Dynamics and asymmetric behavior of loss-induced bound states in the continuum in momentum space

Bound states in the continuum (BICs) are peculiar discrete states embedded in the continuous spectrum. It has been reported that BICs still exist and exhibit new features when parity-time (PT)-symmetric perturbation is applied to the system supporting BICs. Here, we further study the PT unbalanced system, especially a purely passive one, and find that a BIC with a divergent radiative Q factor also exists when differential loss is introduced. Meanwhile, merging of two BICs is observed when varying the strength of the differential loss. Different from the ordinary BIC, this loss-induced BIC can be excited by an external plane wave, although it will not radiate to infinity. On the contrary, another mode at the same frequency but opposite wavevector can radiate, but cannot be excited by an external plane wave, manifesting the asymmetric behavior in momentum space. As the gain is introduced, the net loss can even be compensated precisely, giving rise to a PT-BICs. The Q-factor divergence rate of the PT-BICs is anisotropic in the parameter space. These results can be extended to other systems favorable for experimental implementation and may facilitate applications in light trapping and lasing.