https://onlinelibrary.wiley.com/doi/pdf/10.1002/adpr.202000205

Tunable Metasurfaces: The Path to Fully Active Nanophotonics

Abstract Optical metasurfaces is a rapidly developing research field driven by its exceptional applications for creating easy-to-integrate ultrathin planar optical devices. The tight confinement of the local electromagnetic fields in resonant photonic nanostructures can boost many optical effects and offer novel opportunities for the nanoscale control of light–matter interactions. However, once the structure-only metasurfaces are fabricated, their functions will be fixed, which limits it to make breakthroughs in practical applications. Recently, persistent efforts have led to functional multiplexing. Besides, dynamic light manipulation based on metasurfaces has been demonstrated, providing a footing ground for arbitrary light control in full space-time dimensions. Here, we review the latest research progress in multifunctional and tunable metasurfaces. Firstly, we introduce the evolution of metasurfaces and then present the concepts, the basic principles, and the design methods of multifunctional metasurface. Then with more details, we discuss how to realize metasurfaces with both multifunctionality and tunability. Finally, we also foresee various future research directions and applications of metasurfaces including innovative design methods, new material platforms, and tunable metasurfaces based metadevices.

Optical metasurfaces towards multifunctionality and tunability

This Review focuses on the integration of plasmonic and dielectric metasurfaces with emissive or stimuli-responsive materials for manipulating light-matter interactions at the nanoscale. Metasurfaces, engineered planar structures with rationally designed building blocks, can change the local phase and intensity of electromagnetic waves at the subwavelength unit level and offers more degrees of freedom to control the flow of light. A combination of metasurfaces and nanoscale emitters facilitates access to weak and strong coupling regimes for enhanced photoluminescence, nanoscale lasing, controlled quantum emission, and formation of exciton-polaritons. In addition to emissive materials, functional materials that respond to external stimuli can be combined with metasurfaces to engineer tunable nanophotonic devices. Emerging metasurface designs including surface-functionalized, chemically tunable, and multilayer hybrid metasurfaces open prospects for diverse applications, including photocatalysis, sensing, displays, and quantum information.

Light-Matter Interactions in Hybrid Material Metasurfaces.

Angular‐Multiplexing Metasurface: Building Up Independent‐Encoded Amplitude/Phase Dictionary for Angular Illumination

Conventional human-driven methods face limitations in designing complex functional metasurfaces. Inverse design is poised to empower metasurface research by embracing fastgrowing artificial intelligence. In recent years, many research efforts have been devoted to enriching inverse design principles and applications. In this perspective, we review most commonly used metasurface inverse design strategies including topology optimization, evolutionary optimization, and machine learning techniques. We elaborate on their concepts and working principles, as well as examples of their implementations. We also discuss two emerging research trends: scaling up inverse design for large-area aperiodic metasurfaces and end-to-end inverse design that co-optimizes photonic hardware and post-image processing. Furthermore, recent demonstrations of inverse-designed metasurfaces showing great potential in real-world applications are highlighted. Finally, we discuss challenges in future inverse design advancement, suggest potential research directions, and outlook opportunities for implementing inverse design in nonlocal metasurfaces, reconfigurable metasurfaces, quantum optics, and nonlinear metasurfaces.

https://dspace.mit.edu/bitstream/1721.1/145692/2/acsphotonics.1c01850.pdf

Empowering Metasurfaces with Inverse Design: Principles and Applications

We report an array-level inverse design approach to optimize the beam steering performance of active metasurfaces, thus overcoming the limitations posed by nonideal metasurface phase and amplitude tuning. In contrast to device-level topology optimization of passive metasurfaces, the outlined system-level optimization framework relies on the electrical tunability of geometrically identical nanoantennas, enabling the design of active antenna arrays with variable spatial phase and amplitude profiles. Based on this method, we demonstrate high-directivity, continuous beam steering up to 70° for phased arrays with realistic tunable antenna designs, despite nonidealities such as strong covariation of scattered light amplitude with phase. Nonintuitive array phase and amplitude profiles further facilitate beam steering with a phase modulation range as low as 180°. Furthermore, we use the device geometries presented in this work for experimental validation of the system-level inverse design approach of active beam steering metasurfaces. The proposed method offers a framework to optimize nanophotonic structures at the array level that is potentially applicable to a wide variety of objective functions and actively tunable metasurface antenna array platforms.

Array-Level Inverse Design of Beam Steering Active Metasurfaces.

We develop an inverse design approach to optimize array architectures of reconfigurable metasurfaces and report dramatically improved beam steering performances with non-ideal antenna components. The versatility is enhanced by enabling continuous steering up to 70°.

/pdf/array-scale-inverse-design-of-active-metasurfaces-wb7a9d8qsa.pdf

Prachi Thureja

Papers

Array-Level Inverse Design of Beam Steering Active Metasurfaces.

Array-Scale Inverse Design of Active Metasurfaces