Tunable quality factors (Q-factor) in topological photonics devices have gathered great attention in recent years. The devices with high-refractive-index (HRI) dielectric metasurfaces or non-symmetric nano-photonics devices have demonstrated the concept of bound states in the continuum, which allow the ultra-sharp resonance and extreme large Q-factor. By controlling the structures, the quality factor of resonance would be tunable. Ker-Ker effects, coupled mode theories, and symetric proteced semi bound states in the continuum will be presentated. The effect could be applied in advance design in narrow-band filters, sensors, photodetectors, etc.
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