Giant enhancement of the Goos–Hänchen shift assisted by merging bound states in the continuum

Bound states in the continuum (BICs) have received considerable attention in the field of nanophotonics due to their highly confined resonance and high Q factor, which effectively eliminates radiation loss. Various periodic structures have been studied to achieve BICs, with photonic crystal slabs (PCSs) being a prominent example. In PCS, multiple BICs can be merged to strongly suppress out-of-plane-scattering losses caused by fabrication imperfections. In this paper, we investigate the impact of reflection-type merging BICs on the Goos–Hänchen shift (GH shift) and demonstrate a remarkable enhancement of the GH shift, exceeding five orders of wavelength. We show the dynamic changes of the GH shifts with the isolated, merging, and merged BICs, achieving positive and negative GH shifts in different angles of peak reflectance for the same frequency. Our research highlights that even minor fabrication imperfections can result in a significant change in the GH shift, which can serve as a means for detecting manufacturing defects. Furthermore, we propose an ultrasensitive environmental refractive index sensor based on the enhanced GH shift by an isolated BIC. Our study contributes to the understanding of BICs and their potential applications in nanophotonics, including advanced optical communication devices, nanodevice fabrication, and highly sensitive sensors.