Periodic Reporting for period 1 - UltraCoherentCL (Ultrafast Cathodoluminescence Spectroscopy with Coherent Electron-Driven Photon Sources)
Reporting period: 2024-02-01 to 2025-07-31
The project aims at providing the first prototype of an instrument to probe optical excitations at the nanoscale using cathodoluminescence spectroscopy. We particularly aim to develop a fiber-based cathodoluminescence detectors and collector to analyse the propagation dynamics of polaritonic systems and hybrid materials system, and combine it with electron-driven photon sources to explore coherence dynamics.
Instead of using parabolic mirrors above the structures to gather the entire photons from the interaction of electron beams with structures, as typically is done, we develop a fiber collector with a 3D-printed lens on its end, to enhance the numerical aperture and provide cite selectivity in determining the location from which the emission occurs. Both electron beam impact position and the relative position of the fiber with the electron beam over the sample are controlled with nanometer precision, allowing us to fully determine the spatial profile of the emission as well as the position of defects and discontinuities in the sample.
The goal has been to apply this instrument to different materials systems to examine the suitability of this approach for exploring different metallic and semiconducting materials systems, as well as its exploitation in nanophotonic devices.
Instead of using parabolic mirrors above the structures to gather the entire photons from the interaction of electron beams with structures, as typically is done, we develop a fiber collector with a 3D-printed lens on its end, to enhance the numerical aperture and provide cite selectivity in determining the location from which the emission occurs. Both electron beam impact position and the relative position of the fiber with the electron beam over the sample are controlled with nanometer precision, allowing us to fully determine the spatial profile of the emission as well as the position of defects and discontinuities in the sample.
The goal has been to apply this instrument to different materials systems to examine the suitability of this approach for exploring different metallic and semiconducting materials systems, as well as its exploitation in nanophotonic devices.
The fiber-based cathodoluminescence (CL) detector was realized within the first six months of the project. The fiber holder is mounted on a three-axis piezo stage that precisely positions the fiber collector with respect to the sample stage (Fig. 1). On the analysis path, we employ two measurement scenarios: (1) an intensity correlator—comprising an interferometer and two single-photon detectors—to measure the second-order correlation
g(2)(τ), and (2) a spectrometer for recording emission spectra. All optical setups on the analysis side were realized within the UltraCoherentCL ERC grant during the first project year. Details of the setup are available in arXiv:2501.17723 which is currently under review.
The system has also been adapted to include a second piezo stage that holds the electron-driven photon source above the sample. This configuration has been successfully installed (Fig. 2), and several measurements have been performed to enable comparison with our previously employed mirror-based CL system using a similar electron-driven photon source. Related results have been published in Nature Communications 16, 2326 (2025) and Nature Physics 19(6), 869–876 (2023). Data analysis—and the corresponding manuscript on the fiber-based CL measurements—are in progress.
g(2)(τ), and (2) a spectrometer for recording emission spectra. All optical setups on the analysis side were realized within the UltraCoherentCL ERC grant during the first project year. Details of the setup are available in arXiv:2501.17723 which is currently under review.
The system has also been adapted to include a second piezo stage that holds the electron-driven photon source above the sample. This configuration has been successfully installed (Fig. 2), and several measurements have been performed to enable comparison with our previously employed mirror-based CL system using a similar electron-driven photon source. Related results have been published in Nature Communications 16, 2326 (2025) and Nature Physics 19(6), 869–876 (2023). Data analysis—and the corresponding manuscript on the fiber-based CL measurements—are in progress.
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.
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.