Solitary high-Q resonance in a grating waveguide via parity symmetry breaking at mode crossing

Abstract A phenomenon of a solitary narrow high-Q resonance in planar grating waveguide associated with bound state the continuum (BIC) is revealed analytically. BICs appear when dispersion curves leaking even-parity and non-leaking odd-parity infinite-grating eigenmodes intersect. These intersections engender parity symmetry break eigenmode, causing it to decouple from spatial Fourier harmonic that emitted out waveguide. As result, otherwise low-Q comprised those two aforementioned some other eigenmodes, acquires very high quality factor vicinity mode-crossing frequency. Implementation such mechanism can be instructive for designing photonic crystals structures.