Observation of multiple quasi-bound states in the continuum by symmetry breaking in a photonic crystal slab

Bound states in the continuum (BICs) open up a unique avenue of enhancing light–matter interactions due to their extreme field confinement and infinite quality ( Q ) factors. Although tremendous progress has been made in the past 10 years, the majority of previous works focused on either a single BIC or dual BICs. In this work, we present both theoretical investigation and experimental demonstration on multiple BICs in a photonic crystal slab with a hexagonal lattice. All of these BICs at Γ-point can be categorized as symmetry-protected (SP) BICs. Furthermore, two BICs belong to merging BICs with topological charges q =−2. Breaking the structural symmetry will split these BICs with q =−2 into two accidental BICs with q =−1. While the other two are different from the former two, the Q -factors of both modes at the Γ-point retain a stably ultrahigh value ( Q >10 8 ) when the circular hole is transformed into a rotated elliptical hole with different size ratios of semi-long and semi-short axes. In addition, the Q -factors of the latter two BICs decrease rapidly with k x , indicating that the quasi-BICs become accessible at an ultra-small incident angle. We also show that the Q -factors of the former two BICs exhibit different dependence on the asymmetry parameters, suggesting a viable way of realizing high- Q resonances at multi-wavelengths. Finally, we presented experimental demonstration of four high- Q quasi-BICs at four different wavelengths in the near infrared by fabricating a series of photonic crystal slabs made of rotated elliptical holes and characterizing their reflection spectra. We showed that most of the measured Q -factors are above 1000 for four quasi-BICs, and the highest one can reach 16,764. Our results may find promising applications in sum-frequency generation, four-wave mixing, multiband sensing, lasing, etc.