Few-Cycle Infrared Pulse Evolving in FEL Oscillators and Its Application to High-Harmonic Generation for Attosecond Ultraviolet and X-ray Pulses

Abstract

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 emphasis on 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.

Full characterization of superradiant pulses generated from a free-electron laser oscillator

“…They confirmed the N e 2 dependence of the peak power in an FEL oscillator, FELIX. There have been several analytical and numerical studies on SR in FEL oscillators to discuss few-cycle pulse generation with BC ringing 27 , 29 – 31 . Measurements of SR pulses in FEL oscillators have also been conducted but are limited to interpretation of autocorrelation waveforms without phase recovery 28 , 32 – 34 or to reconstruction of a pulse with coarse resolution 35 .…”

Three-Dimensional, Time-Dependent Analysis of High- and Low-Q Free-Electron Laser Oscillators

“…In particular, dynamic cavity desynchronization [87] can be used to first set a cavity length corresponding to maximum gain for a quick initial growth of the optical pulse energy and then switched to a cavity length for maximum extraction efficiency that is obtained at small cavity desynchronization. The latter is typically also associated with the generation of short optical pulses [19,88,92,[129][130][131][132][133][134]. For short pulse FEL oscillators, i.e., where the root-mean-square (rms) width of the electron bunch is smaller than the total slippage distance, the oscillator is operating in the superradiant regime [132,[134][135][136][137][138][139][140][141][142]].…”

“…The latter is typically also associated with the generation of short optical pulses [19,88,92,[129][130][131][132][133][134]. For short pulse FEL oscillators, i.e., where the root-mean-square (rms) width of the electron bunch is smaller than the total slippage distance, the oscillator is operating in the superradiant regime [132,[134][135][136][137][138][139][140][141][142]]. An analytic theory for this regime predicts a scaling of the optical pulse energy E L with the bunch charge q as E L ∝ q 3/2 and a scaling of the temporal width τ L of the optical pulse as τ L ∝ q −1/2 [138], which has been experimentally confirmed [139], although higher extraction efficiencies have also been observed for a perfectly synchronized cavity length, where the analytic theory breaks down [92,132].…”

Three-dimensional, time-dependent analysis of high- and low-Q free-electron laser oscillators

“…In particular, dynamic cavity desynchronization [86] can be used to first set a cavity length corresponding to maximum gain for a quick initial growth of the optical pulse energy and then switched to a cavity length for maximum extraction efficiency that is obtained at small cavity desynchronization. The latter is typically also associated with the generation of short optical pulses [18,87,91,[121][122][123][124][125][126]. For short pulse FEL oscillators, i.e, where the root-mean-square (rms) width of the electron bunch is smaller than the total slippage distance, the oscillator is operating in the superradiant regime [124,[126][127][128][129][130][131][132][133].…”

“…The latter is typically also associated with the generation of short optical pulses [18,87,91,[121][122][123][124][125][126]. For short pulse FEL oscillators, i.e, where the root-mean-square (rms) width of the electron bunch is smaller than the total slippage distance, the oscillator is operating in the superradiant regime [124,[126][127][128][129][130][131][132][133]. An analytic theory for this regime predicts a scaling of the optical pulse energy 𝐸 𝐿 with the bunch charge 𝑞 as 𝐸 𝐿 ∝ 𝑞 3/2 and a scaling of the temporal width 𝜏 𝐿 of the optical pulse as 𝜏 𝐿 ∝ 𝑞 −1/2 [129] which has been experimentally confirmed [130], although higher extraction efficiencies have also been observed for a perfectly synchronized cavity length where the analytic theory brakes down [91,124].…”