Bulk‐Plasmon‐Mediated Free‐Electron Radiation Beyond the Conventional Formation Time

Free‐electron radiation is a fundamental photon emission process that is induced by fast‐moving electrons interacting with optical media. Historically, it has been understood that, just like any other photon emission process, free‐electron radiation must be constrained within a finite time interval known as the “formation time,” whose concept is applicable to both Cherenkov radiation and transition radiation, the two basic mechanisms describing radiation from a bulk medium and from an interface, respectively. Here, this work reveals an alternative mechanism of free‐electron radiation far beyond the previously defined formation time. It occurs when a fast electron crosses the interface between vacuum and a plasmonic medium supporting bulk plasmons. While emitted continuously from the crossing point on the interface—thus consistent with the features of transition radiation—the extra radiation beyond the conventional formation time is supported by a long tail of bulk plasmons following the electron's trajectory deep into the plasmonic medium. Such a plasmonic tail mixes surface and bulk effects, and provides a sustained channel for electron–interface interaction. These results also settle the historical debate in Ferrell radiation, regarding whether it is a surface or bulk effect, from transition radiation or plasmonic oscillation.