@article{oai:repo.qst.go.jp:00082498,
 author = {Hajima, Ryoichi and Ryoichi, Hajima},
 issue = {1},
 journal = {Atoms},
 month = {Mar},
 note = {Generation of few-cycle optical pulses in
free-electron laser (FEL) oscillators
has been experimentally demonstrated in FEL facilities
based on normal-conducting and superconducting linear accelerators.
Analytical and numerical studies have revealed that
the few-cycle FEL lasing can be explained in the frame of superradiance,
cooperative emission from self-bunched systems.
In the present paper, we review historical remarks of superradiance FEL
experiments in short-pulse FEL oscillators
with emphasizing the few-cycle pulse generation
and discuss the application of the few-cycle FEL pulses to
the scheme of FEL-HHG,
utilization of infrared FEL pulses to drive
high-harmonic generation (HHG) from gas and solid targets.
The FEL-HHG enables one to explore ultrafast science with
attosecond ultraviolet and X-ray pulses with a MHz repetition rate,
which is difficult with HHG driven by solid-state lasers.
A research program has been launched to develop technologies
for the FEL-HHG and to conduct a proof-of-concept experiment of FEL-HHG.},
 title = {Few-Cycle Infrared Pulse Evolving in FEL Oscillators and Its Application to High-Harmonic Generation for Attosecond Ultraviolet and X-ray Pulses},
 volume = {9},
 year = {2021}
}