We report a new approach for making a reconfigurable terahertz digital metasurface that is created with vanadium dioxide (VO2) integrated metasurface unit cells. Such a metasurface achieves terahertz wave beam splitting and switching functionalities for the terahertz wave normal incidence by inducing VO2 conductivity change under a different external temperature. The three-dimensional (3D) far-field scattering patterns and normalized electric field distribution obtained by using full-wave numerical simulations verify the behavior of the terahertz waves in each of the cases and illustrate our general theoretical predictions. This scheme provides a new effective method for the design of terahertz multifunctional devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Owing to locating in a special spectral position of electromagnetic spectrum, terahertz wave shows the extraordinary electromagnetic characteristics, such as coherence, transient and broadband [1,2]. It has attracted massive attention due to numerous potential applications in the fields of spectroscopy, imaging, security checking, biomedicine, astronomy, high data rate wireless communication and so on [3–5]. For realizing such diverse applications, flexible manipulating terahertz wave becomes a key technique. In recent years, T. Cui et al. has proposed the concept of coding metasurface to control electromagnetic waves [6–8]. Most of the reported coding metasurfaces fixed their functionalities without adjustability once they are constructed or fabricated [9,10]. More recently, the programmable coding metasurfaces has been introduced and expanded the application scope of coding metasurface [11–14]. However, these programmable coding metamaterials which need pin diodes can only operate in microwave region, but not in terahertz frequency regime due to lack of active elements (pin diodes) operating at high frequencies . Fortunately, VO2, graphene, liquid crystal hybrid metamaterials [16–21] provide an efficient solution for real-time manipulating terahertz waves. In 2019, Li et al. proposed using double splitting resonators combined with photosensitive conductor germanium to design P-B coding metasurface for dynamical amplitude modulation of the terahertz wave reflection . In 2020, they designed a reflective chiral structure as a switching metasurfaces by changing the conductivity and Fermi level of the doped graphene . Owing to having reversible transition between the insulating monoclinic phase and the metallic tetragonal phase with transition time of 80 fs above temperature 68°C [24,25], VO2-assisted metamaterial devices have developed fantastic applications in terahertz frequencies such as tunable terahertz absorbers [26,27], terahertz filters [28,29], and terahertz antennas [30,31]. Recently, H. Li et al proposed VO2 and graphene combined metamaterials [32–34]. By adjusting the Fermi energy level of graphene or the conductivity of VO2, it can not only control the frequency of the absorber, but also adjust the absorption amplitude to achieve functional switching. Although these applications have made some progress, terahertz wavefront manipulation devices still need to be further explored. It is beneﬁcial for practical applications to design a VO2-assisted metasurface to control terahertz radiation more efficiently and conveniently without changing its geometric dimension and coding consequences.
In this letter, we propose a new approach for designing terahertz wave beam splitting and switch based on reconfigurable terahertz digital metasurface which consists of VO2-integrated metasurface layer, dielectric substrate, and a gold plate to prevent terahertz wave energy transmission through the structure. The VO2-assisted coding particles are elaborately constructed whose operational statuses can be dynamically flexible switched between two states “0” and “1” for 1-bit coding, and four states of “00”, “01”, “10”, and “11” for 2-bit coding by applying external temperature without changing the meta particles geometrical parameters. By arranged various coding sequence samples, we obtained real-time manipulating terahertz devices with different functions from beam splitter toward terahertz switching without redesigning the structure. We believe that our proposed structure has great potential applications in terahertz stealth and other terahertz systems.
2. Design a coding unit cell
Three-dimensional schematic diagram of the unit cell of the proposed digital metasurface based on phase change material is shown in Fig. 1(a). The top layer is composed of rectangular metal frame (marked yellow), one side of which is VO2 (VO2 is marked blue). The bottom layer is gold metal plate to realize complete reflection. The dielectric layer between the top and the bottom layer is polyimide (PI) with permittivity of ɛ=3.5 and thickness of h=40 µm. The geometric parameters of the unit cell are optimized by using the commercially available Computer Software Technology (CST) microwave studio. The optimized dimensions of the unit cell are as follows: P=79 µm, a=48 µm, and b=46 µm. The coding unit cell array layer can be fabricated by large-scale synthesis, transfer and etching techniques, and the electron beam lithography can be employed to produce the subwavelength unit cell layer. The chemical vapor deposition method is one of the beneficial methods for fabricating the bottom layer. Figure 1(b) depicts the rotation angle of the top pattern is α, and the unit cells will generate a ±2α phase shift, where “+” and “-” represent left circularly polarized (LCP) and right circularly polarized (RCP) waves, respectively. For 1-bit metasurface, the unit cells (i.e. “0” and “1”) have 180° reflection phase difference. For the 2-bit metasurface, the phase difference of the adjacent unit cells (i.e. “00”, “01”, “10” and “11”) is fixed as 45°. The rotation angle α of the unit cell varies from 0° to 135° with step width of 45°. Figure 2(a) illustrates the circularly polarized reflection amplitude of unit cells “00”, “01”, “10” and “11” with σ=100 S/m in terahertz frequency. The basic coding unit cells have the reflection amplitude of the normal incidence cross-polarization are larger than 0.9 in the terahertz frequency range of 0.4∼1.6 THz. Figure 2(b) depicts the cross-polarization reflection phase of coding unit cells at σ=100 S/m with different rotation angles α of the top pattern under normal incidence of the LCP wave. The phase difference between adjacent coding unit cells in the terahertz region is about 90°. To decrease the couple effect of the adjacent coding particles, we employ 4×4 coding unit cells as a super coding unit cell. When the circularly polarized wave is vertically incident on the top layer structure with rotation angle α, the complex amplitude of the electric field in the reflected wave can be expressed by Jones matrix :
Similarly, when the conductivity of VO2 increases to 180000 S/m, VO2 bar changes from insulating state to metallic state, and the structure of top-level resonator becomes from anisotropic open resonator to isotropic closed resonator. As shown in Fig. 3(a), the cross-polarization reflection amplitude decreases sharply to 0.2 and the co-polarization reflection amplitude becomes 0.8. Figure 3(b) shows the phase of co-polarization reflection as the conductivity of VO2 is 180000 S/m. It can be observed that the phase difference of adjacent coding unit cells is basically coincident, which reflects the incidence of LCP wave to the opposite direction.
In order to further understand the changes of the reflection amplitude and phase of the unit cells, we simulated the surface current of the coding unit cells at 1.2 THz with different conductivities as illustrated in Fig. 4. According to the surface current distributions, the resonance mode similar to magnetic dipole is excited, and the electromagnetic energy enters into the dielectric layer, resulting in the dielectric loss. The combination of dielectric loss and metal loss leads to the strong absorption of terahertz wave by unit cells. One can see from Fig. 4(a) that when VO2 becomes metal state, the metal open resonance frame and VO2 appears surface current distribution. At this time, the resonance structure of the coding unit cell is isotropic and symmetrical. Alternatively, when the conductivity of VO2 is 100 S/m (i.e. VO2 becomes insulating state), VO2 has no effect on the resonance of U-shaped metal structure, and the surface current is mainly distributed along the metal resonance frame, as shown in Fig. 4(b).
3. Switchable digital metasurface with multi-functions
Here, we calculated the 3D far-field scattering patterns and normalized electric field distribution of the VO2-assisted digital metasurface under the normal incidence of LCP polarization wave. Terahertz beam splitting and switching are realized by change of VO2 conductivity under different operating temperatures. According to the generalized Snell's law, the angle of reflected beam can be calculated by the simplified formula, which is compared with the simulation results to verify the beam splitting function of the metasurface.
3.1 Terahertz beam splitting
Figures 5(a)∼5(d) illustrate the 3D far-field scattering patterns and normalized electric field distribution of 1-bit coding metasurface under LCP wave normal incidence at 1.2 THz with the VO2 conductivity of σ=100 S/m and σ=180000 S/m. It can be noted from Figs. 5(a)∼5 (d) that the vertical incident terahertz wave is reflected as two symmetric RCP waves with the angle of (θ, φ)=(13.1°, 90°) and (θ, φ)=(13.1°, 270°), (Here, φ is the azimuth of the reflected beam, θ=arcsin(λ/Γ), Γ is the length of a gradient period of the coding metasurface and λ is the terahertz wavelength), as the VO2 is insulating monoclinic phase (i.e. σ=100 S/m) and 1-bit coding sequence is ‘00110011…’. But, when the VO2 becomes metallic tetragonal phase (i.e. σ=180000 S/m) and 1-bit coding sequence is still ‘00110011…’, the incident terahertz wave is reflected vertically with reflectivity of 95%. Similarly, the 3D far-field scattering patterns of 1-bit coding metasurface for ‘01010101/101010…’ periodically distributed in y direction under LCP wave normal incidence at 1.2THz with the VO2 conductivity of σ=100 S/m and σ=180000 S/m are shown in Figs. 5(e) and 5(f), respectively. It can also be noted that the incident terahertz wave is divided into four symmetric RCP waves with the angle of (θ, φ)=(39.87°, 45°), (θ, φ)=(39.87°, 135°), (θ, φ)=(39.87°, 225°) and (θ, φ)=(39.87°, 315°), when the VO2 is insulating monoclinic phase (i.e. σ=100 S/m). However, while the VO2 changes from insulating state to metallic state, the incident terahertz wave is almost completely reflected. In order to clearly see the terahertz beam splitting of the proposed metasurface, a two-dimensional normalized amplitude of reflected waves with VO2 in insulating and metallic states is provided, as plotted in Figs. 5(g) and 5(h).
From Figs. 6(a)–6(d), one sees that the 3D far-field scattering patterns and normalized electric field distribution of the 2-bit coding metasurface arranged along y direction with coding sequence of ‘0001101100011011…’ under LCP wave normal incidence at 1.2THz with σ=100 S/m and σ=180000 S/m. It can be observed from the figure that the terahertz wave energy is mainly reflected in the direction of (θ, φ)=(13.06°, 0°) when the VO2 conductivity is σ=100 S/m. However, when the sequence of coding metasurface is ‘111011101110/000100010001…’ distributed along y direction for the VO2 conductivity of σ=100 S/m under normal incidence of LCP wave at 1.2 THz, the normal incidence terahertz wave is divided into four symmetrical RCP waves direction with angles of (θ, φ)=(24.71°, 0°), (θ, φ)=(24.71°, 90°), (θ, φ)=(24.71°, 180°) and (θ, φ)=(24.71°, 270°), as shown in Fig. 6(e). If the VO2 conductivity increases to 180000S/m, the incident terahertz wave is reflected vertically, as shown in Figs. 6(b), 6(d), 6(f) and 6(h). According to the above analysis (sees Fig. 5 and Fig. 6), it is concluded that the numbers and directions of reflected terahertz beam can be well regulated by the phase transition of VO2.
3.2 Terahertz wave switching
In order to further describe the terahertz wave switching function of the proposed VO2 hybrid metasurface, MATLAB is employed to generate random digital sequences to construct the reconfigurable coding metasurfaces. Figures 7(a) and 7(b) depict 1-bit and 2-bit random coding sequences and the insets is 4×4 basic unit cells with different rotation angles.
The 3D far-field scattering patterns of 1-bit and 2-bit random coding metasurfaces under normal incident terahertz waves and different VO2 conductivity is plotted in Fig. 8. Figures 8(a)∼8(b) shows the 3D far-field scattering patterns of 1-bit random coding sequence metasurface under normal incidence of LCP wave at 1.2THz with VO2 conductivity of σ=100 S/m and σ=180000 S/m, respectively. Figures 8(c)∼8(d) illustrates the 3D far-field scattering patterns of 2-bit random metasurface under normal incidence of LCP wave at 1.2 THz with σ=100 S/m and σ=180000 S/m, respectively. Figure 8(e) depicts the 3D far-field scattering patterns of the same size metal plate at 1.2 THz. When the conductivity of VO2 is 100S/m, the incident terahertz wave energy is reflected to numerous directions by the 1-bit and 2-bit random coding metasurfaces. For 1-bit random coding metasurface, the maximum terahertz reflection intensity is 17.4% and greatly suppress the backscattering wave, as shown in Fig. 8(a). In Fig. 8(c), one can see that the maximum terahertz reflection intensity is 40.8% for 2-bit random coding metasurface. Similarly, the reflected terahertz wave intensity is sharply suppressed. However, as the conductivity of VO2 moves up to 180000 S/m, the reflected terahertz wave amplitude of the 1-bit and 2-bit random coding metasurfaces is the same as that of the bare metal plate, as displayed in Figs. 8(b), 8(d) and 8(e). That the maximum terahertz reflection intensity equals 95%. To simplify the analysis and confirm the advantage of the designed terahertz switching, we summarized the switching extinction ratio performance characteristics as shown in Table 1. The maximum extinction ratio of the proposed structure is 13.6 dB.
The switchable metasurface proposed in this paper realizes the functions of beam splitting and switching, which makes it possible to be used in the fields of terahertz sensors and beam splitters.
In conclusion, we propose a switchable terahertz digital metasurface based on VO2 phase-change material with the function of terahertz beam splitting and switching based on hybrid metasurface. The simulated results illustrate that, by predesigned coding sequence, the proposed VO2-assisted metasurface can divide the incident terahertz wave to multiple reflection directions according to our anticipation. Furthermore, by generating random coding sequences, we can indirectly adjust the operating temperature of VO2 (i.e. VO2 conductivity change) to tune the reflected terahertz wave intensity. Thus, a reflected type terahertz switching is obtained with extinction ratio of 13.6 dB. This work shows potential application prospects for space terahertz wave manipulation using reconfigurable VO2 assisted metasurfaces.
National Natural Science Foundation of China (61871355, 61831012); Zhejiang Key R & D Project of China (2021C03153); Zhejiang Lab (2019LC0AB03).
The authors declare no conflicts of interest.
1. T. Masayoshi, “Cutting-edge terahertz technology,” Nat. Photonics 1(2), 97–105 (2007). [CrossRef]
2. J. Federici and L. Moeller, “Review of terahertz and subterahertz wireless communications,” J. Appl. Phys. 107(11), 111101 (2010). [CrossRef]
3. R. Piesiewicz, T. Kleine-Ostmann, N. Krumbholz, D. Mittleman, M. Koch, J. Schoebel, and T. Kurner, “Short-range ultra-broadband terahertz communications: Concepts and perspectives,” IEEE Antennas Propag. Mag. 49(6), 24–39 (2007). [CrossRef]
4. P. H. Siegel, “Terahertz technology in biology and medicine,” IEEE Trans. Microwave Theory Tech. 52(10), 2438–2447 (2004). [CrossRef]
5. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, and R. D. Averitt, “Active terahertz metamaterial devices,” Nature 444(7119), 597–600 (2006). [CrossRef]
6. T. Cui, M. Qi, X. Wan, J. Zhao, and Q. Cheng, “Coding metamaterials, digital metamaterials and programmable metamaterials,” Light: Sci. Appl. 3(10), e218 (2014). [CrossRef]
7. S. Liu and T. Cui, “Concepts, working principles, and applications of coding and programmable metamaterials,” Adv. Opt. Mater. 5(22), 1700624 (2017). [CrossRef]
8. T. Cui, S. Liu, and L. Li, “Information entropy of coding metasurface,” Light: Sci. Appl. 5(11), e16172 (2016). [CrossRef]
9. Y. Saifullah, A. Waqas, G. Yang, and F. Xu, “Multi-bit dielectric coding metasurface for EM wave manipulation and anomalous reflection,” Opt. Express 28(2), 1139–1149 (2020). [CrossRef]
10. L. Shao, W. Zhu, M. Y. Leonov, and I. D. Rukhlenko, “Dielectric 2-bit coding metasurface for electromagnetic wave manipulation,” J. Appl. Phys. 125(20), 203101 (2019). [CrossRef]
11. Q. Ma, G. D. Bai, H. B. Jing, C. Yang, and T. J. Cui, “Smart metasurface with self-adaptively reprogrammable functions,” Light: Sci. Appl. 8(1), 1–12 (2019). [CrossRef]
12. J. Zhao, X. Yang, J. Y. Dai, Q. Cheng, X. Li, N. Qi, J. Ke, G. Bai, S. Liu, S. Jin, A. Alù, and T. Cui, “Programmable time-domain digital-coding metasurface for non-linear harmonic manipulation and new wireless communication systems,” Natl. Sci. Rev. 6(2), 231–238 (2019). [CrossRef]
13. Y. Li, L. Li, B. Xu, W. Wu, R. Wu, X. Wan, Q. Cheng, and T. Cui, “Transmission-type 2-bit programmable metasurface for single-sensor and single-frequency microwave imaging,” Sci. Rep. 6(1), 23731 (2016). [CrossRef]
14. L. Li, H. Ruan, C. Liu, Y. Li, Y. Shuang, A. Alù, C. W. Qiu, and T. J. Cui, “Machine-learning reprogrammable metasurface imager,” Nat. Commun. 10(1), 1–8 (2019). [CrossRef]
15. H. Yang, X. Cao, F. Yang, J. Gao, S. Xu, M. Li, X. Chen, Y. Zhao, Y. Zheng, and S. Li, “A programmable metasurface with dynamic polarization, scattering and focusing control,” Sci. Rep. 6(1), 35692 (2016). [CrossRef]
16. M. Zhang and Z. Song, “Terahertz bifunctional absorber based on a graphene-spacer-vanadium dioxide-spacer-metal configuration,” Opt. Express 28(8), 11780–11788 (2020). [CrossRef]
17. L. Chen and Z. Song, “Simultaneous realizations of absorber and transparent conducting metal in a single metamaterial,” Opt. Express 28(5), 6565–6571 (2020). [CrossRef]
18. R. Deshpande, F. Ding, and S. I. Bozhevolnyi, “Dual-band metasurfaces using multiple gap-surface plasmon resonances,” ACS Appl. Mater. Interfaces 12(1), 1250–1256 (2020). [CrossRef]
19. Y. Fan, N. Shen, T. Koschny, and C. Soukoulis, “Tunable terahertz metasurface with graphene cut-wires,” ACS Photonics 2(1), 151–156 (2015). [CrossRef]
20. C. Lin, Y. Li, C. Hsieh, R. Pan, and C. Pan, “Manipulating terahertz wave by a magnetically tunable liquid crystal phase grating,” Opt. Express 16(5), 2995–3001 (2008). [CrossRef]
21. R. Wang, L. Li, J. Liu, Y. Fei, and W. Sun, “Triple-band tunable perfect terahertz metamaterial absorber with liquid crystal,” Opt. Express 25(26), 32280–32289 (2017). [CrossRef]
22. J. Li, Y. Zhang, J. Li, X. Yan, L. Liang, Z. Zhang, J. Huang, J. Li, Y. Yang, and J. Yao, “Amplitude modulation of anomalously reflected terahertz beams using all-optical active Pancharatnam–Berry coding metasurfaces,” Nanoscale 11(12), 5746–5753 (2019). [CrossRef]
23. J. Li, J. Li, Y. Yang, J. Li, Y. Zhang, L. Wu, Z. Zhang, M. Yang, C. Zheng, J. Li, J. Huang, F. Li, T. Tang, H. Dai, and J. Yao, “Metal-graphene hybrid active chiral metasurfaces for dynamic terahertz wavefront modulation and hear field imaging,” Carbon 163, 34–42 (2020). [CrossRef]
24. Z. Song, A. Chen, J. Zhang, and J. Wang, “Integrated metamaterial with functionalities of absorption and electromagnetically induced transparency,” Opt. Express 27(18), 25196–25204 (2019). [CrossRef]
25. A. Cavalleri, T. Dekorsy, H. Chong, J. Kieffer, and R. Schoenlein, “Evidence for a structurally-driven insulator-to-metal transition in VO2: A view from the ultrafast timescale,” Phys. Rev. B 70(16), 161102 (2004). [CrossRef]
26. Y. Zhao, Q. Huang, H. Cai, X. Lin, and Y. Lu, “A broadband and switchable VO2-based perfect absorber at the THz frequency,” Opt. Commun. 426, 443–449 (2018). [CrossRef]
27. F. Ding, S. Zhong, and S. Bozhevolnyi, “Vanadium dioxide integrated metasurfaces with switchable functionalities at terahertz frequencies,” Adv. Opt. Mater. 6(9), 1701204 (2018). [CrossRef]
28. N. Born, A. Crunteanu, G. Humbert, A. Bessaudou, M. Koch, and B. Fischer, “Switchable THz filter based on a vanadium dioxide layer inside a Fabry-Pérot cavity,” IEEE Trans. THz Sci. Technol. 5(6), 1035–1039 (2015). [CrossRef]
29. N. Tayyab, H. J. Hyun, F. Mohd, L. Kye-Jeong, N. Do-Young, and J. Jae-Hyung, “Vanadium dioxide based frequency tunable metasurface filters for realizing reconfigurable terahertz optical phase and polarization control,” Opt. Express 26(10), 12922–12929 (2018). [CrossRef]
30. Y. G. Jeong, H. Bernien, J. S. Kyoung, H. Park, H. Kim, J. Choi, B. Kim, H. Kim, K. Ahn, and D. Kim, “Electrical control of terahertz nano antennas on VO2 thin film,” Opt. Express 19(22), 21211–21215 (2011). [CrossRef]
31. H. Liu, Y. Fan, H. Chen, L. Li, and Z. Tao, “Active tunable terahertz resonators based on hybrid vanadium oxide metasurface,” Opt. Commun. 445, 227–283 (2019). [CrossRef]
32. H. Li, W. Xu, Q. Cui, Y. Wang, and J. Yu, “Theoretical design of a reconfigurable broadband integrated metamaterial terahertz device,” Opt. Express 28(26), 40060–40074 (2020). [CrossRef]
33. H. Li and J. Yu, “Bifunctional terahertz absorber with tunable and switchable property between broadband and dual-band,” Opt. Express 28(17), 25225–25237 (2020). [CrossRef]
34. H. Li and J. Yu, “Active dual-tunable broadband terahertz absorber based on hybrid graphene-vanadium dioxide metamaterial,” OSA Continuum 3(8), 2143–2155 (2020). [CrossRef]
35. W. Luo, S. Xiao, Q. He, S. Sun, and L. Zhou, “Photonic spin Hall effect with nearly 100% efficiency,” Adv. Opt. Mater. 3(8), 1102–1108 (2015). [CrossRef]