Fig. 2: Unit cell method and variations by modifying boundary conditions.

a Unit cell method. The top metasurfaces represent the actual metasurfaces, while the three metasurfaces below depict the assumed surrounding conditions for each unit cell. Although the metasurfaces are actually nonperiodic, the electromagnetic behavior of each unit cell is calculated by presuming that the surrounding unit cells are identical, effectively assuming periodicity³⁷. This method allows for faster metasurface design because it does not simulate the entire area. b Increasing the unit cell size can reduce the incorrect prediction of coupling with surrounding meta-atoms⁴⁰. c Modifying the periodic boundary condition to the perfectly matched layer (PML) in inverse design produces more efficient results³⁰. d Introducing a spatial overlap region instead of a PML right next to the target cell allows for more efficient simulations (a: basic unit cell and d: area encompassing both the unit cell and overlapping domain regions)⁴⁷.