High photon flux table-top coherent extreme-ultraviolet source

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(2-4). 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 x 10(12) photons s(-1), which constitutes an increase by more than one order of magnitude in that wavelength range(6-8). The strongest harmonic (H25, 30 eV) has an average power of 143 mu W (3x10(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).