Speaker
Description
Free electrons are powerful nanoscale probes of light–matter interactions, providing both high spatial and temporal resolution. Recent advances in time-resolved cathodoluminescence (CL) spectroscopy have enabled the study of coherent ultrafast dynamics in quantum emitters. In addition, electron-beam excitation of defects can lead to controllable nonclassical effects, such as photon antibunching, in CL emission. These developments highlight the role of free electrons both in generating quantum states of light and in probing quantum coherence in materials.
In this contribution, we discuss theoretical models describing the interaction of free electrons with quantum systems that support recent experimental findings. For quantum optical phenomena such as photon antibunching (or bunching) in incoherent CL, the interaction can be modeled using a master-equation approach, treating the electron-beam excitation as an incoherent broadband field source. Interestingly, we predict the possibility of quantum interference effects in incoherent CL emission from multilevel systems driven by free electrons. Interference induced between different relaxation pathways can lead to either enhancement or suppression of the cathodoluminescence signal.
Quantum interference effects in CL can also arise from the coherent interaction between a free electron and a quantum emitter, as recently demonstrated using electron-driven photon sources. In this regime, a fully quantum, time-dependent theory is required. We show that a free electron can induce transient coherent oscillations in the emitter populations when the quantum emitter is initially prepared in a coherent superposition state. These coherence effects are reflected in both the electron energy-loss spectroscopy (EELS) and CL spectra. The initial coherence of the emitter manifests as Ramsey-like fringes in the coherent CL radiation, demonstrating the ability of free electrons to probe quantum coherence at the nanoscale.