A new first-principles approach provides a unified foundation for studying complex band structure and light confinement in periodic media Representation of the mathematics behind complex band structure and light confinement in periodic media (Credit: Dezhuan Han) Photonic crystal slabs are periodic structures that confine light in two dimensions while allowing it to leak in the third. Their in‑plane periodicity forces light to behave like an electron in a crystal, forming bands rather than isolated modes. These objects can host an array of novel physical phenomena, from ultra‑sharp resonances to exotic singularities such as exceptional points. Among the most intriguing are bound states in the continuum (BICs). These are modes that, despite lying in an energy range where radiation is allowed, remain perfectly confined. In a new theoretical study, a team of researchers from China showed that this leakage, and its surprising absence in certain cases, can be understood from a single first‑principles viewpoint.…