A key advantage of the fiber-based detector is its ability to probe the cathodoluminescence (CL) signal nonlocally and identify the specific points in the sample that launch radiation into the far field. This capability is particularly important because electron–matter interactions generate polarization that drives charge carriers, optical waves, and polaritons; these can scatter from defects and edges, creating secondary radiation pathways. As in scanning near-field optical microscopy, rigorous modeling is therefore required to trace propagation paths within the sample and quantify their contributions to the far-field spectrum.
Within the UltraCoherentCL project, we address this challenge with a dual-probe approach: the fiber scans the sample while the electron impact position is held fixed, enabling local identification of radiation origins. As a representative application, we use this method to probe exciton energy-transfer mechanisms in hBN/perovskite heterostructures. The results allow us to determine propagation lengths and to resolve emissions from networks of defects in hexagonal boron nitride (hBN) coupled to excitons in Ruddlesden–Popper perovskites. Figure 3 summarizes the main findings, including selective emissions from regions comprising pure perovskite, pure hBN, and their heterostructures. In particular, we demonstrate that perovskite excitons couple to hBN defects and hop across the defect network. This mechanism is highly efficient due to Coulomb interactions among defects, enabling energy transfer over distances up to 142 µm in extruded regions of hBN. These findings are reported in arXiv:2504.12024 currently under review.
