The echo-enabled harmonic generation (EEHG) scheme holds promising prospects for efficiently generating intense coherent radiation at very high harmonics of a conventional ultraviolet seed laser. We report the lasing of the EEHG free-electron laser (FEL) at an extreme ultraviolet (EUV) wavelength with a seeded FEL facility, the Shanghai soft x-ray FEL. For the first time, we have benchmarked the basic theory of EEHG by measuring the bunching factor distributions over one octave down to the EUV region. Our results demonstrated the key advantages of the EEHG FEL, i.e., generation of very high harmonics with a small laser-induced energy spread and insensitivity to beam imperfections, and marks a great step towards fully coherent x rays with the EEHG scheme.

No abstract

It is a long-standing challenge for laser technologies to generate intense fully coherent pulses in the x-ray regime. Here, we demonstrate an external seeding mechanism, termed echo-enabled harmonic cascade (EEHC) for generating coherent and ultrashort soft x-ray pulses. The mechanism uses echo-enabled harmonic generation as the first stage, producing intense extreme ultraviolet pulses that seed the second stage x-ray free-electron laser (FEL) with the high-gain harmonic generation setup. Benefiting from the low sensitivity to electron-beam imperfections of EEHC, we generated high-power nearly transform-limited soft x-ray pulses. We have also demonstrated a unique feature of EEHC in generating isolated few-femtosecond-long x-ray pulses. The supreme up-frequency conversion efficiency and flexible pulse length control of this EEHC mechanism allow us to exceed the current limitations of seeded FELs while preserving the coherence of the seed. Our results are a step towards fully coherent and ultrashort x-ray lasers and could enable the extension of nonlinear optical techniques to shorter wavelengths.

It is very important to increase the quantum efficiency (QE) and prolong the lifetime of the photocathode in a variety of applications. We have succeeded in preparing a high QE cesium potassium antimonide (K–Cs–Sb) photocathode by K and Cs co-evaporation in the photocathode preparation facility. In order to better understand the effect of the substrate (photocathode) temperature on the photocathode performance, the photocathode preparation, photocathode performance degradation, photocathode performance recovery and photocathode removal are studied in detail.