Transition and tunability of bound states in the continuum in metastructures composed of subwavelength gratings and distributed Bragg reflectors.

Metastructures that are composed of high-contrast gratings (HCGs) and distributed Bragg reflectors (DBRs) exhibit unique optical properties and have potential applications in surface-emitting lasers and sensors. This paper studies the transition and tunability of bound states in the continuum (BICs) in the HCG-DBR metastructure. At the high-symmetry point, the metastructure has a symmetric mode with radiation loss and an antisymmetric mode (BIC) without radiation loss. The bandgap closure and band flipping occur by modulating the filling factor (FF) of the HCG. The band flipping is essentially related to the transition of the BIC. The topological charge characterizing the far-field polarization of the BIC changes during the band flipping. Furthermore, the HCG-DBR metastructure with a dual-period grating is proposed based on the BICs induced by the Brillouin zone folding. BIC can be tuned by applying different perturbations to the dual-period grating. Both single- and dual-band edge quasi-BICs (QBICs) can be realized. Finally, by employing the dramatic phase change of dual QBICs at the resonant wavelength, giant enhancements of both positive and negative Goos-Hänchen (GH) shifts are achieved. This property can be used to design an ultrasensitive refractive index sensor based on the HCG-DBR metastructure with the Bloch surface wave (BSW) and QBIC-enhanced GH shift. This study is useful for the realization of surface-emitting lasers and high-sensitivity optical sensors based on the HCG-DBR metastructure.