Challenges toward monocycle free electron lasers

Shortening the laser pulse length, ultimately toward the monocycle time scale, is one of the most important subject in the laser technology. Although this is also the case for free electron lasers (FELs), there is a fundamental and theoretical lower limit to the attainable pulse length in FELs, which comes from the so-called slippage effect intrinsic to the FEL amplification process. In this paper, a new FEL concept, which has been proposed to overcome the theoretical limit and realize monocycle FELs, is explained together with reviewing two schemes to implement the concept in the actual FEL system.