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)

High harmonic generation (HHG) at a high repetition rate requires tight focusing of the moderate peak power driving pulses. So far the conversion efficiencies that have been achieved in this regime are orders of magnitude behind the values that have been demonstrated with loose focusing of high energy (high peak power) lasers. In this contribution, we discuss the scaling laws for the main physical quantities of HHG and in particular analyze the limiting effects: dephasing, absorption and plasma defocusing. It turns out that phase-matched and absorption-limited HHG can be achieved even for very small focal spot sizes using a target gas provided with an adequately high density. Experimentally, we investigate HHG in a gas jet of argon, krypton and xenon. By analyzing the pressure dependence we are able to disentangle the dephasing and absorption effects and prove that the generated high order harmonics are phase-matched and absorption-limited. The obtained conversion efficiency is as high as 8 × 10 −6 for the 17th harmonic generated in xenon and 1.4 × 10 −6 for the 27th harmonic generated in argon. Our findings pave the way for highly efficient harmonic generation at megahertz repetition rates. New J. Phys. 16 (2014) 033022 J Rothhardt et al New J. Phys. 16 (2014) 033022 J Rothhardt et al 3 New J. Phys. 16 (2014) 033022 J Rothhardt et al New J. Phys. 16 (2014) 033022 J Rothhardt et al 5

The strong-field process of high-harmonic generation is the foundation for generating isolated attosecond pulses, which are the fastest controllable events ever induced. This coherent extreme-ultraviolet radiation has become an indispensable tool for resolving ultrafast motion in atoms and molecules. Despite numerous spectacular developments in the new field of attoscience, the low data-acquisition rates imposed by low-repetition-rate (maximum of 3 kHz) laser systems hamper the advancement of these sophisticated experiments. Consequently, the availability of high-repetition-rate sources will overcome a major obstacle in this young field. Here, we present the first megahertz-level source of extreme-ultraviolet continua with evidence of isolated attosecond pulses using a fibre laser-pumped optical parametric amplifier for high-harmonic generation at 0.6 MHz. This 200-fold increase in repetition rate will enable and promote a vast variety of new applications, such as attosecond-resolution coincidence and photoelectron spectroscopy, or even video-rate acquisition for spatially resolved pump-probe measurements

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.

We report on a few-cycle laser system delivering sub-8-fs pulses with 353 μJ pulse energy and 25 GW of peak power at up to 150 kHz repetition rate. The corresponding average output power is as high as 53 W, which represents the highest average power obtained from any few-cycle laser architecture so far. The combination of both high average and high peak power provides unique opportunities for applications. We demonstrate high harmonic generation up to the water window and record-high photon flux in the soft x-ray spectral region. This tabletop source of high-photon flux soft x rays will, for example, enable coherent diffractive imaging with sub-10-nm resolution in the near future.