Bound states in the continuum (BICs), exceptional points (EPs), and poles of PT-symmetric structures are three distinct types of physical singularities. BICs represent modes that are completely decoupled from radiation channels, enabling optical resonant modes with theoretically infinite Q factors, which makes them excellent candidates for overcoming current limitations in cavity mode technology. EPs and poles arise from PT-symmetric structures. An EP serves as a phase transition point that delineates the boundary between PT-symmetric and PT-broken phases. A pole occurs within the PT-broken phase and corresponds to two reciprocal eigenvalues of the structure’s scattering matrix.

Many interesting physical phenomena result from the coupling of different modes. BICs can lead to strong enhancement of the electromagnetic field but cannot amplify energy. In contrast, poles can lead to lasing but suffer from limited Q factors and high lasing thresholds. Given the unusual properties of both kinds of singularities, an intriguing question arises: can they coalesce into a new singularity? Beyond fundamental interest, such a “BIC-pole” double singularity would combine their respective features and enable lasing at an extremely low threshold.

Finding a suitable structural platform that supports both a BIC and a PT-symmetric pole is challenging. To achieve coupling between a BIC and a PT-symmetric pole point, we propose and design a novel composite structure consisting of a pair of gratings and a pair of waveguide cavities arranged in a PT-symmetric configuration. Through numerical calculations and simulations, we demonstrate that the pole effect of PT symmetry, combined with the two-fold “BIC-resonance”, generates a special “BIC-pole” coupled resonance effect. First, the compound structure induces a pair of “BIC-resonance” modes. The two “BIC-resonance” two-fold modes can approach each other and become degenerate by changing the thickness of the coupling layer. The coupled resonance effect involving the degenerate “BIC-resonance” two-fold modes differs from that of the non-degenerate ones. The latter can only lower the threshold for achieving pole lasing. The former enables a perfect absorption mode of the pole effect; with proper parameters, it can achieve a rapid transition from unidirectional invisibility to perfect absorption within a frequency shift of 0.02 THz.