• Chinese Journal of Lasers
  • Vol. 48, Issue 20, 2014003 (2021)
jiaoyan Guo1、2、3, Wenyu Li1、2、3, Ran Sun1、2、3, and Guozhong Zhao1、2、3、*
Author Affiliations
  • 1Department of Physics, Capital Normal University, Beijing 100048, China
  • 2Beijing Advanced Innovation Center for Imaging Theory and Technology, Beijing 100048, China
  • 3Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Capital Normal University, Beijing 100048, China
  • show less
    DOI: 10.3788/CJL202148.2014003 Cite this Article Set citation alerts
    jiaoyan Guo, Wenyu Li, Ran Sun, Guozhong Zhao. Generation of Broadband Terahertz Vortex Beam Based on Double-Arrow Metasurface[J]. Chinese Journal of Lasers, 2021, 48(20): 2014003 Copy Citation Text show less

    Abstract

    Objective An electromagnetic wave with a spiral wavefront is a vortex beam with an orbital angular momentum(OAM). The OAM vortex beam has a phase factor exp(i), φ is the azimuth angle, and l is the topological charge (integer or non-integer). The vortex beam’s wavefront propagates in a spiral forward, and the intensity of the beam center is close to zero. This unique electromagnetic characteristic has potential applications in optics, atomic physics, and communications. In recent years, the massive capability potential has benefited humans, and the broadband characteristics are the key to effectively improving the channel capacity. Most of the current vortex beam generating devices have shortcomings such as narrow bandwidth and low working efficiency. Methods like spiral phase plates and spatial light modulators are employed in previous studies to generate vortex beams. These methods produce optical devices with issues such as large size and high price. Because metasurfaces can control the phase, amplitude, and polarization of electromagnetic wavefronts, using metasurfaces to generate vortex beams has become a novel method. This method is simple in structure and low in loss, making it ideal for integration development.

    Methods This paper proposes a terahertz broadband vortex beam generator based on the metasurface of a single-layer double-arrow unit structure. The device is composed of a three-layer metasurface-dielectric-metal structure. The top layer is a metasurface consisting of double-arrow metal structural units, the middle layer is a polyimide, and the bottom metal layer is gold. CST MICROWAVE STUDIO is employed to simulate the reflection performance of the unit structure. Better performance is obtained after optimizing the structural parameters, and a set of optimized structural parameters is determined(Fig. 2). The reflection characteristics of the top single-layer double-arrow unit with its center as the origin and the z-axis as the rotation axis are analyzed when the incident light is set as a circularly polarized wave. Reflective broadband metasurfaces with various topological charges are designed using the unit structure’s ordered arrangement to generate the corresponding vortex beams based on the Pancharatnam-Berry (P-B) phase principle. Metasurfaces with topological charges of 1 and 2 are designed in this study. CST MICROWAVE STUDIO software simulates the reflection amplitudes and phases of the circularly polarized terahertz beams perpendicular to the metasurface and verifies the designed reflective metasurface’s broadband and propagation characteristics.

    Results and Discussions When the incident wave is circularly polarized, the center of the metal structure of the top double-arrow unit is setted as the origin, and the z-axis is the rotation axis. The reflection amplitude change is relatively stable in the broadband range of 0.9--1.8 THz under different rotation angles, and the phase change with angle is approximately equal to twice the rotation angle(Fig. 4). The broadband and propagation characteristics of the designed reflective metasurface are verified to observe the generation of vortex beams in the entire operating frequency band. The operating frequency of a circularly polarized wave incident perpendicular to a metasurface with a topological charge of 1 is f=0.9, 1.3, 1.8 THz, and the vortex beam’s reflection phases and amplitudes at a distance of 500 μm from the metasurface is given(Fig. 6). It reveals that the reflection phases at the three frequencies are continuously distributed at 0--2π, with a spiral distribution and rotating in a counterclockwise direction. The central dark ring pattern of the near field can be seen in Fig. 6, confirming the existence of the origin vortex and the wave’s singularity in the whole beam. When the working frequency of the metasurface with a topological charge of 2 is f=0.9 THz, the vortex beam’s reflection phases and amplitudes at the propagation distance of 50, 250, 500, and 1000 μm are given(Fig. 7), and the phase diagrams gradually spiral, while the amplitude distribution diagrams gradually tend to a ring, indicating that the vortex beam has better propagation characteristics in the operating frequency range.

    Conclusions The simulation results show that the designed unit structure can achieve reflection amplitudes with over 90% efficiency through various rotation angles. The metasurface is designed in the frequency range of 0.90--1.80 THz, which can effectively convert circular polarized terahertz beams into vortex beams with OAM with different topological charges. It has the advantages of high efficiency, broadband, and simple structure and has a particular reference value for using metasurfaces to generate terahertz vortex beams.

    jiaoyan Guo, Wenyu Li, Ran Sun, Guozhong Zhao. Generation of Broadband Terahertz Vortex Beam Based on Double-Arrow Metasurface[J]. Chinese Journal of Lasers, 2021, 48(20): 2014003
    Download Citation