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 > 10⁸) 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.