Fig. 5. Comparison of far-field polarization states in momentum space of two twisted PhCSs supporting BIC with the same relative angle between top and bottom layer ∆α = α_top−α_bot = 18°.

a Schematic of the structure with point symmetry against the center of the PhCS at α_top = 9° and α_bot = −9°. Distribution far-field polarization states in (b), c upward and d, e downward radiation, showing the same χ distribution. f Schematic of the PhCS with the same relative angle of ∆α = 18° but the different absolute angle of α_top = 18° and α_bot = 0°. g distribution of upward far-field polarization states shows circularly polarized radiation at every point in vicinity of BIC. h The ellipticity angle (χ) of polarization states in (g) is 45° indicating circular polarization. i Polarization states of downward radiation. j The ellipticity angle is 13°, different from upward radiation. Insets in (h), j show the enlarged view of polarization ellipses