image: (a, b) Under LP incidence, the schematic functions for conversion and focusing of the metalens with different rotation angles; (c) with incident LCP and RCP, the schematic functions for conversion and focusing of the metalens; (d) the schematic THz imaging. Respectively, the helix directions of LCP and RCP waves correspond to counterclockwise and clockwise rotation of electric field vector when viewed along wave propagation direction. view more
Credit: OEA
In a new publication from Opto-Electronic Advances; DOI 10.29026/oea.2022.210062 , researchers from Tianjin University, China and Capital Normal University, China, discuss dynamic phase assembled terahertz metalens for reversible conversion between linear polarization and arbitrary circular polarization.
Metalens is a class of metasurface devices with beam focusing function. If metalens integrates the function of single-handed circular polarization (CP) wave conversion, the focusing lens together with CP lens in traditional cameras may be replaced by a metalens. Several techniques have achieved CP conversion and focusing, for example, introducing chiral optics and spin-decoupling mechanisms into metalens. However, the physics of these techniques do not allow for a clean and efficient conversion of linear polarization (LP) to single-handed CP waves.
The metalens must depend on several different unit cells those phase delay can cover 0-2p. Traditionally, the principle of Pancharatnam-Berry (PB) phase is in whole or in part introduced in the phase design of the metalens, even for those metalenses containing chiral optics and spin decoupling mechanisms. For instance, the general operation is that several unit cells with different structural parameters are firstly designed to meet the phase change of 0-p, and then the phases of the remaining p-2p are obtained by rotating these structures at 90o. Although an unit cell can realize the conversion of LP to single-handed CP, the helix of CP will be opposite direction after this cell being rotated by 90o due to the spin dependence of PB phase. Therefore, the output CP wave contains non-negligible left-handed and right-handed components, so that the LP wave component synthesized by the two CP components is also non-negligible, which is not applicable to advanced cameras.
Meanwhile, the polarization conversion of the existing metalenses is usually not reversible. Although a LP wave through those metalenses is converted into other polarization states and focused, wave with other polarization states can no longer be converted into the original LP wave and focused. These shortcomings also limit the multifunctional applications of the metalens to some extent.
The authors of this article propose an all-dielectric silicon-based terahertz metalens, which is assembled using dynamic phase and getting rid of the PB phase. The problems considered in this work are: 1) metalens integrated with single-handed CP wave conversion have practical application value in terahertz (THz) cameras, and how metalens converts LP THz wave into arbitrary single-handed THz wave as perfectly as possible; 2) how THz metalens achieves reversible conversion between LP wave and arbitrary single-handed CP wave to expand their application scope as much as possible.
Article reference: Li JT, Wang GC, Yue Z, Liu JY, Li J et al. Dynamic phase assembled terahertz metalens for reversible conversion between linear polarization and arbitrary circular polarization. Opto-Electron Adv 5, 210062 (2022) . doi: 10.29026/oea.2022.210062
Keywords: terahertz metalens / metamaterials / metasurfaces / dynamic phase / single-handed circular polarization / reversible conversion
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Professor Jianquan Yao, academician of Chinese Academy of Sciences, is working at Tianjin University, China. The group of Professor Yao is mainly engaged in theoretical and applied research in nonlinear optics, laser technology, optical communication, terahertz science and technology, etc. This group is supported by more than 30 projects including National Natural Science Foundation of China, National Key Research and Development Program of China, etc. Professor Yao has won National Invention Award of China, and the Grand Prize of the Chinese Academy of Sciences. To date, he has published 700+ papers, and 20+ invention patents. http://laser.tju.edu.cn/index.htm
Professor Yan Zhang, Fellow of the Optical Society of America, is working at Capital Normal University, China. He is also supported by Program for New Century Excellent Talents in University, China. He has successively engaged in research work at Yamagata University in Japan, Hong Kong Polytechnic University, University of Stuttgart in Germany, Hong Kong University of Science and Technology, Leinster University of Technology in the United States, and University of Konstanz in Germany. Currently, the group of Professor Zhang is mainly engaged in research on terahertz optoelectronics, micro-nano optics, optical information processing, etc., and has published more than 270 journal papers in Nature Photonics, Nature Communications, Advance Optical Materials, etc. https://physics.cnu.edu.cn/people/faculty/teacher/120067.htm
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