Q-factor optimization in dielectric oligomers

In this paper we propose an optimization procedure for quality-factor (Q-factor) enhancement in dielectric oligomers. The oligomers are introduced as linear periodic chains of dielectric elements the individual Q factors of which are optimized via multipolar conversion in an avoided crossing of two resonant eigenfrequencies. It is demonstrated that when such dielectric elements are assembled into an infinite periodic array the coupling between individual resonances gives rise to an in-$\Gamma$ optical bound state in the continuum (BIC) unprotected by symmetry. By setting up the oligomers of finite numbers of dielectric elements we observe high quality resonances which occur as traces of the infinite array band structure. The resulting high quality resonances exhibit light localization spots of a few squared wavelengths with Q factors $Q>{10}^4$. The localization is provided by similarity of the field pattern to that of the BIC with the losses at the edges of the array suppressed due to an almost flat BIC host band. The scaling law of the Q factor against the dielectric permittivity is derived.