Smith–Purcell_effect
The Smith–Purcell effect was the precursor of the free-electron laser (FEL). It was studied by Steve Smith, a graduate student under the guidance of Edward Purcell. In their experiment, they sent an energetic beam of electrons very closely parallel to the surface of a ruled optical diffraction grating, and thereby generated visible light.[1] Smith showed there was negligible effect on the trajectory of the inducing electrons. Essentially, this is a form of Cherenkov radiation where the phase velocity of the light has been altered by the periodic grating. However, unlike Cherenkov radiation, there is no minimum or threshold particle velocity.
Smith–Purcell radiation is particularly attractive for applications involving non-destructive beam diagnostics (bunch-length diagnostics in accelerators for example) and especially as a viable THz radiation source, which has further broad-range uses in diverse and high-impact fields like materials sciences, biotechnology, security and communications, manufacturing and medicine. Operating at THz frequencies also allows for potentially large accelerating gradients (~10s GeV/m[2]) to be realised. This, paired with plasma-wakefield acceleration methods under development and linear accelerator (linac) technology, could pave the way to next-generation, compact (and hence cheaper), less prone to RF breakdown (current limits for surface E fields are of the order of 10s-100 MV/m[3]), high energy output linacs.