Chiral exceptional point in optical metasurfaces induced by phase change of GST films [Invited]

Non-Hermitian (NH) systems have attracted considerable attention, and implementation in photonic systems has realized a variety of intriguing phenomena and functionalities, leading to the emergence of NH photonics. In particular, a chiral exceptional point (EP) offers a route to controlling chiral optical response without relying on intrinsically three-dimensional structures. In this study, we propose and numerically investigate a reconfigurable chiral EP in optical metasurfaces by exploiting the phase-change material Ge 2 Sb 2 Te 5 (GST). At telecommunication wavelengths, GST exhibits a pronounced contrast in the material loss (the imaginary part of the refractive index) between amorphous and crystalline phases. The amorphous GST (aGST) is nearly lossless, whereas the crystalline GST (cGST) is significantly lossy. Our system consists of coupled dipolar resonators, where a GST film is selectively loaded on only one resonator. The selective loading of the GST film introduces a controllable loss imbalance and makes the system passive parity–time ( PT ) symmetric only in the cGST state. In this case, the metasurface supports a coalesced circular-polarization eigenstate associated with a chiral EP and exhibits a PT phase transition in polarization space. Such a behavior is absent in the aGST state, where the system is effectively Hermitian. The qualitative switching between Hermitian and non-Hermitian behavior enabled by the GST phase change provides a practical design strategy for reconfigurable chiral metasurfaces based on controllable non-Hermiticity.