Open Access
Add to BibSonomy
Abstract
Currently, most reported terahertz metasurfaces can only respond to terahertz waves with a single polarization state (linearly polarized or circularly polarized wave), which poses a problem of limited degrees of freedom in regulation. Facing this challenging issue, we design a functionally switchable terahertz metasurface that can achieve independent control of linearly polarized and circularly polarized terahertz waves. It comprises a top layer photosensitive silicon and metal composite structure, a middle polyimide medium, and a bottom layer metal plate. By changing the conductivity of photosensitive silicon, the same metasurface structure can achieve two different functional mode switching under different polarization wave incidences, such as switching between beam splitting and vortex beam, switching between RCS and multi vortex beams, switching between single focus and multi focus. This structure provides a new idea for the flexible regulation of terahertz waves.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Metasurface is a two-dimensional planar structure composed of artificial atoms with special electromagnetic properties arranged in a certain way. It can flexibly regulate the amplitude, phase, polarization and other characteristics of incident electromagnetic waves [1–8], providing convenience for device integration and miniaturization, and has received widespread attention from researchers. In recent years, there have been some reports based on metasurface devices, such as polarization conversion [9], holographic imaging [10], superlenses [11,12], beam deflection [13,14], etc. In 2019, Chen et al. [15] proposed a cascade of three open square rings to construct a metasurface for achieving unidirectional anomalous refraction, unidirectional focusing, asymmetric focusing, and holographic imaging of incoming ray polarization waves. In 2020, Saifullah et al. [16] presented a reflective metasurfaces based on different sizes of square hole to control electromagnetic wave scattering and anomalous reflection. Tian et al. [17] designed titanium dioxide columns to form an all dielectric metasurface and achieve adjustable orbital angular momentum beams. In 2021, Jiang et al. [18] utilized an orthogonal grating and an intermediate arrow shaped structure to form a metasurface for polarization conversion in the range of 0.73∼2.24 THz, with an efficiency greater than 99.5%, while generating anomalous refraction, focusing lenses, and vortex beams. In 2022, Li et al. [19] demonstrated an arrow metal pattern metasurface to generate four vortex beams. However, once the above metasurfaces are made, their functions are relatively fixed and can only respond to linearly polarized or circularly polarized waves. Recently, some researchers have achieved active control of metasurfaces by embedding tunable materials such as graphene [20], vanadium dioxide [21], GST (tellurium antimony germanium Ge2Sb2Te5) [22], photosensitive silicon [23], etc. Among them, photosensitive silicon can change its conductivity through pumping laser [24]. When there is no pump excitation, the conductivity of photosensitive silicon is 0 S/m and the dielectric constant is 11.7. When the pump light power is 600 µJ/cm2, the conductivity of photosensitive silicon is 2.5 × 105 S/m; As the pump light power is 790 µJ/cm2, the conductivity of photosensitive silicon equals 5.0 × 105 S/m. Due to the fact that the conductivity of photosensitive silicon changes with the intensity of the external pump laser, it is favored by researchers.
In this article, we propose a photosensitive silicon and metal patterns hybrid metasurface, which consists of a substrate layer, a middle polyimide dielectric layer, and a top layer of photosensitive silicon and metal octagonal composite patterns. By utilizing light radiation to change the conductivity of photosensitive silicon, this metasurface can achieve independent control of linearly polarized and circularly polarized incident terahertz waves. As the polarization states of the incident beam change, the designed metasurface achieves multi-functions such as beam splitting, vortex beam, RCS, multi vortex beams, single focus, and multi focus. A single metasurface can simultaneously regulate linearly polarized and circularly polarized electromagnetic waves. This method can be extended and applied to regulate different polarizations of microwaves and light waves by changing the size parameters of the metasurface.
2. Structure design
Figure 1 shows the function schematic diagram of the proposed metasurface under linearly polarized and circularly polarized waves incidence, respectively. The bottom layer of the metasurface is a metal layer, and the middle layer is a polyimide dielectric layer. The relative dielectric constant of polyimide ε=3.5 with a thickness of 40 µm. The top layer is a composite pattern of photosensitive silicon and metal octagonal shape, with a thickness of 1 µm for both metal and photosensitive silicon. The encoding unit cycle is P = 120 µm. The width of the internal photosensitive silicon ring is 5 µm. The line width of the composite pattern is 1 µm. The thickness of the metallic parts in both top and bottom layers is 1µm. Herein, the coding particles are optimized by using the commercial CST Microwave Studio. Both x-axis and y-axis are set as periodic boundary conditions and the z-axis is set as open space. Figures 2(a) and 2(b) show the amplitude and phase curves generated by linearly polarized terahertz waves incidence under external laser irradiation. It can be seen that the reflection coefficients of the four encoding units exceed 0.8 at 1.3 THz, and the corresponding phase parameters are given in Table 1 and 2. Figures 2(c) and 2(d) display the amplitude and phase curves generated by circularly polarized terahertz waves incidence under external laser irradiation. Similarly, Figs. 3(a) and (b) give the amplitude and phase curves of circularly polarized terahertz waves incidence without external laser irradiation. It can be found that the terahertz wave reflectivity is greater than 0.8 between 0.6 THz and 1.2 THz. Figures 3(c) and (d) present the amplitude and phase curves generated by linearly polarized terahertz waves incidence without external laser irradiation. The corresponding phase parameters and top view of the unit cells are shown in Table 1 and Table 2. Comparing Fig. 2 and Fig. 3, it can be noted that the designed metasurface structure can achieve dynamic regulation between linear polarization and circular polarization of the incident wave by adjusting the external laser irradiation.
Fig. 1. (a) Function schematic diagram of the proposed metasurface, (b) top view of the unit cell, (c) three-dimensional view of the unit cell, (d) Poncare Ball.
Fig. 2. (a-b) Amplitude and phase curves of the unit structure under linearly polarized terahertz waves incidence without external laser irradiation, (c-d) Amplitude and phase curves of the unit structure under linearly polarized terahertz waves with external laser irradiation.
Fig. 3. (a-b) Amplitude and phase curves of the unit structure under circularly polarized terahertz waves incidence without external laser irradiation, (c-d) Amplitude and phase curves of the unit structure under circularly polarized terahertz waves with external laser irradiation.
Table 1. Phase and top view of unit structure under linear polarization normal incidence
Table 2. Phase and top view of unit cell under circular polarization normal incidence
3. Simulation results and analysis
3.1 Switching between beam splitting and vortex beam
According to the generalized Snell's theorem and the far-field function of metasurface scattering [25], the deviation angle (pitch angle) of the designed metasurface under terahertz wave incidence can be expressed by
where λ is the free space wavelength, Г is the period of the gradient encoding sequence. The azimuth angle of terahertz waves generated by the designed metasurface can be calculated byTo generate vortex beam with different topological charges, the proposed metasurface should exhibit a spiral phase profile with exp(-ilφ), where l is the topological charge at operation frequency, and φ is azimuthal angle around the beam axis. To achieve the desired vortex beam, the coding particle need be arranged in a spiral shape and corresponding phase distribution at each position (x, y) can be given by
Figure 4(a) and 4(b) display the calculated phase distribution of the proposed beam splitting metasurface and vortex beam metasurface with the topological charge of l =+2, respectively. The metasurface is made of 24 × 24 coding particle. Figures 5(a) and 5(b) show the three-dimensional far-field and normalized amplitude curves of terahertz beam splitting waves generated by the designed metasurface (encoding units in Table 1 arranged according to the sequence of 00002222…at 1.3 THz) under linearly polarized wave incidence with external laser irradiation. According to Eq. (1), we can obtain that the calculated deflection angle is θ=14.14° (the encoded period Γ=960µm), which are good agreement with the simulated deflection angle of 14° (see Fig. 5(b)). Figures 5(c)∼5(f) represent the amplitude electric field, phase distribution, mode purity (approximately 84%), and three-dimensional far-field images of the vortex beam generated by the designed metasurface under circularly polarized wave incidence without external laser irradiation, respectively. As shown in Fig. 5, the photosensitive silicon metal patterned composite metasurface can achieve switching beam splitting and vortex beam functions with or without external laser irradiation.
Fig. 4. The calculated phase distribution of (a) beam splitting metasurface, (b) vortex beam metasurface with topological charge of l =+2.
Fig. 5. Switching beam splitting and vortex beam, (a-b) Beam splitting far-field and its normalized amplitude curve under linearly polarized wave incidence with external laser irradiation; (c-f) Amplitude, phase distribution, mode purity and three-dimensional far-field diagram under circular polarized wave incidence without external laser irradiation.
3.2 Switching between RCS and multi vortex beams
Figures 6(a) and (b) illustrate the calculated phase distribution of radar cross-section metasurface and multi vortex beams metasurface, respectively. The metasurface is made of 24 × 24 coding particle. Figure 7(a) shows the three-dimensional far-field scattering of diffuse reflectance RCS (radar cross-section) generated by the designed metasurface under linearly polarized waves incidence with external laser irradiation. Figure 7(b) illustrates the RCS attenuation characteristics at a frequency of 1.3 THz. It can be seen that the RCS value of the metal reflector is −42 dB. The RCS value of the designed metasurface is −62 dB, achieving a peak RCS reduction of over −20 dB. Figures 7(c-e) display the far-field two-dimensional electric field, corresponding phase distribution, and three-dimensional far-field of the multi vortex beam generated by the designed metasurface under the circularly polarized wave incidence without external laser irradiation. From Fig. 7, it can be clearly seen that with or without external laser irradiation, the metasurface can achieve the switching function between RCS and multi vortex beams.
Fig. 6. The calculated phase distribution of (a) radar cross-section metasurface, (b) multi vortex beams metasurface.
Fig. 7. RCS and multi-vortex beam switching function. (a-b) RCS far field and RCS curve under linearly polarized wave incidence with external laser irradiation; (c-e) Two-dimensional electric field distribution of multi vortex beam far-field, phase distribution and three-dimensional far-field diagram under circularly polarized wave incidence without external laser irradiation.
3.3 Switching between single focus and multi focus
The phase distribution corresponding to single focus and multi focus focusing on the designed metasurface can be calculated by the following equation [26]
Figure 8(a) and 8(b) show the calculated phase distribution of single focus metasurface and multi focus metasurface, respectively. The metasurface is made of 24 × 24 coding particle. The two-dimensional electric field effect of the single focus at 0.8 THz under the linearly polarized wave incidence with external laser irradiation, the normalized electric field intensity on xoy plane, and the corresponding three-dimensional electric field intensity are shown in Figs. 9(a-c). Figures 10(a-c) indicate the normalized electric field intensity on xoy plane and the corresponding three-dimensional electric field intensity generated by the multi focal focusing two-dimensional electric field effect under the circularly polarized wave incidence without external laser irradiation. It can be clearly observed that multiple focal points generate four focal spots offset from the central axis from top to bottom, left to right. At a distance of 1000 µm from the metasurface, the four focal points of electric field energy focusing on xoy plane is 700 µm away from the center point. From Figs. 9 and 10, it can be seen that the designed metasurface can achieve switching between single focus and multi focus under the simultaneous incidence of linearly polarized and circularly polarized waves, with or without external laser irradiation.
Fig. 8. The calculated phase distribution of (a) single focus metasurface, (b) multi focus metasurface.
Fig. 9. (a) Two-dimensional electric field of single-focuse under linearly polarized wave incidence with external laser irradiation, (b) Normalized electric field intensity in xoy plane, (c)Three-dimensional electric field intensity.
Fig. 10. (a) Two-dimensional electric field of multi-focal under circular-polarized wave incidence without external laser irradiation, (b) normalized electric field intensity in xoy plane, (c) three-dimensional electric field intensity.
4. Conclusion
To sum up, we proposes a terahertz metasurface composed of photosensitive silicon and metal patterns, which can simultaneously regulate linearly polarized and circularly polarized incident waves, and achieve multi-functions such as switching between terahertz beam splitting and vortex beam, terahertz RCS and multi vortex beams, and terahertz single focus and multi focus. The results indicate that it is only necessary to change the conductivity of photosensitive silicon through external lasers irradiation to achieve multi-functional independent control of different polarization incident terahertz waves on the same metasurface. This composite metasurface structure provides a flexible and innovative approach for expanding the design of terahertz based multifunctional devices, and can also be extended to the fields of microwave and optics.
Funding
Natural Science Foundation of Xinjiang Uygur Autonomous Region (2021D01A73).
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.
References
1. J. Liao, S. Guo, L. Yuan, et al., “Independent manipulation of reflection amplitude and phase by a single-layer reconfigurable metasurface,” Adv. Opt. Mater. 10(4), 2101551 (2022). [CrossRef]
2. R. Nie, C. He, R. Zhang, et al., “Vanadium dioxide-based terahertz metasurfaces for manipulating wavefronts with switchable polarization,” Opt. Laser Technol. 159, 109010 (2023). [CrossRef]
3. Z. Pan, Z. Zhou, and Z. Song, “Terahertz generations of transmissive deflection, focusing, and orbital angular momentum with polarization conversion,” Opt. Laser Technol. 159, 109036 (2023). [CrossRef]
4. Z. Zhou and Z. Song, “Terahertz mode switching of spin reflection and vortex beams based on graphene metasurfaces,” Opt. Laser Technol. 153, 108278 (2022). [CrossRef]
5. J. Xin, Z. Du, Z. Zhou, et al., “Optical reflective metasurfaces enable spin-decoupled OAM and focusing,” Phys. Chem. Chem. Phys. 25(40), 27008–27016 (2023). [CrossRef]
6. C. He, Z. Pan, and Z. Song, “Manipulating spin-dependent wavefronts of vortex beams via plasmonic metasurfaces,” Ann. Phys. 535(10), 2300235 (2023). [CrossRef]
7. J. Xu, D. Liao, M. Gupta, et al., “Terahertz microfluidic sensing with dual-torus toroidal metasurfaces,” Adv. Opt. Mater. 9(15), 2100024 (2021). [CrossRef]
8. J. Lv, S. Shen, L. Chen, et al., “Frequency selective fingerprint sensor: the Terahertz unity platform for broadband chiral enantiomers multiplexed signals and narrowband molecular AIT enhancement,” Photoni X 4(1), 28 (2023). [CrossRef]
9. Y. Cheng, Z. Gong, and L. Wu, “Ultra-broadband linear polarization conversion via diode-like asymmetric transmission with composite metamaterial for terahertz waves,” Plasmonics 12(4), 1113–1120 (2017). [CrossRef]
10. Z. Liu, W. Zhu, P. Huo, et al., “Multifunctional metasurfaces enabled by simultaneous and independent control of phase and amplitude for orthogonal polarization states,” Light: Sci. Appl. 10(1), 107 (2021). [CrossRef]
11. C. Zheng, J. Li, G. Wang, et al., “All-dielectric metasurfaces capable of dual-channel complex amplitude modulation,” Nanophotonics 10(11), 2959–2968 (2021). [CrossRef]
12. J. Basilio-Ortiz and I. Moreno, “Multilayer dielectric metalens,” Opt. Lett. 47(20), 5333–5336 (2022). [CrossRef]
13. J. Wei, Y. Qi, B. Zhang, et al., “Research on beam manipulate and RCS reduction based on terahertz ultra-wideband polarization conversion metasurface,” Opt. Commun. 502, 127425 (2022). [CrossRef]
14. F. Yu, Z. Zhao, J. Chen, et al., “Reconfigurable metasurface with tunable and achromatic beam deflections,” Opt. Mater. Express 12(1), 49–58 (2022). [CrossRef]
15. K. Chen, G. Ding, G. Hu, et al., “Directional janus metasurface,” Adv. Mater. 32(2), 1906352 (2020). [CrossRef]
16. Y. Saifullah, A. Waqas, G. Yang, et al., “Multi-bit dielectric coding metasurface for EM wave manipulation and anomalous reflection,” Opt. Express 28(2), 1139–1149 (2020). [CrossRef]
17. Y. Tian, X. Jing, H. Yu, et al., “Manipulation of the arbitrary scattering angle based on all-dielectric transmissive Pancharatnam Berry phase coding metasurfaces in the visible range,” Opt. Express 28(21), 32107–32123 (2020). [CrossRef]
18. X. Jiang, W. Fan, C. Qin, et al., “Ultra-broadband polarization conversion metasurface with high transmission for efficient multi-functional wavefront manipulation in the terahertz range,” Nanophotonics 11(11), 2895 (2021). [CrossRef]
19. Z. Li, S. Li, G. Huang, et al., “Quad-OAM-beam based on a coding transmissive metasurface,” Opt. Mater. Express 12(9), 34616–3428 (2022). [CrossRef]
20. C. He and Z. Song, “Terahertz graphene metasurfaces for cross-polarized deflection, focusing, and orbital angular momentum,” Opt. Express 30(14), 25498–25508 (2022). [CrossRef]
21. B. Ren, Y. Feng, S. Tang, et al., “Dynamic control of THz polarization modulation and multi-channel beam generation using a programmable metasurface,” Opt. Express 29(11), 17258–17268 (2021). [CrossRef]
22. Q. Lin, H. Wong, L. Huitema, et al., “Coding metasurfaces with reconfiguration capabilities based on optical activation of phase-change materials for terahertz beam manipulations,” Adv. Opt. Mater. 10(1), 2101699 (2022). [CrossRef]
23. Z. Guo, Y. Sun, L. Zeng, et al., “Temperature-controlled and photoexcited multitasking Janus metasurface in the terahertz region,” Ann. Phys. 534(5), 2100499 (2022). [CrossRef]
24. J. Li, J. Li, Y. Zhang, et al., “All-optical switchable terahertz spin-photonic devices based on vanadium dioxide integrated metasurfaces,” Opt. Commun. 460, 124986 (2020). [CrossRef]
25. C. Li, S. Zhang, H. Chen, et al., “On the generalized Snell's law for the design of elastic metasurfaces,” J. Appl. Phys. 133(9), 095104 (2023). [CrossRef]
26. L. Zhang, S. Liu, and T. Cui, “Theory and application of coding metamaterials,” Chinese Optics 10(1), 1–12 (2017). [CrossRef]
