Injection of harmonics generated in gas in a free-electron laser providing intense and coherent extreme-ultraviolet light

Conventional synchrotron radiation sources enable the structure of matter to be studied at near-atomic spatial resolution and picosecond temporal resolution. Free-electron lasers promise to extend this down to femtosecond timescales. The process by which free-electron lasers amplify synchrotron light-known as self-amplified spontaneous emission(1-3) - is only partially temporally coherent, but this can be improved by seeding it with an external laser(4,5). Here we explore the use of seed light produced by high-order harmonic generation in a gas(6-9), covering wavelengths from the ultraviolet to soft X-rays. Using the SPring-8 Compact SASE Source test accelerator(10-12), we demonstrate an increase of three orders of magnitude in the intensity of the fundamental radiation at 160 nm, halving of the free-electron laser saturation length, and the generation of nonlinear harmonics(13) at 54 nm and 32 nm. The low seed level used in this demonstration suggests that nonlinear harmonic schemes should enable the generation of fully coherent soft X-rays at wavelengths down to the so-called 'water window', vital for the study of biological samples.