Optimal Polarization Conversion Using a Toroidal-Fano-Resonant Metasurface

There has been plenty of interest and investigation in plasmonic metasurfaces to realize high-performance flat optical components and polarization converter devices [1]. Although, these highly-efficient meta-devices are either mainly pivoted on lossless microwave regimes or focused on a reflection scheme at optical frequencies [2]. Most reported plasmonic metasurfaces are optimized under the realm of highly-radiative lossy electric and magnetic multipoles, which in fact limit their transmission efficiency [3]. As a result, plasmonic metasurfaces seem not very ideal for real applications in particular after introducing their dielectric counterparts. Nonetheless, it has been shown that dielectric metasurfaces cannot interact with the incident light as strongly as plasmonic structures [4]. The requirement of a high aspect ratio in dielectric metasurfaces further devaluates their practical applications.<br/>In this work, we draw our attention to modifying the model that is commonly used for designing a plasmonic metasurface for decades. We innovate a strategy to enhance the transmission efficiency of plasmonic metasurface to the highest attainable level [5]. We take the advantage of Fano coupling between toroidal dipole and toroidal quadrupole that introduces the least radiative loss and strong field confinement as inherently existing in toroidal resonance. With the assistance of relative low-loss toroidal multipoles, a giant cross-polarization converter with a transmission efficiency of 22.9% in a single-layer plasmonic metasurface comparable to the theoretical bound (25%) has been experimentally demonstrated which is the highest available to date. The success of this work paves the way for highly-efficiency plasmonic metasurface components and systems that could be potentially developed for low-profile applications in the real world.<br/><b>References</b><br/>[1] A. Ahmadivand, B. Gerislioglu, R. Ahuja, Y. K. Mishra, <i>Laser Photon. Rev.</i> 2020, <b>14</b>, 1900326.<br/>[2] X. Ding, F. Monticone, K. Zhang, L. Zhang, D. Gao, S. Nawaz Burokur, A. De Lustrac, Q. Wu, C. W. Qiu, A. AlùProf, <i>Adv. Mater.</i> 2015,<b> 27</b>, 1195.<br/>[3] M. Li, L. Guo, J. Dong, H. Yang, <i>J. Phys. D: Appl. Phys.</i> 2014, <b>47</b>, 185102.<br/>[4] Y. Li, Z. Huang, Z. Sui, H. Chen, X. Zhang, H. Guan, W. Qiu, J. Dong, W. Zhu, J. Yu, H. Lu, Z. Chen, <i>Nanophotonics</i> 2020,<b> 9</b>, 3575.<br/>[5] A. Hassanfiroozi, P.-S. Huang, S.-H. Huang, K.-I. Lin, Y.-T. Lin, C.-F. Chien, Y. Shi, W.-J. Lee, P. C. Wu, <i>Adv. Optical Mater.</i> 2021 https://doi.org/10.1002/adom.202101007