Abstract
Abstract Bound states in the continuum (BIC) have been at the forefront of research in optics and photonics over the past decade. It is of great interest to study the effects associated with quasi-BICs in the simplest structures, where quasi-BICs are very pronounced. An example is a dielectric cylinder, and in a number of works, quasi-BICs have been studied both in single cylinders and in structures composed of cylinders. In this work, we studied the properties of quasi-BICs during the transition from a homogeneous dielectric cylinder in an air environment to a ring with narrow walls while increasing the diameter of the inner air cylinder gradually. The results demonstrate the quasi-BIC crossover from the strong-coupling to the weak-coupling regime, which manifests itself in the transition from the avoided crossing of branches to their intersection with the quasi-BIC being preserved on only one straight branch. In the regime of strong-coupling and quasi-BIC, three waves interfere in the far-field zone: two waves corresponding to the resonant modes of the structure and the wave scattered by the structure as a whole. The validity of the Fano resonance concept is discussed since it describes the interference of only two waves under weak coupling conditions.
Highlights
Localization of electromagnetic waves is important both for fundamental research and for many important applications
In the quasi-Bound states in the continuum (BIC) region of the avoided crossing, we have the interference of three waves in the far-field, two waves are determined by the resonant modes of a cylinder or ring (TE1,1,0 and TM1,1,1), and the third wave is associated with a nonresonant background
We have revealed that high-Q quasi-BIC exists in dielectric cylindrical resonators and in dielectric ring resonators with high robustness over a wide range of internal hole sizes
Summary
Localization of electromagnetic waves is important both for fundamental research and for many important applications. BIC is a general wave phenomenon that was first mathematically proposed in 1929 by von Neumann and Wigner [1] for electronic states. Note that the initial proposal of von Neumann and Wigner was never implemented in practice, but the idea of the appearance of such a state turned out to be very fruitful; a number of other mechanisms of the BIC formation were theoretically proposed and experimentally demonstrated [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. Photonic BICs coexist with propagating electromagnetic waves and lie in a continuum, but theoretically remain completely confined in the structure without any radiation [8]. Due to the finite length of the structures, material loss, and imperfection, the BICs collapse to a quasi-BIC with a limited radiation Q-factor [18]. There are BICs protected by symmetry and separability; second, these are BICs built using inverse construction (for example, potential, hopping rate, or shape engineering); and there are BICs, achieved by setting parameters
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