Destruction of symmetry protected optical bound state in the continuum by high-index substrate and roughnesses

We experimentally and theoretically analyze the role of substrate on the optical bound states in the continuum (BICs). We reveal that a high-index substrate could destroy even in-plane symmetry protected BIC due to leakage into the diffraction channels opening in the substrate. We show how two concurrent loss mechanisms scattering due to surface roughness and leakage into substrate contribute to the suppression of the resonance lifetime.