The properties of wavefunctions, such as the ones encoded in the Berry curvature, play a fundamental role in topological photonics. In open photonic systems, where light can radiate into the far field, an intriguing question arises: to what extent can the topology of bulk modes be inferred from the emitted light?

In this work, we employ an effective non-Hermitian model that captures both guided and radiative modes in photonic crystal slabs with diffractive and radiative couplings. Within this framework, we calculate the radiation Berry curvature — a topological quantity defined directly from the far-field emission — and systematically compare it to the conventional bulk Berry curvature from Bloch states.

We show that bulk and radiation Berry curvatures do not always coincide. In fact, their correspondence is fundamentally non-universal and can break down in the presence of polarization singularities such as bound states in the continuum!

Read more on Physical Review Research.