Abstract
Semiconducting transition‐metal dichalcogenides (TMDCs) provide a fascinating discovery platform for strong light–matter interaction effects in the visible spectrum at ambient conditions. While most of the works have focused on hybridizing excitons with resonant photonic modes of external mirrors, cavities, or nanostructures, intriguingly, TMDC flakes of subwavelength thickness can themselves act as nanocavities. Herein, the optical response of such freestanding planar waveguides of WSe2 by means of cathodoluminescence spectroscopy is determined. Strong exciton–photon interaction effects that foster long‐range propagating exciton–polaritons and enable direct imaging of the energy transfer dynamics originating from cavity‐like Fabry–Pérot resonances are revealed. Furthermore, confinement effects due to discontinuities in the flakes are demonstrated as an efficient means to tailor mode energies, spin–momentum couplings, and the exciton–photon coupling strength, as well as to promote photon‐mediated exciton–exciton interactions. The combined experimental and theoretical results provide a deeper understanding of exciton–photon self‐hybridization in semiconducting TMDCs and may pave the way to optoelectronic nanocircuits exploiting exciton–photon interaction beyond the routinely employed two‐oscillator coupling effects.
Highlights
Electron microscopy has continuously been advanced as a probe of both structural and optical properties of materials, with arguably the highest spatial, temporal, and energy resolutions.[42]
We will show that CL can probe exciton polaritons and Rabi splitting in atomically flat singlecrystalline TMDC flakes of subwavelength thicknesses
We ascertain that the presence of spontaneous coherence supported by the excitation of exciton–polaritons, provides evidence for CL spectroscopy to be able to probe Rabi oscillations and nonlinear exciton– photon interactions
Summary
The phenomenon of spontaneous collective coherence in condensed matter quantum systems, as most notably manifested by Bose–Einstein condensation, continues to be a subject of increasing interest.[1,2,3,4,5] Whereas temperature is the typical control knob to drive a system of collective excitations to its ground. We investigate the spontaneous optical properties of thin WSe2 flakes by means of cathodoluminescence (CL) spectroscopy.[40,41] Using an electron beam-based probe, we rule out the possibility of lasing as the underlying coherence mechanism. Optical modes propagating in thin films are quasi-propagating modes, transversally confined between the film boundaries and propagating along the symmetry axis of the film This effect results in strong exciton–photon coupling as manifested in a Rabi energy splitting of 0.24 eV (corresponding to a wavelength splitting of %110 nm) and the formation of exciton–polaritons whose spatial coherence and propagation mechanisms are investigated here at a high spatial resolution. Dispersion control of the optical modes via film thickness and higher-dimensional confinements, leading to excitation of edge exciton polaritons, is used to tailor the exciton–photon coupling strength. Multi-oscillator coupling physics is explored on the basis of photon-mediated exciton–exciton couplings between A and B excitons, that is, polariton–polariton interactions
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