High photon flux table-top coherent extreme-ultraviolet source

High photon flux with up to 10 12 photons in the 25–40 eV range has been achieved in a new table-top coherent extreme ultraviolet (EUV) source based on phase-matched high-harmonic generation using a fibre laser. Intense and compact EUV sources are needed for certain types of spectroscopic and imaging applications. High harmonic generation (HHG) enables extreme-ultraviolet radiation with table-top set-ups 1 . Its exceptional properties, such as coherence and (sub)-femtosecond pulse durations, have led to a diversity of applications 1 . Some of these require a high photon flux and megahertz repetition rates, for example, to avoid space charge effects in photoelectron spectroscopy 2 , 3 , 4 . To date, this has only been achieved with enhancement cavities 5 . Here, we establish a novel route towards powerful HHG sources. By achieving phase-matched HHG of a megahertz fibre laser we generate a broad plateau (25 eV–40 eV) of strong harmonics, each containing more than 1 × 10 12 photons s –1 , which constitutes an increase by more than one order of magnitude in that wavelength range 6 , 7 , 8 . The strongest harmonic (H25, 30 eV) has an average power of 143 μW (3 × 10 13 photons s –1 ). This concept will greatly advance and facilitate applications in photoelectron or coincidence spectroscopy 9 , coherent diffractive imaging 10 or (multidimensional) surface science 2 .