig. 8(a1), which is counterproductive.

 

Fig. 8. (a1) The transmission spectrum as h_B increase from 30µm to 40µm, (a2) the absorption spectrum as h_B increase from 30µm to 40µm, (b) the transmission spectrum as h_D increase from 10µm to 20µm, (c) the absorption spectrum as c increase from 5µm to 25µm.

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When the metasurface device is used as a transmission device, the VO₂ layer acts as an insulator. In this case, the transmission performance is primarily influenced by the presence of the SiO₂ layer. As shown in Fig. 8(b), although SiO₂ as a non-loss material does not cause the loss of incident light, the change of the thickness of the layer will affect the intensity of the quasi-FP resonance. As the thickness of the SiO₂ layer increases, the transmission peaks in the high-frequency band undergo a redshift, while the transmission peaks in the low-frequency band experience a blueshift. Thereafter, the two transmission peaks gradually converge. However, excessive thickness of the SiO₂ layer will lead to abnormal coupling between the resonance modes and unstable transmission spectrum.

When the metasurface device functions as an absorption device, the SiO₂ layer only exists as a substrate. Therefore, the influence on the absorption performance is primarily analyzed based on the aperture parameter, denoted as c in Fig. 8(c). The aperture parameter c shows no influence on the absorption peak in the low-frequency band, as the absorption curves under all parameters completely overlap. However, with a gradual increase in the incident light frequency, particularly when c reaches 25µm, the absorption peak exhibits red-shift. In the frequency band where the incident frequency exceeds 2THz, the absorption efficiency becomes unstable.

3.3 Effect of incident angle on device performance

Optical devices that can maintain excellent performance across a range of incidence angles are highly valuable in modern optical technology. In Figs. 9(a) and (b), we present how the performance of the device changes as the incidence angle increases from 0 to 90 degrees when the structure is used as either a transmission or absorption device. The relationship between the transmission spectrum and the incident angle is shown in Fig. 9(a). As the incidence angle of THz wave increases, the transmission bandwidth decreases overall and the transmission efficiency two transmission peaks is attenuated. The low frequency band of the transmission spectrum is getting shrink, while the high frequency peak of the transmission spectrum experiences a redshift. As incidence angle of THz wave exceeds around 50°, the third transmission peak will be triggered in vicinity of the high frequency band. Turning to the study of the sensitivity of absorption performance to incident angle, the result is shown in Fig. 9(b). As the incidence angle increases, the absorption peak in the low frequency region narrows towards the high frequency band. On the other hand, the high-frequency band remains relatively stable with minor changes. It is noticed that as incidence angle of THz wave exceeds around 20°, a new absorption peak appears in the high frequency band. The eigenmode and the dipole resonance of a metasurface is primarily determined by the coupling that occurs within its aperture. This coupling is not only sensitive to the angle of incident deflection but also affected by higher-order diffraction resulting from the deflection angle. As shown in Fig. 9(a) and (b), the peak in the low frequency band changes more obviously with the deflection angle of the incident wave. When the incident light angle is too large, due to the influence of high-order diffraction, the diffraction peak will appear in the high frequency band. This shows that the proposed device has a very outstanding tolerance within 50° of incident angle for transmission, and within 20° of incident angle for absorption performance. Furthermore, due to the C4 rotational symmetry of the structure, the device is inherently insensitive to polarization when the incidence angle is small [36]. This polarization insensitivity adds to the stability and practicality of the device, making it suitable for addressing practical engineering challenges.

 

Fig. 9. When the incidence angle increases from 0° to 90°, (a) transmission spectrum and (b) absorption spectrum of the structure.

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3.4 Local electric field enhancement

Building upon the observation of the ‘vacuum region’ in the charge distribution depicted in Figs. 4 and 5, we introduce bow-tie apertures on both sides to attain local enhancement factors without affecting the transmission and absorption properties. By iteratively optimizing the parameters, we successfully achieve a substantial local electric field enhancement while preserving the transmission and absorption properties. The gap size is expressed by g, setting D_x= 10µm, D_y= 15µm and g = 100µm, the multi-functional metasurface with local enhancement is shown in Fig. 10. Based on Fig. 10(b), the maximum achievable enhancement factor reaches 1105 when the gap size of the bow-tie aperture approaches 40 nm. Notably, even with a reduction in the gap size to only 100 nm, it remains feasible to achieve an enhancement factor exceeding 400. In previous studies, we have shown that width of bow bow-tie aperture D_y has a significant gain effect on the enhancement factor because it affects the inclination at the gap [47]. Through parameter optimization, when D_y is set to 15µm, the “vacuum region” can be maximally utilized. In conventional cases, the presence of an aperture in a metasurface can often lead to undesirable effects, such as dampening of transmission or absorption properties [32]. However, the design principle we have proposed offers a comprehensive solution to completely overcome this issue. In this study, we find that not all regions in the metasurface excite the required resonance modes of transmission and absorption, and that effective use of the “vacuum region” can fully realize new functions without sacrificing performance, which undoubtedly provides a new way of thinking for subsequent optical device design.

 

Fig. 10. (a) Modified top view of hybrid-patterned metasurface, (b) the relationship between local enhancement factor and gap size and the near-field electric field distribution of bow-tie aperture, (c) modified device perspective.

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4. Conclusion

In summary, we propose a novel THz device capable of switching between extraordinary terahertz transmission and bi-directional perfect absorption while simultaneously realizing a huge local enhancement factor. The structure breaks the limitation of dual function in conventional optical devices and can realize multiple functions. We add bow-tie apertures on both sides of the hybrid-patterned metasurfaces to achieve a significant enhancement factor without affecting the transmission and absorption capabilities by analyzing the electric field distribution and charge distribution. Furthermore, we display that the structure can achieve wide-angle transmission and absorption, demonstrating its extraordinary applicability in solving practical engineering problems. The proposed structure provides a novel approach for the creation of adjustable, multifunctional THz metasurface device with potential applications in fields such as sensing, cloaking, filtering and single-molecule detection.