Nearly bound states in the radiation continuum in a circular array of dielectric rods

Bulgakov, Evgeny N.; Sadreev, Almas F. PHYSICAL REVIEW A, 97 (3):10.1103/PhysRevA.97.033834 MAR 16 2018

We consider E-polarized bound states in the radiation continuum (BICs) in circular periodical arrays of N infinitely long dielectric rods. We find that each true BIC which occurs in an infinite linear array has its counterpart in the circular array as a near-BIC with extremely large quality factor. We argue analytically as well as numerically that the quality factor of the symmetry-protected near-BICs diverges as eλN, where λ is a material parameter dependent on the radius and the refraction index of the rods. By tuning of the radius of rods, we also find numerically non-symmetry-protected near-BICs. These near-BICs are localized with exponential accuracy outside the circular array, but fill the whole inner space of the array carrying orbital angular momentum.