Abstract
We propose and demonstrate the realization of dual Friedrich-Wintgen (F-W) bound states in the continuum (BICs) in a terahertz all-silicon photonic crystal (PhC) slab. A symmetric PhC slab with cylindrical hole array supports an electric quadrupole-SP BIC, which transitions into a quasi-SP BIC upon the breaking of its C2v symmetry. By adjusting the asymmetry parameter, the quasi-SP BIC can be transformed into two F-W BICs due to its destructive interference with two guided mode resonances (GMRs), respectively. Notably, the frequency of one F-W BIC exhibits robustness to the thickness of the PhC slab over an extremely wide range, which is very useful for its practical applications.