Seeding of extreme ultraviolet free electron laser with high-order harmonic

Frontiers in optical science in the short-wavelength region have greatly been expanded by the advent of intense, single-pass free-electron lasers (FELs) based on a self-amplified spontaneous emission (SASE) scheme. However, their temporal profile and frequency-domain spectra are composed of random and uncontrollable spikes, exhibiting shot-to-shot fluctuation originating from the stochastic start-up process intrinsic to spontaneous radiation. The most straightforward method for improving the temporal coherence of FEL is to inject the high-order harmonic (HH) generated by a laser into an FEL amplifier. In this paper, the authors first demonstrate the 13th harmonic (61 nm) of a Ti:Sapphire laser in the plateau region was injected as the SPring-8 Compact SASE source test accelerator.