Terahertz bound states in the continuum on-and-off-<i>Г</i> point of a moiré photonic superlattice

Moiré metasurfaces exhibit high optical tuneabilities and versatile light manipulation capabilities. Both infinite quality factor (Q factor) and topological vortex configurations in momentum space (<i>k</i>-space) of the bound state in the continuum (BIC) have introduced new dimensions for light modulation. Herein, we propose a moiré metasurface comprising two identical square photonic lattices superimposed with a commensurate angle of 12.68°. By tuning the incidence angle, the symmetric-protected BICs, Friedrich-Wintgen BIC, and accidental BIC can be achieved simultaneously in our moiré metasurfaces. It is found that the quasi-BICs maintain an ultrahigh Q factor beyond 10⁷. The photonic band structures manifest that the three types of BICs are at the center of far-field polarization vortices in <i>k</i>-space, which have their own topological charges. We experimentally show that these BICs exhibit high sensitivity to subtle changes in analyte refractive index for thin-film sensor application. Our discovery predicts an approach to a highly sensitive multi-channel terahertz biosensor.