Bulgakov, Evgeny; Sadreev, Almas Source: PHYSICAL REVIEW B, 98 (8):10.1103/PhysRevB.98.085301 AUG 1 2018

We show that a circular periodic array of silicon dielectric cylinders supports nearly bound states in the continuum (BICs) propagating along the cylinders. These propagating nearly BICs with extremely large-Q factors are surrounded by resonant modes weakly leaking into the radiation continuum. We present leaky zones in the form of dispersion curves for complex eigenfrequencies dependent on propagation constant kz, with the wave vector directed along the cylinders in the vicinity of different types of BICs. Symmetry-protected nearly BICs have the resonant width proportional to squared propagation constant Γ∼k2z; the widths of non-symmetry-protected nearly BICs behave as Γ∼(kz−kc)2, where kc and non-symmetry-protected nearly BICs have the resonant width proportional to k4z. The latter propagating nearly BICs can serve for the transmission of a electromagnetic signal paving a way to a different type of optical fiber. We also demonstrate weakly leaking resonant modes which carry orbital angular momentum.