Isolated spectrally discrete bound states in the continuum in an open system

Bound states in the continuum (BICs) exist in a variety of physical systems where they appear as lossless propagating states surrounded by radiating modes. In the case of open systems, they coexist with continuous families of guided states, which may be modes or other BICs, located in different regions of the frequency-momentum parameter space. Here we report anisotropic waveguiding structures where guided modes and BICs protected by symmetry are not possible whatsoever, though an isolated, single interference BIC emerges as a lossless, solitary needle from a sea of radiating states. The needle BIC is the unique possible bound state, which originates from the interplay of the two different radiation channels present in the structure, and remarkably, it exists at a single frequency and a precise propagation direction as a spectrally discrete bound state for any practical range of frequencies.