Observation of Coherently-Enhanced Tunable Narrow-Band Terahertz   Transition Radiation from a Relativistic Sub-Picosecond Electron Bunch Train
  We experimentally demonstrate the production of narrow-band ($\delta f/f \simeq20$% at $f\simeq 0.5$ THz) THz transition radiation with tunable frequency over [0.37, 0.86] THz. The radiation is produced as a train of sub-picosecond relativistic electron bunches transits at the vacuum-aluminum interface of an aluminum converter screen. We also show a possible application of modulated beams to extend the dynamical range of a popular bunch length diagnostic technique based on the spectral analysis of coherent radiation. 
P. Piot, Y.-E Sun, T. J. Maxwell, J. Ruan, A. H. Lumpkin, M. M.   Rihaoui, R. Thurman-Keup
physics.acc-ph