Dynamically-tunable Metasurfaces - From Forward to Automatic Inverse Design Approach
Pin Chieh Wu1*
1Department of Photonics, National Cheng Kung University, Tainan, Taiwan
* Presenter:Pin Chieh Wu, email:pcwu@gs.ncku.edu.tw
Metasurfaces have attracted a lot of attention because of their great capability to manipulate the attributes of an electromagnetic wave on demand. Thus, nanophotonic metasurfaces provide great flexibility in light management that highly benefits the development of low-profile optical systems [1, 2]. In this talk, I will present two strategies to realize actively-tunable high-performance metasurfaces for feasible applications. Firstly, I will demonstrate that the polarization state of the light reflected from an indium tin oxide (ITO)-based active metasurface can be dynamically controlled [3]. By suitably biasing between the ITO layer and back reflector, the linearly-polarized incident light can be actively converted to a cross-polarized, circularly-polarized, or elliptically-polarized light. Secondly, I will report an unsupervised genetic-type tree search algorithm in a virtual space wherein an automatic inverse design of metasurfaces is achieved with low computational complexity [4]. With the developed method, we design and characterize beam steering metasurfaces, which are capable of deflecting normally incident light into target directions with minimal sidelobes; other functions of metasurfaces could also be achieved by revising the learning criterion in the virtual space. Due to the co-optimization of amplitude and phase shift, the inversely-designed metasurface possesses higher optical directivity than the forward-designed metasurface.

1. Q. He, S. Sun, S. Xiao, L. Zhou, "High-efficiency metasurfaces: Principles, realizations, and applications," Advanced Optical Materials 6, 1800415 (2018)
2. A. Hassanfiroozi, P.-S. Huang, S.-H. Huang, K.-I Lin, Y.-T. Lin, C.-F. Chien, Y. Shi, W.-J. Lee, and P. C. Wu, "A toroidal-Fano-resonant metasurface with optimal cross-polarization efficiency and switchable nonlinearity in the near-infrared," Advanced Optical Materials 9, 2101007 (2021)
3. P. C. Wu, R. Sokhoyan, G. Kafaie Shirmanesh, W.-H. Cheng, and H. A. Atwater, "Near-infrared active metasurface for dynamic polarization conversion," Advanced Optical Materials 9, 2100230 (2021)
4. C.-H. Lin, Y.-S. Chen, J.-T. Lin, H. C. Wu, H.-T. Kuo, C.-F. Lin, P. Chen, and P. C. Wu "Automatic inverse design of high-performance beam-steering metasurfaces via genetic-type tree optimization," Nano Letters 21, 4981-4989 (2021)

Keywords: Tunable metasurface, Polarization control, Beam steering, Inverse design, High-performance optics