1.The Institute of Laser and Opto-electronics, School of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China 2.Key Laboratory of the Ministry of Education on Optoelectronic Information Technology, Tianjin University, Tianjin 300072, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0700202) and National Natural Science Foundation of China (Grant Nos. 61675147, 61735010, 91838301)
Received Date:11 June 2020
Accepted Date:30 June 2020
Available Online:02 December 2020
Published Online:20 November 2020
Abstract:Terahertz (THz) has the characteristics of non ionization, penetration, water absorption, high resolution, etc. It has shown an important application prospect in many fields, such as non-destructive testing, imaging and communication. However, THz is in the transition frequency band ranges from macro-electronics to micro-photonics, so, it belongs to the interdisciplinary field, forming the “terahertz gap” in electromagnetic wave. In recent years, with the continuous development and improvement of THz radiation source and detection technology, the THz modulation technology has gradually aroused the interest of researchers. Metamaterials with many properties that natural materials do not possess provide a common way to control THz. The two-dimensional structure of a metamaterial is called a metasurface. The coding metasurface encodes the phase digitally so that the electromagnetic wave can be regulated. It is proposed that it is first in the microwave band and then extended to the THz band. In the microwave band, the number, direction and amplitude of the far-field beams can be changed dynamically by programming, which is connected with the integrated circuit such as FPGA by using diodes, but due to the limitation of diode size and micro-nano manufacturing technology, the programmable metasurface in microwave band cannot be well used in THz band. In order to improve the flexibility of THz coding metasurface, in this paper we choose the phase change material vanadium dioxide (VO2) to active modulation coding metasurface. In this paper, we analyze the VO2’s insulating state before the phase transformation and metallic state after the phase transformation. Then designing an active control 1 bit coding metasurface by using the influence of the two states on the amplitude and phase of the unit structure, which is composed of VO2, polyimide and aluminum, can not only realize the basic function of coding metasurface adjusting the electromagnetic wave beams, but alsoimplement the switching of two kinds of far-field beams at 1.1 THz for the same coding sequence by thermal stimulated VO2. The coding metasurface also realizes the switching between two near-field focal points at 1.1 THz for the same coding sequence. Based on the effect of VO2 on the phase, this coding metasurface provides a new way to adjust and control the THz wave flexibly, and will have a great application prospect in THz transmission, imaging and communication. Keywords:vanadium dioxide/ terahertz/ coding metasurface
3.仿真结果与讨论为了实现主动调控, 将编码超表面和二氧化钒相结合, 如图3所示. 除了顶层外, 其余部分和图1的结构相同, 单元“0”顶层圆环内半径c2 = 27 μm, 外半径c1 = 34.5 μm, 其余部分全部为二氧化钒, 而对于单元“1”, 顶层铝的直径s2 = 68 μm, 其余部分为二氧化钒. 单元“0”和“1”的幅值相位图如图2(c)和图2(d). 当二氧化钒为绝缘态时, 单元“0”和“1”在1.1 THz的幅值相差无几, 均在0.7以上, 而相位相差约为180°, 满足编码超表面的条件. 而当单元“0”和“1”的二氧化钒转化为金属态时, 幅值均大于0.85, 相位基本相同. 这样通过改变二氧化钒, 可以实现反射波束的变化, 如图4所示. 当平面波垂直入射时(偏振方向为x方向), 二氧化钒为绝缘态时, 棋盘序列(0101/1010)的反射波束为四波束, 和基本的编码超表面相同; 当二氧化钒转化为金属态时, 反射波束为垂直的波束. 这样通过温控二氧化钒等方式就能够实现反射波束的动态变化. 图 3 1 bit编码超表面示意图 (a) 单元“0”的结构示意图; (b) 单元“1”的结构示意图; (c) 单元“0”和“1”在二氧化钒不同态时的反射振幅; (d) 单元“0”和“1”在二氧化钒不同态时的反射相位 Figure3. Schematic diagram of 1-bit coding metasurface: (a) Schematic diagram of unit “0”; (b) schematic diagram of unit “1”; (c) reflection amplitude of units “0” and “1” in different vanadium dioxide states; (d) reflection phase of units “0” and “1” in different vanadium dioxide states.
图 4 棋盘序列(0101/1010)在1.1 THz的远场示意图 (a) 二氧化钒为绝缘态时的远场示意图; (b) 二氧化钒为金属态时的远场示意图 Figure4. Far-field diagram of chessboard sequence (0101/1010) at 1.1 THz: (a) Far-field diagram when vanadium dioxide is in insulating state; (b) far-field diagram when vanadium dioxide is in metallic state.
值得一提的是, 图3所展示的二氧化钒的变化, 只是将单元“0”和“1”的相位转变为相同的相位, 从而让任意编码序列的反射波束转变为垂直波束, 无法实现各种波束间的转化. 为此设计了新的二氧化钒-金属结构如图5所示, 单元“0”中q1 = 8 μm, w = 31 μm, b = 71 μm, 单元“1”中q2 = 14 μm, e = 80 μm, a = 108 μm, 红色部分为二氧化钒. 幅值和相位如图5(c)和图5(d)所示, 在1.1 THz处, 当二氧化钒都为绝缘态时, 单元“0”和“1”的幅值相差不大, 相位相差约180°, 当二氧化钒转化为金属态时, 单元“0”和“1”的幅值同样在0.7以上, 相位差也相差180°, 即相当于单元“0”变成了单元“1”, 单元“1”变成了单元“0”. 图6展示了在1.1 THz时, 棋盘序列的远场方向图, 可以看出, 无论二氧化钒是金属态还是绝缘态, 入射波都被反射到了四个方向, 不同的是编码序列由0101/1010序列转换成了1010/0101序列. 图 5 1 bit编码超表面示意图 (a) 单元“0”的结构示意图; (b) 单元“1”的结构示意图; (c) 单元“0”和“1”在二氧化钒不同态时的反射振幅; (d)单元“0”和“1”在二氧化钒不同态时的反射相位 Figure5. Schematic diagram of 1 bit coding metasurface: (a) Schematic diagram of unit "0"; (b) schematic diagram of unit "1"; (c) reflection amplitude of units "0" and "1" in differ rent vanadium dioxide states; (d) reflection phase of units "0" and "1" in different vanadium dioxide states.
图 6 1.1 THz棋盘序列的远场方向图 (a) 二氧化钒为绝缘态; (b) 二氧化钒为金属态 Figure6. Far-field pattern of chessboard sequence at 1.1 THz: (a) Vanadium dioxide is in insulating state; (b) vanadium dioxide is in metallic state.
图 9 整体结构和电场图 二氧化钒为金属态时 (a) 超表面结构; (b)在z = 900 μm平面的x方向的归一化电场的x分量图; (c)在z = 900 μm平面的y方向的归一化电场的x分量图. 二氧化钒为绝缘时 (d) 超表面结构; (e) 在z = 800 μm平面的x方向的归一化电场的x分量图; (f) 在z = 800 μm平面的y方向的归一化电场的x分量图 Figure9. Overall structure and electric field diagram. When vanadium dioxide is metallic state: (a) Metasurface structure; (b) x component diagram of normalized electric field in X direction of z = 900 μm plane; (c) x component diagram of normalized electric field in Y direction of z = 900 μm plane. When vanadium dioxide is insulating state (d) metasurface structure; (E) x component diagram of normalized electric field in X direction of z = 800 μm plane; (f) x component diagram of normalized electric field in Y direction of z = 800 μm plane.