Armin Feist, Guanhao Huang, Germaine Arend, Yujia Yang · 14 authors
Electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) are both powerful tools for the optical characterization of materials such as plasmonic and resonant dielectric nano-structures, probing electronic transitions and the local photonic density of states with nanometric spatial resolution [1]. Up to now, the correlations between electron energy-loss and photon generation have only scarcely been considered, which however has huge potential of increasing measurements sensitivity as recently demonstrated for x-ray & core-loss EELS [2]. In this work, we demonstrate two-order of magnitude contrast enhanced cavity mode imaging enabled by the generation and coincident detection of energy-shifted electrons and intracavity photons, produced in a spontaneous scattering process [3]. We combine a transmission electron microscope (TEM) with a fibre-coupled high-Q silicon nitride (Si3N4) resonator with top air cladding, enabling access to the optical near field for free electrons. The continuous electron beam at 120keV with 25nm focal diameter is passed by the ring resonator in an aloof geometry visualized in Fig. 1a. The stream of single electrons interacts with the vacuum fields of the waveguide, resulting in the generation of single photons by inelastic electron-light scattering [4,5]. In this spontaneous process the electron will lose the corresponding photon quanta of energy. We achieve coincidence detection of generated photons and energy-shifted electrons by employing a single-photon avalanche diode and an event-based electron detector behind an imaging spectrometer (cf. Fig 1a). Analysing the correlation data in time delay to the next photon event and electron energy, we observe a time-independent uncorrelated background around zero energy-loss and a strong coincidence peak at zero time-delay and an electron energy-loss of 0.8 eV, corresponding to a generated photon wavelength around 1550 nm (cf. Fig. 1b). The latter demonstrates the measurement of correlated electron-photon pairs on a single particle level. These pairs can be harnessed for correlation-enhanced imaging by enabling the separation of physical scattering events from independent noise at the detectors. In a proof of concept experiment we demonstrate correlation-enhanced mapping of an optical mode's exponential decay in vacuum. To this end, we scan the focused electron beam across the area in front of the resonator's waveguide and perform an electron-photon correlation measurement for every beam position. When analysing the electron and photon data separately, we observe the known behaviour of tracing the exponential decay up to a point where the detector counts level off at a constant background value, thereby limiting the dynamic range of the measurement (cf. Fig. 1c, blue and orange data). In contrast, the correlated events resolve the evanescent decay of the cavity field over longer distances, due to the strongly reduced background (cf. Fig. 1c, purple). Including the slight decrease in signal intensity, this corresponds to a two-order of magnitude improved dynamic range. In conclusion, we demonstrate contrast-enhanced optical cavity mode imaging, by harnessing the generation and detection of electron-photon pairs. This quantum enhanced imaging allows background-suppression in optical field measurements of nano-structures without requiring prior knowledge of the noise causing processes. Additionally, it improves the differentiation between CL pathways [6]. Moreover, the generation and detection of electron-photon pairs constitutes a crucial step towards future experiments on the quantum nature of free electron-light interactions, investigating the entanglement between photons and electrons. Correlation-enhanced cavity mode imaging. a) Experimental setup: Free electrons in a continuous electron beam traverse a fibre-coupled high-Q Si3N4 microring resonator. In a spontaneous scattering process a photon is emitted into a resonator mode, while the electron experiences energy loss. The generated photons are detected with a single photon counter and the electron energy and arrival time are analysed with an event-based electron detector behind an imaging spectrometer. Both signals are considered in a coincidence scheme, with the photon arrival time used for time tagging. b) Correlation histogram over the electron energy and relative time delay to a photon event, showing a clear coincidence peak at an energy-loss of one photon energy and zero time-delay. c) Imaging of the distant dependent exponential decay of an optical resonator mode. Every data point is acquired by integrating over a line parallel to the resonator's surface. The correlated events show a much smaller background and, thus, a higher dynamic range (DR) compared to the sole electron data and the sole photon data.