Multiple toroidal dipole symmetry-protected bound states in the continuum in all-dielectric metasurfaces

In nanophotonics, exciting multiple high quality factor (Q-factor) and high tunable Fano resonances is important for many applications. In this paper, an all-dielectric metasurface consists of Si layer and three Si rectangular bars with different widths and lengths is proposed. The triple high Q-factor and high tunable the quasi bound states in the continuum (quasi-BIC) modes with distinct Fano features can be achieved by breaking the C2 rotational symmetry of the unit cell. With adjusting the asymmetry parameter, the modulation depth, resonant frequency and linewidth of the sharp triple Fano resonances can be engineered, and the Q-factor can reach 107. In addition, the triple quasi-BIC are both mainly generated by the toroidal dipole oscillation through analyzing the result of electromagnetic distribution and the far-field scattering of Cartesian multipoles with asymmetric nanostructure. The proposed nanostructure has numerous potential applications on nonlinear optics, filters, and multi-channel biosensors.