Emerging Planar Nanostructures Involving Both Local and Nonlocal Modes

With the capabilities to localize light at the subwavelength scale and flexibly control the full dimensions of electromagnetic fields, planar optical artificial nanostructures have been widely explored to innovate next-generation multifunctional compact photonic devices in recent decades. To further improve the efficiency, quality (Q) factors, and design degrees of freedom of photonic devices, recent research advances have brought in comprehensive consideration of both local and nonlocal modes in nanostructures with blurred distinctions between metasurfaces and photonic crystals. In this Perspective, we review the recent progress in planar nanophotonics involving localized resonances, nonlocal modes, and their couplings. Many intriguing physical effects correlated to local and/or nonlocal modes and the corresponding applications are reviewed, as well as large-area optimization strategies to directly simulate local and nonlocal optical responses. We also summarize several current challenges and foresee various future directions of the attractive field.