High-Q states in subwavelength dielectric resonators forming in strong mode coupling regime

Summary form only given. Resonator is one of the main component of almost any optoelectronic and photonic device. Quality (Q) factor is the most important characteristic of the resonator showing its ability to amplify the external electromagnetic field. There are a lot of types of optical resonator, e.g., photonic crystal cavities, Fabry-Perot resonators, whispering gallery mode resonator and many others. The high record values of Q factor today rich giant values up 1010. However, decrease of the size of any named above resonators results in exponential drop of their Q factor. When the size of the resonator is comparable with vacuum wavelength (the case of the lower Mie-resonance), the value of Q-factors is about 10-20, which is not enough for many practical applications. Therefore, the quest for compact photonic systems makes a new query for small high-Q resonators, using fundamentally new principles of light trapping. In this work we reveal new mechanism of efficient light trapping in subwavelength resonators based on strong coupling between its modes [1,2]. We demonstrate that strong mode coupling results in resonances with high quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich-Wintgen scenario of destructive interference (Fig. 1). We explain the physics of these states in terms of multipole decomposition, and show that their appearance is accompanied by a drastic change in the far-field radiation pattern. We reveal a fundamental link between the formation of the high quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra. Our theoretical findings are confirmed by microwave experiments for the scattering of high-index cylindrical resonators with a tunable aspect ratio.