Topologically enabled giant angle-insensitive Goos-Hänchen shift by tunable merging bound states in the continuum of a quasiflat band

Optical bound states in the continuum (BICs) exist commonly in periodic structures with strong local resonances. Merging multiple BICs provides an excellent way to further enhance the $Q$ factor of nearby quasi-BICs compared with isolated BICs. Here, we report on the giant and angle-insensitive transmitted optical Goos-Hänchen shift (GHS) through a photonic crystal slab, assisted by the ultrahigh-Q quasi-BICs on a quasiflat band with embedded tunable merging BICs at a nearly arbitrary wave vector in the reciprocal space. Even at large angles of incidence, GHS can also be enhanced to >4 orders of wavelength by the designed tunable off-$\Gamma$ merging BICs. Empowered by the angle-insensitive ultrahigh-$Q$ resonances, the wide-angle giant GHS within an extremely narrow bandwidth is realized. Furthermore, we propose an ultrasensitive environmental refractive index sensor and a temperature sensor based on the enhanced GHS by merging BICs. Our work reveals the tremendous potential of tunable merging BICs for various applications based on angular selectivity, such as beam steering, directional vector beams, and angle-multiplexed sensors.