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Abstract
A switchable multi-function terahertz metasurface employing phase transition material vanadium dioxide (VO2) is presented and investigated. By varying the temperature, the hybrid structure can switch between transmission, absorption, and reflection modes in broadband ranges. When the temperature is below 340 K and VO2 is in the insulating state, perfect polarization conversion is demonstrated. Efficient asymmetric transmission (AT) exceeding 0.7 is simultaneously achieved with an ultra-wide bandwidth of 3.6THz. When VO2 is in the metal phase, it shows different modulation characteristics for x and y-polarized waves. The structure can absorb over 90% of y-polarized waves from 3.56 THz to 7.2 THz (bandwidth, 3.64 THz), while 85% of x-polarized waves are reflected from 1 THz to 9 THz (bandwidth, 8 THz). Compared with other related published works, the designed structure makes significant progress in integrated functionalities, operating bandwidth, and working efficiency. It shows great potential for use in terahertz dynamic control and multifunctional integrated systems.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz waves have significant applications in imaging [1], communication [2], and information sensing [3]. However, the development of Terahertz technologies has been limited by the lack of natural materials that have efficient interaction with Terahertz waves. Metasurfaces have been investigated for a few decades as an outstanding alternative to tailor the interaction between electromagnetic waves and matter. Devices with unique functions have been invented and extensively studied, such as polarization conversion [4,5], focusing [6,7], absorption [8–10], holograms [11,12], and asymmetric transmission (AT) [13,14]. As one of the most important properties of electromagnetic waves, it is of great interest to explore polarization-related converters and asymmetric transmission devices. Absorbers have also attracted a vast of attention due to their practical applications in imaging, detecting, and other fields.
In the past, researchers only used metals and dielectrics to construct metasurfaces, where just single and fixed functional properties were obtained. For example, Landy et al demonstrated a narrow absorber with an absorption rate of 96% designed only by metal [15]. Grady et al showed a polarization converter, with a conversion efficiency exceeding 50% from 0.52 to 1.82 THz [16]. Li et al displayed the experimental proof of near-infrared asymmetric transmission with L-shaped and dual antenna nanostructures [17]. However, those designs with a unique but non-tunable function cannot meet the demands for the integration of multi-function devices. To extend the functions of the design of terahertz devices, tunable active materials such as graphene [3], liquid crystal [18], Dirac semimetals [19] and vanadium dioxide (VO2) [20,21] have been introduced into the metasurface. In 2018, Ding et al proposed a metasurface design that allowed switching between the absorber and the half-wave plate via VO2 [22]. In 2021, by introducing graphene and VO2 into a designed THz absorber, Liu et al achieved two different absorption effects (broadband and dual-frequency absorption) in one structure [23]. Ren et al proposed a dual-function metasurface based on VO2 [24]. When VO2 is in metallic state, single-frequency narrow-band perfect absorption at 1.75 THz is shown. Efficient linear asymmetric transmission between 1.5-2.5THz is obtained when VO2 changes to its dielectric state. In 2022, Lv et al demonstrated a THz metasurface design combining AT and polarization conversion functions with a simple three-layer structure embedded VO2 rectangle [25]. Besides, some experimental results based on VO2 with tunable asymmetric transmission functions were presented [26,27]. Various progress has been made in multifunctional integration, however, the functions of reported structures are limited, and the operating bandwidth and general working efficiency are unsatisfactory. Rare results demonstrating four functions (polarization conversion, AT, absorption, and reflection) simultaneously in one THz metasurface are reported, especially for working in broadband.
In this work, we present a terahertz metasurface with four functions based on phase change material VO2. According to some references, the response time of VO2 irradiated by a continuous-wave laser beam can be increased to the order of microseconds [26]. When triggered by a high-power pulse laser, the insulator-to-metal phase transition can be as fast as tens of femtoseconds [28]. When VO2 is in the insulating phase, the metasurface works in transmission mode and enables perfect polarization conversion and broadband AT. The bandwidth is 3.6THz (1.50-4.49 and 4.55-5.16THz) with AT efficiencies over 0.7. When VO2 is converted into the metal phase, the metasurface acts as an absorber for y-polarized incident waves and the absorption bandwidth (absorption rate over 90%) is 3.64 THz, while x-polarized waves are almost completely reflected. Our design takes the number of integrated functions, operating bandwidth, and working efficiency in a single metasurface to a new level. The physical mechanisms behind the high performance and multifunction response are analyzed and explained in detail. The switchable multifunctional metasurface demonstrates the potential for future applications in terahertz splitters, circulators, isolators and optical limiters/diodes [26,27,29].
2. Structural model and method
We design a structure with three different patterned layers to break mirror symmetry in the z/-z direction and improve the transmission efficiency and bandwidth of the metasurface because of the weak interaction between electromagnetic waves and single-layer structure [29,30]. Besides, phase change material VO2 is introduced into the metasurface, and polarization selective absorption is achieved through the patterned structure of Au and VO2. The schematic of the multifunctional metasurface is shown in Fig. 1(a). The period of the unit cell is Px = Py = 28µm. There are five layers in a unit cell, including the top patterned layer, the first dielectric layer, the middle patterned layer, the second dielectric layer, and the bottom patterned layer. The top patterned layer consists of three horizontal Au stripes (σAu = 4.56 × 107S/m [31]) and two VO2 rectangles as shown in Fig. 1(b). The specific geometric parameters are set as m1 = 2µm, m2 = 2µm, m3 = 6µm, t1 = 8µm, and t2 = 5µm. The bottom layer is the same as the top one but rotated clockwise by 90°in Fig. 1(d). A 1-µm-thick VO2 film embedded with an Au rectangle with a length of 30µm and a width of 6µm forms the middle pattern layer (Fig. 1(c)). The three patterned layers are separated from each other by a dielectric material Topas, the relative dielectric permittivity of which is 2.35 [32]. The thickness of both dielectric layers is 6.5µm. Rapid advances in fabrication technology provide a potential way to realize our design. A Cr sacrificial layer on the Si substrate is used to achieve a self-standing structure [33,34]. The three patterned layers composed of Au and VO2 [35–37] can be fabricated by reactive magnetron sputtering, lithography, ion beam milling, and lift-off technique. The dielectric Topas is obtained by spin coating and heat curing. At last, the metasurface is immersed in a Cr etchant to remove the Cr sacrificial layer and Si substrate, forming the proposed multi-function design in Fig. 1.
Fig. 1. (a) Schematic of the periodical unit cell based on VO2, the left side indicates the top layer. (b) Top view of the top patterned layer made up of Au and VO2. (c) Top view of the middle Au-VO2 layer. (d) Top view of the bottom Au-VO2 patterned layer.
The conductivity of VO2 in the terahertz band can be depicted by the Drude model [38]:
3. Results and discussion
3.1 Transmission mode
When the temperature is below 340 K, VO2 is in the insulating phase and the metasurface works in transmission mode. The Jones matrix is introduced to describe the correlation between the incident and transmitted waves [39,40]:
Figure 3(a) displays the polarization conversion rate (PCR) (PCR(y)=txy2/(txy2 + tyy2) and PCR(x)=tyx2/(tyx2 + txx2)) curves of y-polarized(x-polarized) incident waves along -z(z) direction. We can see that the PCR curves for both cases are identical and over 0.9 in 1.38-4.49 and 4.54-5.39 THz, indicating great polarization conversion. The vast difference between txy and tyx indicates that there is a great AT effect in this mode of the proposed structure. Asymmetric transmission (AT) refers to the significant difference in transmission when waves with a certain polarization state propagate along opposite directions, which can be described as [40,41]:
Fig. 3. (a) PCR curves for y-polarized and x-polarized waves. (b) Asymmetric transmission curves for linear and circular polarization waves.
For linear polarization incident waves, ${t_{xx}} = {t_{yy}},{t_{xy}} \ne {t_{yx}}$ are required to achieve asymmetric transmission. It demonstrates efficient broadband AT response in Fig. 3(b) with bandwidth 3.6THz (1.50-4.49 and 4.55-5.16THz) and AT efficiency over 0.7 for y-polarized incident light (red solid line). From Eq. (3), the AT effect is reversed for x- and y-polarization electromagnetic waves when they are incident along the same direction. As shown in Fig. 3(b), the values of AT for different polarization waves are opposite (red and blue lines). For circularly polarized waves, the transmission matrix is expressed as [42]:
We further discuss the physical origin of the metasurface to realize the large polarization conversion and asymmetric transmission. Conventional multipole decomposition shows an overall analysis of near-field electromagnetic distribution and far-field response which weakens the effect of critical vertical coupling between layers [43]. Many studies have shown that electrical and magnetic resonance occur between the two layers due to the coupling effect and that parallel currents cause the electrical resonance and anti-parallel currents lead to magnetic resonance [44,45]. As a typical analytical method, the theory of the distributions of currents and induced dipoles has been widely used in recently published papers [14,19,38,46–48]. The electric distributions and surface current vector directions of three patterned layers are shown in Fig. 4 at four peaks when VO2 is insulating. We can see that the electric field mainly concentrates on different positions of the Au strip border causing the different directions of the current. At 1.6 THz, the current on the top layer is from right to left along the Au stripes (Fig. 4(a1)) while currents on the middle and bottom layer also flow along the disposition of Au rectangles (Fig. 4(a2) and 4(a3)). The current in the middle layer can be decomposed into two mutually perpendicular components along the x-axis and y-axis. Because its current component on the horizontal axis is in the opposite direction as the current in the top layer, it excites the magnetic dipole moment (m1), inducing a magnetic field (H1) (see Fig. 4(a4)). Another magnetic dipole moment (m2) and magnetic field (H2) (see Fig. 4(a4)) are excited by the opposite current direction along the y-axis in the middle and bottom patterned layers. Owing to H1 being parallel to the incident electric field Ey, it takes a critical role in polarization conversion, while H2 is vertical to Ey with no effect on polarization conversion. For 2.51THz, the opposite current between the middle layer and the bottom layer along the y-axis (see Fig. 4(b2) and (b3)) results in the magnetic dipole moment (m1) and induced magnetic field (H1) (see Fig. 4(b4)). However, H1 is perpendicular to the incident electric field Ey and does not contribute to the great AT effect. Therefore, the electric field (E1) (see Fig. 4(b4)) induced by isotropic current between the current on the top and middle layers along the x-axis (see Fig. 4(b1) and (b2)) causes the polarization conversion at 2.51THz. Using the same approach to analyze the physical mechanisms of 4.34THz and 4.64THz, it is found that induced electric field E1 and magnetic field H1, H2 all contribute to excellent AT effect at 4.34THz (Fig. 4(c1-c4)) and only induced magnetic field (H1) (Fig. 4 d1-d4) results in great polarization conversion at 4.64THz.
Fig. 4. The electric field distributions (color section), surface current vector distributions, and the induced electromagnetic field of three patterned layers at (a1-a4) 1.6THz, (b1-b4) 2.51THz, (c1-c4) 4.34THz, (d1-d4) 4.64THz when VO2 is in the insulating state.
The electric fields of the y-z plane and the x-y plane with y-polarized and x-polarized incident THz waves are shown in Fig. 5 at 1.6THz and 4.51THz. We use 1.6THz as an example to explore the electric field distribution for the large AT effect. The y-polarized waves have clear interaction with the patterned layers of the structure in the y-z plane (in Fig. 5(a)) while the x-polarized waves barely transmit through it in Fig. 5(b). The electric field distributions also have a huge difference in the x-y plane for mutually perpendicular polarized waves from Fig. 5(c) and 5(d). However, as for 4.51 THz, an obvious change can be seen in the electric field. When the incident waves are y polarized, there is a strong electric concentration around the middle layer which limits the forward transmission. But for x-polarization waves, the strong electric field distribution in the middle part helps the light transmission. Thus, the asymmetric transmission effect at 4.51 THz decreases.
Fig. 5. The electric field distribution of the y-z plane for (a) y polarization and for (b) x polarization and the x-y plane for (c) y polarization and for (d) x polarization at 1.6THz, the y-z plane for (e) y polarization and for (f) x polarization and the x-y plane for (g) y polarization and for (h) x polarization at 4.51THz.
3.2 Absorption/reflection mode
When the temperature exceeds the critical phase change temperature of VO2, it transforms from the insulating state to the metallic state. Because the thickness of the middle pattern layer is greater than the skin depth of incident THz waves, it almost completely blocks the downward propagation of electromagnetic waves. Therefore, the electromagnetic radiation intensity through the metasurface is almost zero. In this case, the proposed structure works as a three-layer sandwich structure. The absorption rate can be simplified as A = 1-R = 1-|S11|2, where R=|S11|2 represents reflectance obtained by S-parameters. In Fig. 6(a), we find that the absorption bandwidth is about 3.64 THz from 3.56 THz to 7.2 THz with an absorbance over 90% for y-polarization waves (red line), while x-polarized waves are consistently reflected with absorption no more than 0.15 (blue line) from 1 THz to 9 THz. It exhibits a clear polarization-selective absorption/reflection phenomenon. To understand the reasons for the high polarization selectivity, the impedance matching theory is used to analyze the physical mechanism. The relative impedance between the metasurface and the free space can be expressed as [49]:
Fig. 6. (a) The absorption spectrum for x or y polarization illumination. (b) Relative impedance of the designed structure for x and y-polarized incident waves.
To further investigate the physical mechanism of the high absorption of y-polarization waves, we discuss the electric field (color section) and surface current distributions (vector section) at two absorption peaks in Fig. 7. We can see from Fig. 7(a) that the electric field in the first patterned layer mainly concentrates on the middle part of the middle Au strip and slightly distributes in boundaries of the upper and lower Au strips at 4.05THz. Correspondingly, the current flows mainly from the center of the middle Au strip which has the strongest electric location, through the metallic VO2 on both sides, to the lower Au strip. For the intermediate pattern layer which acts as a barrier to electromagnetic waves, there is a negligible localized effect of the electric field with upward flowing current in Fig. 7(c). The appearance of anti-parallel currents in the upper and lower layers indicates the excitation of strong magnetic resonance, resulting in strong absorption at 4.05 THz. For the second peak (6.67THz), a clear electric localization at the upper and lower inner boundaries of the square enclosed by Au and VO2 is found (see Fig. 7(b)). The current direction starts at the middle of the middle Au and flows through the two VO2 rectangular which is similar to the direction of the current at 4.05 THz. The difference is that the current finally converges on the upper edge of the lower Au strip at 6.67 THz, while at 4.05 THz it does not. From Fig. 7(d), the current in the lower layer with weakened electric field has almost the same current direction as that of the upper patterned layer. The parallel current of the absorber means that electrical resonance is the main factor causing high absorption at 6.67 THz.
Fig. 7. The electric field distributions and surface current vector distributions on the top patterned layer at (a) f1 = 4.05THz and (b) f2 = 6.67THz and on the middle patterned layer at (c) f1 = 4.05THz and (d) f2 = 6.67THz as VO2 is in the metal state.
Typical research results combining some of asymmetric transmission, polarization conversion, absorption, and reflection are listed in Table 1. It can be seen that although some active materials are introduced to increase the functionality of metasurfaces, their functions are still confined, and the operation bands and efficiencies are still limited. In this work, we create a structure to combine all four functions based on VO2 and innovatively implement them both running on broadband with high working efficiencies. In transmission mode, the bandwidth of the PCR over 90% and the AT rate exceeding 70% corresponds to 3.6 THz. Switched VO2 to metal phase, more than 90% of y-polarized waves are absorbed within 3.64 THz, while over 85% of x-polarized waves are reflected from 1 THz to 9 THz. The simulation results present great progress in recent related reports.
Table 1. Comprising the Proposed Design with the Previous Related Designs
4. Conclusion
In conclusion, a terahertz device with switchable multiple functions is reported. The metasurface contains three different Au-VO2 patterned layers separated by two dielectric layers. Based on the phase transition property of VO2, it can be flexibly converted between the absorption/reflection and transmission modes. When VO2 is in the insulating state, it realizes a highly efficient polarization conversion effect for linear polarization electromagnetic waves. In the same band (1.50-4.49 and 4.55-5.16 THz), the structure simultaneously shows the AT function whose operating bandwidth reaches 3.6 THz over a 0.7 AT ratio. As temperature increases up to 340 K, VO2 is converted to the metal phase. The metasurface is a broadband absorber (bandwidth 3.64THz) with high selectivity for y-polarization waves while it acts as a reflector for x-polarized waves with a bandwidth of 8 THz. The multifunctional metasurface can potentially be used in advanced nonreciprocal optical device integration.
Funding
The Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB43010000); National Key Research and Development Program of China (No. 2020YFB2206103); National Natural Science Foundation of China (No. 12075244); National Natural Science Foundation of China (No. 61835011).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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