image: Fig 1. | Schematic diagram of the temporal evolution process of plasmonic vortices with the same topological charge generated by different couplers. (a-c) Sample 1 introduces only the geometric phase through varying the orientation angles of the slit resonators. (d-f) Sample 2 introduces both geometric phase and propagation phase through varying the radial position of slit-pairs. view more
Credit: OEA
A new publication from Opto-Electronic Advances, 10.29026/oea.2023.220133 discusses tailoring spatiotemporal dynamics of plasmonic vortices.
Plasmonic vortex is an optical field distribution with topological features formed by interfering surface plasmons, which enriches the class of vortex phenomena in nature. Owing to their special orbital angular momentum feature in the evanescent field region, plasmonic vortices hold great promises for many cutting-edge applications, such as plasmonic tweezers for microparticle manipulations and on-chip quantum information processing. The generation methods and evolution dynamics of plasmonic vortices have thus elicited great research enthusiasm in the last decade, which have provided many insights into the nature of plasmonic vortex and rapidly promoted the related applications forward.
For plasmonic vortex generation, the most common method is constructing special couplers and utilizing the design degrees of freedoms of propagation phase and geometric phase to convert circularly polarized light carrying spin angular momentum into on-chip plasmonic vortex. Despite the sole or combined use of propagation and geometric phase can all achieve plasmonic vortex of target topological charge, the actual differences of their spatiotemporal dynamics have remained unexplored. For characterization methods, the currently used photoemission electron microscopy and nonlinear near-field optical microscopy are limited by the probing principle and optical systems, thus can hardly obtain the exact evolution dynamics. The research documented by this article mainly focused on the objective characterization of plasmonic vortex and the subjectively tailoring of its spatiotemporal dynamics for specific applications has not been achieved.
The research group from Tianjin University, Guilin University of Electronic Technology and authors of this article propose a new method to tailor the spatiotemporal dynamics of plasmonic vortices. It is demonstrated that the plasmonic vortices with the same topological charge can be endowed with distinct spatiotemporal dynamics by simply changing the coupler design (Fig 1). The full amplitude and phase information of surface plasmons fields and the exact evolution dynamics with ultrahigh temporal resolution were directly obtained based on a near-field scanning terahertz microscopy.
Based on the orthogonal slit-pairs, the group designed two plasmonic vortex couplers to generate plasmonic vortices with the same topological charge (l = 4). By introducing different propagation phase and geometric phase, the spatiotemporal dynamics of generated plasmonic vortices can be totally different. In order to numerically reveal the processes of the formation, revolution and decay stages in the lifetime of plasmonic vortex, the group generalized the 2D Huygens-Fresnel principle to time-domain and obtained the time-resolved snapshots of the plasmonic vortices field distributions (Fig 2). Although two couplers’ performances in frequency domain are similar, in terms of intensity and phase distributions, the spatiotemporal dynamics of the two plasmonic vortices have distinct characteristics. As depicted in the figures, the surface plasmon fields reach and decay simultaneously (Sample 1) or successively (Sample 2) at the same revolution orbit, which corresponds to the uniformity or decomposition of the carried orbital angular momentum in spatiotemporal distribution. The results based on near-field scanning terahertz microscopy revealed the exact spatiotemporal evolution dynamics of plasmonic vortices with ultrahigh resolution (~1/66 of the optical-cycle at center frequency), and experimentally verified the feasibility of tailoring plasmonic orbital angular momentum in time-domain based on different plasmonic vortex coupler designs.
This research on the distinct generation and evolution behaviors of plasmonic vortices is of great importance for the practical applications related to time-varying characteristics. The manipulation of orbital angular momentum in both spatial and temporal dimensions will provide a new design degree of freedom and higher precision for plasmonic tweezers and on-chip information processing. In addition, the proposed strategy is general and can be directly applied to the infrared and visible regimes, providing a new approach to explore more intrinsic nature and potential applications of plasmonic vortices.
Keywords: plasmonic vortex / surface plasmon / spatiotemporal dynamics / optical orbital angular momentum
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Relying on the "Key Discipline of Optical Engineering", the "Key Laboratory of Optoelectronic Information Technology of the Ministry of Education" and the "Innovation Team of Basic Frontier and Application of ultrafast Photonics of the Ministry of Education", the Center for Terahertz Wave of Tianjin University currently has 12 professors and more than 50 master and doctoral candidates. In recent years, with the support of the National 973 Program, National 863 Program, National Major Scientific Instrument Project, National Natural Science Foundation of China, Distinguished Young Scholars, Key Projects, etc., the Center for Terahertz Wave has become a world-class research platform for terahertz photonics. Members of the center focus on international frontier areas to carry out systematic research, and the related achievements have been published in Nature Physics, Nature Communications, Science Advances, Light: Science & Applications, Optica, Opto-Electronic Science, Opto-Electronic Advances and other high level academic journals. The center will adhere to the national strategic needs, be committed to the basic research and application promotion of terahertz photonics.
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Article reference Yuan XY, Xu Q, Lang YH, Jiang XH, Xu YH et al. Tailoring spatiotemporal dynamics of plasmonic vortices. Opto-Electron Adv 6, 220133 (2023). doi: 10.29026/oea.2023.220133
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