Bound states in the continuum versus material losses: Ge2Sb2Te5 as an example

Photonic bound states in the continuum (BI\cyrchar\CYRS{}s) were investigated depending on material and radiation losses on the example of the phase-change chalcogenide ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ and two types of resonators in the form of a cylinder and a ring which support quasi-BIC according to the Friedrich-Wintgen mechanism. The complex refractive index for the amorphous and crystalline phases of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$ was measured experimentally. It was found that as the material losses increase, the quasi-BIC becomes practically a dark mode, while the usual resonator eigenmodes continue to remain intense. At the same time the strength of the mode coupling has been not affected, that is, the Rabi splitting remains unchanged in the studied range of losses. Photonic mode switching during the amorphous-crystalline phase transition was also investigated and two mechanisms were identified, including an efficient Fano resonance mechanism that can be used in real devices.