Light-matter coupling in semiconductors is usually tuned by changing the cavity. Adjust the mirror spacing, modify the photonic crystal, reshape the resonator — each alters the photon mode that hybridizes with the exciton. The coupling strength depends on the mode overlap, which depends on the structure. Structural changes are permanent or at best slow.
Ebel and collaborators show that electron velocity is a continuous knob.
When a free electron passes through a thin film, it generates transition radiation — light emitted at the interfaces. In a film of finite thickness, the radiation from the two surfaces interferes, producing a pattern of constructive and destructive fringes that depend on the film thickness and the electron energy. These interference resonances are the “photon modes” that couple to excitons in the film.
By changing the electron energy — which changes the effective wavelength of the transition radiation — the resonance frequencies shift. This detunes the photon mode relative to the exciton. At one electron energy, the photon mode is resonant with the exciton and strong coupling produces a Rabi splitting. At another energy, the mode is detuned and the coupling is weak. The hybridization is continuously tunable.
No structural change is needed. The film is fixed. The microscope does the tuning. The electron beam, which in cathodoluminescence spectroscopy is already the excitation source, becomes simultaneously the control parameter for the coupling strength.
The practical implication is that a single sample can be driven through the full weak-to-strong coupling crossover by sweeping the electron accelerating voltage. What previously required fabricating a series of cavities with different spacings can now be done in situ, in a single measurement, on a single nanostructure.
The new knob was always there. It just needed someone to turn it.