Ultra-widely tunable superradiant terahertz free electron laser from electron microbunch trains

High-intensity and widely tunable sources in the terahertz (THz) frequency range are highly desirable for both fundamental and applied researches. Free electron lasers (FELs) coupling between a relativistic electron beam and a copropagating electromagnetic wave in an undulator can generate intense THz light. However, most THz FEL facilities are still working with a relatively long and low-peak-current electron beam, limiting the emission efficiency and the THz pulse energy. Here, we demonstrate a superradiant THz FEL by pre-modulating the electron beam into microbunches which emit in phase and interact strongly with the generated THz waves in a one-meter-long undulator. The measurements show that the narrow-band radiation frequency can be tuned throughout the 1 THz to 15 THz range at the fundamental harmonic of electron microbunches and up to 20 THz at the second harmonic. The detected pulse energy reaches 150 microjoules at 10 THz without consideration of the transportation loss. Such kind of THz source is stable in terms of shot-to-shot pulse intensity and frequency, and is expected to enable many novel researches.