Optical superradiance from single-digit-femtosecond electron beam structure

We report measurements of superradiant optical transition radiation in the 550-800 nm range produced by ultrashort relativistic electron bunches at a dielectric boundary. In the measured optical spectra, we observe photon production with quadratic charge dependence in the visible range, consistent with optical frequency coherence determined by the longitudinal electron bunch form factor. The measured spectral envelope is reproduced by a theoretical model of coherent transition radiation (CTR), which is consistent with a sub-femtosecond longitudinal feature within the electron bunch with characteristic scale $\tau_{\mathrm{FWHM}} = 1.2~\mathrm{fs}$. These results extend CTR from the terahertz into the visible spectrum without the use of undulators or externally seeded microbunching. This superradiant boundary emission in the optical range opens a route to tunable coherent radiation from charged particle beams and provides a platform for broadband coherent light generation, enabling new opportunities for phase-sensitive optical experiments.