Fig. 2. Numerical analysis of symmetry-protected BICs in momentum and parameter space.
(A) Transverse magnetic (TM) absorbance spectrum of a symmetric (α = 0°) PNM in momentum space illuminated by light under an incident angle θ and of a PNM under symmetric excitation (θ = 0°) in parameter space with a triangle angle α. Three modes are visible in both spaces, which we attribute to first- and second-order symmetry-protected BICs (BIC1 and BIC2) and a SLR (from long to short wavelengths, respectively). (B to D) Electric and magnetic field vectors (arrows) and intensities (in blue and red, respectively) of BIC1, BIC2, and the SLR. The fields were extracted according to the sketched cutting planes, and all correspond to the CC condition of each mode, marked as red lines in (A). (E) Maximal absorbance (gray), Q-factor (blue), and maximal field enhancement (red) of BIC1 in momentum (θ) and parameter (α) space. (F) PNM system illustrated as a single resonator coupled to one radiative and one intrinsic channel. (G) Nanofin sketch indicating the in-plane projection px of the nanofin’s dipole p. px is proportional to the asymmetry factor tanα. (H) The numerically obtained maximal absorbance (gray) is compared to the TCMT analytical model (black), assuming an inverse quadratic relation between the radiative Q-factor and an asymmetry factor tanα (blue).