Multiple toroidal dipole bound states in the continuum in dielectric metasurfaces

= ±1. In particular, the FW BICs and SP BICs are represented as electric or magnetic toroidal dipole (TD) modes characterized by electric or magnetic fields circulating around the surface of a hypothetical torus, with magnetic or electric fields looping inside the torus. The symmetry axis of an electric or magnetic TD can be oriented either normal or tangential to the metasurface, leading to a longitudinal or transverse TD. Occasionally, a few pairs of electric or magnetic TD modes are gathered in the unit cell to form multiple TDs. By carefully arranging the dielectric patches and substrates, the underlying metasurfaces support a combination of multiple electric and magnetic TD BICs or quasi-BICs oriented in longitudinal and transverse directions.