Nonreciprocal transmission based on quasi-bound states in the continuum via scaled lattice constants.

Nonreciprocal devices are key technologies for modern photonic applications, and nonlinearly induced nonreciprocity based on metasurface platforms opens up the potential for compactness and miniaturization of such devices in free-space optical paths. In this work, a design of quasi-bound states in the continuum (QBICs) via scaled lattice constants is employed, featuring a plate-hole tetramer unit cell with single-step etching. Under normal incidence, QBICs exhibit polarization-insensitive excitation, with the scattering dominated by the combination term of magnetic dipoles, magnetic toroidal dipoles, electric quadrupoles as well as electric toroidal quadrupoles - rendering the vertical field distribution sensitive to parametric variations. Embedding Kerr nonlinearity yields nonreciprocal responses under upper and lower port excitation. The steep edges between peaks and valleys in the Fano-line-shaped transmission spectra facilitate balancing the nonreciprocal intensity range (NRIR) and isolation. The NRIR originates from electromagnetic asymmetry introduced by varying the substrate refractive index and can be flexibly tuned via structural parameters - validated by simulations of nonreciprocal responses with varying plate thickness alone. Additionally, Rabi splitting from varying hole depth induces abrupt electromagnetic asymmetry changes in the strong coupling region, offering a new NRIR tuning freedom. This design strategy provides fresh insights for nonreciprocal device research, with the structure holding promise for sensing and nonlinear applications.