Superradiant terahertz free-electron laser driven by electron microbunch trains.

Superradiance, an enhanced radiation phenomenon stemming from the phase synchronization of emitters, features a radiation intensity proportional to the number of emitters squared. The pursuit of superradiance from free electrons has long been a goal for generating intense radiation across a broad spectrum, from terahertz (THz) to the X-ray regime. However, achieving superradiance in the THz spectral range has been hindered by the lack of effective microbunching techniques. Here, we demonstrate an ultra-widely tunable superradiant THz free-electron laser (FEL) driven by high-peak-current electron microbunch trains. The emission efficiency is substantially improved as the ultra-short electron microbunches emit in phase and engage in strong interactions with the generated THz waves within the undulator. We further demonstrate that the implementation of a tapered undulator configuration leads to a two-fold enhancement in emission intensity compared to the non-tapered case, elevating the pulse energy of the narrow-band THz radiation to the millijoule level in a one-meter-long undulator. This experimental breakthrough represents a critical step toward realizing a compact, high-power, narrow-band THz source capable of fully bridging the 'THz gap' and will unlock numerous opportunities across a wide range of scientific disciplines.