Self-hybridized exciton polaritons in layered semiconductors emerge from the hybridization of excitonic resonances with confined photonic modes in films and provide a versatile platform for controlling light--matter interactions at the nanoscale. Yet, the nature of exciton--photon interactions and the collective and synchronous strong coupling of multiple excitons to the same photonic mode remain largely unexplored. Here, we use complementary momentum-resolved photoluminescence and cathodoluminescence spectroscopy to compare self-hybridized exciton polaritons in Ruddlesden--Popper perovskite flakes under optical and electron-beam excitation. We find that cathodoluminescence exhibits remarkably larger polaritonic level repulsion than photoluminescence, revealing an enhanced effective coupling strength under electron-beam excitation. We attribute this enhancement to the collective coherent excitation of multiple excitons by fast electrons, which couple to the same photonic mode and increase the interaction strength according to a gN=g1N scaling, with an effective number of up to N≈23 coherently coupled excitonic oscillators. By reducing the electron kinetic energy, we further uncover a crossover to a partially incoherent excitation regime in which higher-order Fabry--Pérot exciton-polariton resonances disappear, leaving only the lowest polariton branches while coherent transition radiation persists. This transition arises because low-energy electrons excite excitons over extended spatial volumes, washing out the phase coherence required to sustain collective coupling and form polaritonic resonances. Our work shows that electron beams can actively modify collective light--matter coupling in layered semiconductors and establishes cathodoluminescence as a route to access excitation regimes beyond all-optical excitation schemes.
Excitonic nonlinearity in wide-gap semiconductors has a strong possibility that we can realize efficient ultrafast optical interactions. Recently, we have observed very large third-order nonlinear phase shift in ZnSe layers grown by MBE on GaAs substrate [1], The third-order nonlinear susceptibility at exciton resonance is more than 10–2 esu at 10 K. Because of the biaxial strain, the valence band splits into a heavy- and a light-hole branches at Γ point [2]. To clarify the intrinsic nature of excitonic nonlinearity, the materials which are free from such strain are preferable. In this report, we present our experiments on excitonic nonlinearity using ZnSe homoepitaxial layers which are free from the strain effects [3], and discuss the origin of enhanced nonlinearity at exciton resonance in ZnSe.
The coherent excitation of a Wannier-exciton system by a short resonant light pulse is described through a simple model that takes into account explicitly the phase-space-filling effects due to the fermion nature of the electrons and holes composing the excitons. This formalism can describe both coherent-transient experiments, such as photon echoes, and optical observations at time scales longer than dephasing, such as absorption saturation. At high excitation densities the optical response of hydrogenic excitons deviates significantly from the response of the two-level model used traditionally to describe resonant effects. This suggests that coherent-optical-transient experiments, such as photon echoes, may be used to probe exciton interactions due to their internal structure, in addition to the relaxation processes that they probe in traditional experimental arrangements.