Publication date: 8th June 2021
Semiconducting transition metal dichalcogenides (TMDC) combined with photonic cavities, exhibit strong light-matter interactions, leading to the emergence of new quasi particles [1], namely exciton-polaritons (EP). EPs have been extensively studied using near-field techniques such as SNOM and EELS due to their fascinating fundamental properties [2-3]. In this work, we used cathodoluminescence (CL) spectroscopy for the first time to probe EPs and stemming spatial correlations in atomically-flat TMDCs such as Wse2 [4]. Being based on spontaneous interactions, the resolved spatial interference maps are the direct proof of the spontaneous coherences associated with propagating exciton polaritons.
Here, we demonstrate that the transversal one-dimensional optical confinement within the thin film and the propagation of the optical waves along the longitudinal orientation, allow for strong exciton-photon couplings leading to an energy split and spatially-resolved interference patterns, associated with propagating optical modes. In particular the combination of both aspects confirm the excitation of exciton polaritons. Experimental results followed by numerical simulations, provide deep inside into the electron-photon interaction mechanisms inside the specimen.
CL radiation collected from thin WSe2 flakes demonstrate a wavelength splitting on the order of 100 nm to 250 nm, depending on the thickness of the film, comparable to the predicted spectral and spatial maps. A deep at the A exciton wavelength of 751 nm and broad peaks associated with lower and upper polariton branches were observed, respectively; below and above the A exciton. Numerous interference fringes in the wavelength-distance CL map are clear signature of the spontaneous coherence caused by the excitation of EPs. Spatial interference fringes up to several orders confined along the edges revealed the excitation of edge exciton polaritons. Our results demonstrate that CL spectroscopy can be used to probe coherent optical modes of semiconducting van der Waals materials.
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program, Grant Agreement No. 802130 (Kiel, NanoBeam) and Grant Agreement No. 101017720 (EBEAM). Financial support from Deutsche Forschungsgemeinschaft under the Art. 91 b GG Grant Agreement No. 447330010 and Grant Agreement No. 440395346 is acknowledged.
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