Dynamical Evolution of BIC‐Fano Dual Modes under Misalignment of Two Waveguide‐Coupled Gratings

Applications of bound states in the continuum (BIC) are limited by the challenges of dynamical tuning and coupling to external excitation. In this work, a well‐thought‐out structure model of two gratings coupled by a waveguide is reported. The structure results in unique BIC‐Fano dual modes which undergo an unusual degeneration and inversion by either the tuning of structure parameters or through misalignment of the bilayer gratings, accompanied by the merging and splitting of Quasi(Q)‐BIC and Fano peaks in the transport spectra of the structure. The BIC‐Fano dual mode degeneration leads to the infinite Q factor from BIC and the coupling channel to external excitation from resonance. The results suggest an elegant way for smart engineering of resonances for many applications in optical switching and enhanced light−matter interactions. Especially, the dual‐mode structure gives a highly efficient coupling to a planar waveguide, and is also useful for displacement sensing with an extremely high resolution.