Some mechanisms and applications of nonlocal effects. (a) Physical diagram of local and nonlocal effect, both in real space and in momentum space (These are idealizations, as the response is never perfectly local) [8]. (b) Normalized distribution of the electric field amplitude for the focusing of a TE-polarized plane wave by a local idealized metalens, and the same metalens followed by nonlocal metasurfaces with 5 layers and 10 layers [14]. (c) Local and nonlocal meta-lenses are generally limited by broadband responses and circular polarization conversion efficiency TLR of ~25%, respectively. Nonlocal Huygens’ meta-lenses can simultaneously acquire narrowband wavefront shaping and efficiency TLR exceeding 25% [17]. Inset: tilted scanning electron microscope (SEM) image of the fabricated sample. The scale bar is 500 nm. (d) Schematic of the metalaser. Each unit cell is depicted as an inset. Here the lattice size of a unit cell is a = 360 nm. The radius and pillar height of the Si₃N₄ nanodisk are R = 135 nm and h = 150 nm, respectively [18]. The eccentric hole is positioned at L = 60 nm with a radius of r = 20 nm and variable rotation angle θ. With the control of the rotation angle of each hole, different laser profiles such as Gaussian beam, donut beam, focus spot, focus line, as well as hologram can be generated. The polarization angle θF of farfield radiation at the resonant wavelength as a function of rotation angle θ. Bottom insets show the electric field (left) and power (right) distributions of quasi-BIC in one unit cell. The top inset illustrates the geometric phase acquired by the emission from each Si₃N₄ nanodisk.