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)

An ultrafast laser delivering 10.4 kW average output power based on coherent combination of twelve stepindex fiber amplifiers is presented. The system emits close-to-transform-limited 254 fs pulses at 80 MHz repetition rate, has a high beam quality (M 2 ≤1.2), and a low relative intensity noise of 0.56% in the frequency range of from 1 Hz to 1 MHz. Automated spatiotemporal alignment allows for hands-off operation.

The process of high harmonic generation (HHG) enables the development of table-top sources of coherent extreme ultraviolet (XUV) light. Although these are now matured sources, they still mostly rely on bulk laser technology that limits the attainable repetition rate to the low kilohertz regime. Moreover, many of the emerging applications of such light sources (e.g., photoelectron spectroscopy and microscopy, coherent diffractive imaging, or frequency metrology in the XUV spectral region) require an increase in the repetition rate. Ideally, these sources are operated with a multi-MHz repetition rate and deliver a high photon flux simultaneously. So far, this regime has been solely addressed using passive enhancement cavities together with low energy and high repetition rate lasers. Here, a novel route with significantly reduced complexity (omitting the requirement of an external actively stabilized resonator) is demonstrated that achieves the previously mentioned demanding parameters. A krypton-filled Kagome photonic crystal fiber is used for efficient nonlinear compression of 9 mJ, 250 fs pulses leading to ,7 mJ, 31 fs pulses at 10.7 MHz repetition rate. The compressed pulses are used for HHG in a gas jet. Particular attention is devoted to achieving phase-matched (transiently) generation yielding .10 13 photons s -1 (.50 mW) at 27.7 eV. This new spatially coherent XUV source improved the photon flux by four orders of magnitude for direct multi-MHZ experiments, thus demonstrating the considerable potential of this source.