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Abstract
We propose a metasurface based on hybrid phase change materials GeTe and vanadium dioxide (VO2), which can manipulate the transmission /reflection /absorption of terahertz waves independently. By changing the external temperature from 25°C to 160°C, the function of this structure can be dramatically changed. When VO2 is in a dielectric state (i.e. 25°C), the designed structure behaves as a transmission-mode terahertz vortex beams manipulator. When VO2 is in a dielectric state (i.e. 68°C), the proposed structure serves as a reflection-mode terahertz vortex beams controller. When GeTe is in crystalline state (i.e. 160°C), the designed structure becomes as a terahertz perfect absorber at a frequency of 1.98THz within the incident angle of 30°. The proposed structure provides a new method toward the use of multifunctional terahertz devices for their potential in applications including terahertz wireless communication and detection.
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1. Introduction
Due to their small size, negligible thickness, extremely flexible regulation of electromagnetic wave and easy integration with other devices, metasurfaces have great applications in the field of polarization conversion [1,2], focusing [3], absorption [4,5], and so on. More interesting, in 2014, Cui [6] proposed the concept of digitally coding metasurface, which further improved the flexibility and adjustability. Recently, researchers have realized beam deflection [7–11], vortex control [12–17], focusing [18–21] by using the digitally coding metasurface. However, the above-mentioned structures generally worked in transmissive mode [22–27] or reflective mode [28]. In 2020, Li et al. [29] proposed the switchable encoding metasurface with the transmission and absorption modes by adding a diode to the encoding unit in microwave region. Yet, the structure cannot manipulate the reflected electromagnetic waves. In 2021, Liu et al. [30] suggested a dual-function terahertz metasurface based on graphene and VO2 to achieve terahertz reflective splitting and absorption, without transmission beam manipulattion. Until today, there are rare studies about simultaneously manipulation the transmission, reflection and absorption of terahertz waves. However, it is very necessary to use a single device to regulate these functions in electromagnetic wave applications. No one has put forward a structure combining these functionalities in terahertz wave regime so far.
In this paper, we propose a multi-functional metasurface with the aid of phase change materials GeTe and VO2, which can control terahertz wave transmission, reflection and absorption by changing the working temperature. When VO2 is in dielectric state (at 25°C), the designed structure controls the transmitted terahertz vortex beams. While VO2 is in metallic state (at 68°C), the structure manipulates the reflected terahertz vortex beams. Furthermore, as the working temperature rises to 160°C (i.e. GeTe becomes crystalline state), the designed structure behaves as perfect narrow-band absorption at 1.98THz. Our work show that these functions can be freely switched by changing the temperature. Our design provides a new idea to realize multifunctional terahertz devices. Due to high space utilization and temperature controllable features, such a multifunctional device has promising applications in radar stealth, design of ultra-thin reconfigurable terahertz devices, imaging, and communication.
2. Structure design
Figure 1 shows the designed metasurface achieving multiple-functions terahertz wave manipulation including transmissive vortex control, reflective vortex control and absorption. Different functions can be switched by changing the external operating temperature. Figure 1(d) displays a multi-layer schematic diagram of the designed unit cell, which is composed of six layers including mixed GeTe-metal pattern layer, polymide dielectric layer, a VO2 thin film layer, a metal ring layer, a polymide dielectric layer and a metal ring layer. Figure 1(e) is the top view of the unit cell, which is composed of metallic aluminum arcs in yellow and GeTe bar in red. The optimized structural parameters are as follows: r1 = 10µm, r2 = 10.2µm, r3 = 42µm, r4 = 44µm, w1 = 1.6µm, w2 = 8µm, w3 = 6µm, h = 27µm, and P = 100µm. Figure 1(f) is a schematic diagram of the second and third layers of metal patterns with r5 = 41µm, r6 = 43µm. The thickness of the VO2 thin film layer is 2 µm.
Fig. 1. Schematic diagram of the designed multiple functions metasurface. (a) Transmitted terahertz vortex beam at 25°C; (b) Reflected terahertz vortex beam at 68°C; (c)Terahertz absorber at 160°C; (d) Schematic diagram of the designed unit cell; (e) Top view of the unit cell, (f) The second and third layer’s metal patterns.
According to the Pancharatnam-Berry phase theory, an encoding phase difference will be produced by rotating the metal patterns. The corresponding phase and rotation angles of the eight encoding unit cells are shown in Table 1. When the external temperature is 25°C, VO2 is in the dielectric state and GeTe is in amorphous state. The terahertz wave transmission amplitude and phase of the unit cell are shown in Fig. 2(a, b). It can be clearly found that the transmission amplitude is close to 0.9 at 1.26 THz and the phase difference of adjacent unit cells is 45°. When the temperature increases to 68°C, VO2 becomes in metallic state and GeTe is still in amorphous state. At this time, the terahertz wave reflection amplitude and phase of the designed unit cell are shown in Fig. 2 (c, d). It can be noted that the reflection amplitude is greater than 0.8 at 1.26 THz and the reflection amplitude is evenly distributed in the range of 360°. But, as the temperature rises to 160°C, GeTe becomes crystalline. Simultaneously, the designed metasurface structure behaves as a terahertz perfect absorber.
Fig. 2. Transmission amplitude and phase curves of the proposed unit cell, (a, b) Transmission amplitude and phase of the proposed unit cell at 25°C, (c, d) Reflection amplitude and phase of the proposed unit cell at 68°C.
Table 1. Coding unit cell parameters and corresponding phases
3. Transmission mode vortex wave beams
Figure 3 (a) shows a metasurface pattern S1 composed of 1 × 1 “6 4 / 0 2” lattices with 12×12 unit cells. A matesurface pattern S2 is made up of 8 × 8 “0 2 4 6/ 4 6 0 2” lattices with 12×12 unit cells, as shown in Fig. 3(b). Figure 3(c) illustrates that the convolution operation is the addition of metasurface pattern S1 and matesurface pattern S2. The structure is simulated by a commercial software-COMSOL Multiphysics (i.e. Finite element method). Perfect matched layers (PMLs) are set for far-field calculation along all directions and excitation source is a plane wave. When VO2 is in dielectric state, the designed structure behaves as a transmission-mode terahertz vortex beams manipulator. Figures 3(d) and 3(f) display the simulated 3D far-field and 2D electric field scattering patterns of the designed metasurface S3 under circularly polarized wave normal incidence at 1.26THz. Figure 3(e) shows the phase of the vortex beam. We clearly see that the designed metasurface generates two transmissive vortex beams with a certain deflection angle. According to the generalized Snell’s law, the abnormal deflection angle of the transmitted beam can be given by
where λ is the wavelength, and Г is the coding period. As depicted in Figs. 3(d) and 3(e), the deflection angle of the transmissive vortex beams appears at θ=26.33°, which is in accordance with the theoretical prediction of θ=26°. The simulation result is consistent with the theoretical expectation.Fig. 3. Schematic diagram of the proposed metasurface with two deflection vortex wave beams at 25°C. (a) Metasurface S1 pattern with encoding sequence of “6 4/0 2”; (b) Metasurface S2 with encoding sequence of “0 2 4 6 /4 6 0 2”; (c) Metasurface S3 is obtained by convolving S1 and S2; (d) 3D far field scattering patterns, (e) phase distribution of the vortex beam, and (f) 2D electric field scattering patterns of metasurface S3.
Secondly, we design a metasurface S5 which is generated by using convolution operation metasurface S1 and chessboard-like metasurface S4 composing of 8×8 “4 0…/0 4…” lattices with 3×3 unit cells (Γ=424.2µm), as shown in Figs. 4(a-c). Figures 4(d) and 4(f) illustrate the simulated 3D far-field and 2D electric field scattering patterns of the designed metasurface S5 under circularly polarized wave normal incidence at 1.26THz. Figure 4(e) shows the phase of the vortex beam. We clearly observe that the designed metasurface produces four transmissive vortex beams with a deflection angle of θ=34°. According to Eq.(1), the deflection angle of the vortex beam is θ=34.13°, which is consistent with simulation result.
Fig. 4. Schematic diagram of the metasurface with four deflected vortex waves at 25°C, (a) Metasurface S1 pattern with encoding sequence of “6 4/0 2”, (b) Chessboard-like metasurface S4 with coding sequence of “4 0…/ 0 4…”, (c) Metasurface S5 is obtained by convolving S1 and S2, (d) 3D far field scattering patterns, (e) phase distribution of the vortex beam, and (f) 2D electric field scattering patterns of metasurface S5.
Here, we design a metasurface S7 which is obtained by superposition of S6 (coding pattern is composed of 1×1 “0 6/2 4” lattices with 2×2 unit cells) and S2 (coding pattern is made up of 8×8 “0 2 4 6/ 4 6 0 2” lattices with 2×2 unit cells), as shown in Figs. 5(a-c). Figures 5(d) and 5(f) display the simulated 3D far-field and 2D electric field scattering patterns of the designed metasurface S7 under circularly polarized wave normal incidence at 1.26THz. Figure 5(e) shows the phase of the vortex beam. It can be seen from the Fig. that the designed metasurface generates a transmission vortex beam and two deflection beams (deflection angle of θ=180°-138°=42°). According to Eq. (1), the deflection angle of the two deflection beams can be calculated as θ=41.70° (Г=357.77µm). Simulated result from finite element method is in great agreement with that from the theoretical calculation Eq. (1) with very small deviation.
Fig. 5. Schematic diagram of the metasurface with vortex beam and deflection beam at 25°C, (a) Metasurface S6 pattern with encoding sequence of “0 6/2 4”, (b) Metasurface S2 with encoding sequence of “0 2 4 6 /4 6 0 2”, (c) Metasurface S7 is obtained by superposition S6 and S2, (d) 3D far field scattering patterns, (e) phase distribution of the vortex beam, and (f) 2D electric field scattering patterns of metasurface S7.
4. Reflection mode vortex wave beams
When the external temperature is 68°C, the phase-change material VO2 is in metallic state and the proposed structure serves as a reflection-mode terahertz vortex beams controller. Figures 6(a) and 6(b) show 3D far-field radiation patterns of metasurface S3 under circularly polarized wave normal incidence at 1.26THz and the phase of the vortex beam. It can be seen from the Fig. that the above-mentioned metasurface S3 generate two reflected deflection vortex wave beams. From the Fig. 6(c), we can infer that the deflection angle is about θ=26°. According to Eq. (1), we obtain the deflection angle of θ=26.33°. Similarly, 3D far-field and 2D electric field scatter patterns of the metasurface S5 generated by the multi-vortex beams a 1.26THz is clearly illustrated in Figs. 7(a) and 7(c). Figure 7(b) shows the phase of the vortex beam. According to Eq. (1), the calculated deflection angle of vortex beam is 34.13°. It can be seen from Fig. 7 that the simulated deflection angle is close to θ=34°, which is consistent with the theoretical expectation. Similarly, Fig. 8 shows reflection 3D far-field, the phase of the vortex beam and 2D electric field diagram of the metasurface S7 under circularly polarized terahertz wave normal incidence (1.26THz) at 68°C.The results demonstrate that the designed encoding metasurface achieves the superposition effect of a vortex beam and two deflected beams. According to Eq. (1), the deflection angle of the terahertz wave is of θ=41.70°. The simulated deflection angle of the terahertz wave beam is at 42° which is in good agreement with the theoretically expected deflection angle. From the above simulation results, one can see that the coded metasurface realizes the regulation function of transmission and reflection vortex wave beams, and effectively improves the freedom degree of terahertz wave manipulation by the change of working temperature.
Fig. 6. (a) Reflection 3D far-field, (b) phase distribution of the vortex beam, and (c) 2D electric field diagram of the metasurface S3 at 68°C.
Fig. 7. (a) Reflection 3D far-field, (b)phase distribution of the vortex beam, and (c) 2D electric field diagram of the metasurface S5 at 68°C.
Fig. 8. (a) Reflection 3D far-field, (b) phase distribution of the vortex beams, and (c) 2D electric field diagram of the metasurface S7 at 68°C.
5. Terahertz wave absorber
When the temperature reaches 160°C, GST is converted to crystalline state, and the designed structure becomes a terahertz wave absorber. The structure is also simulated by using finite element method with periodic boundary conditions for unit cell along x and y directions, and PML for unit cell along z direction. Excitation source is set as the floquet-mode port. Figures 9(a-b) show the absorption characteristics and the electric field distributions diagram of the proposed structure. It can be found that the absorption of the meta-elements “0”, “2”, “4”, and “6” are more than 99.2% at f = 1.98THz. Moreover, one can see that the electric field is mainly localized at the edge of individual metal-ring. The formula of metasurface absorptivity is A(ω) = 1-R(ω)-T(ω), where R(ω) is the reflectance, T(ω) is the transmittance. Obviously, the thickness of the bottom vanadium dioxide film is thicker than the skin depth of terahertz wave, the transmittance is almost zero. So the transmittance T(ω) can be ignored and the absorption rate formula can be simplified as A(ω) = 1-R(ω).
Fig. 9. Absorption curve and electric field distribution of the corresponding unit cells “0”, “2”, “4”,"6”.
Assuming the relative equivalent impedance of the proposed structure is Zr, and the relative impedance of free space is Z0, then the equivalent impedance can be given by the following formula
Fig. 10. Equivalent impedance real (a) and (b) imaginary parts of the unit cell “0”, “2”, “4”, “6” at 1.98THz
Fig. 11. (a) Absorption rate as a function of incident angle, (b) Absorption rate as a function of polarization angles
6. Conclusion
To sum up, we propose a multifunctional metasurface to control terahertz wave transmission -reflection -absorption based on hybrid GeTe-VO2 medium. The structure consists of a GeTe-metal pattern layer, polymide dielectric layer, VO2 thin film layer, metal ring layer, a polymide dielectric layer and metal ring layer. By changing the external temperature from 25°C to 160°C, the function of this structure can be dramatically changed. At room temperature, the designed structure serves as a transmissive-mode vortex beams regulator. When the temperature rises to 68°C, our proposed structure presented as reflective-mode vortex beams controller. When the external temperature becomes 160°C, the structure registers as narrow band terahertz absorber. Our work realizes multiple functionalities in a single structure, which shows great potential applications in the fields of terahertz communication and switching.
Funding
National Natural Science Foundation of China (61831012, 61871355); Talent project of Zhejiang Provincial Department of science and technology (2018R52043); Zhejiang Key R & D Project of China (2021C03153, 2022C03166); Research Funds for the Provincial Universities of Zhejiang (2020YW20, 2021YW86); Zhejiang Lab (2019LC0AB03).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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