Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources

OPEN Light: Science & Applications (2015) 4, e320; doi:10.1038/lsa.2015.93 ß 2015 CIOMP. All rights reserved 2047-7538/15 www.nature.com/lsa ORIGINAL ARTICLE Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources Steffen Hädrich1,2, Manuel Krebs1, Armin Hoffmann1, Arno Klenke1,2, Jan Rothhardt1,2, Jens Limpert1,2 and Andreas Tünnermann1,2,3 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 .1013 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. Light: Science & Applications (2015) 4, e320; doi:10.1038/lsa.2015.93; published online 28 August 2015 Keywords: coherent extreme ultraviolet sources; high average power; high harmonic generation; nonlinear compression; ultrafast laser INTRODUCTION High harmonic generation (HHG) offers an elegant approach to achieve table-top sources of coherent extreme ultraviolet (XUV) or even soft X-ray radiation1. Although this process has been known for almost 30 years, it is still a subject of intense research because of the ever-increasing number of possible applications. Moreover, these sources are an alternative to large-scale facilities, such as free electron lasers or synchrotrons, which provide limited user access. All of these sources are used in diverse fields, e.g., in physics, biology, chemistry, material science, and others. This application-oriented approach has defined two major challenges in the field of HHG, which involved either increasing the energy per XUV pulse or the repetition rate2,3. The latter challenge is particularly important for photoelectron spectroscopy and microscopy in which space-charge effects need to be avoided3,4. Other possible applications are coherent diffractive imaging5,6, coincidence experiments7, or frequency metrology, in which multi-10 MHz lasers with a stabilized frequency comb are used8. Generally, there is a great demand for compact, cost-effective and reliable sources that can also be used by scientists who are not laser experts, which would severely broaden their applications. HHG relies on the interaction of a high-intensity (1013–1015 W cm22) ultrashort laser pulse with noble gases. The efficiency of the HHG process critically depends on the possibility of achieving phase-matching, i.e., matching the phase velocity of the driving laser field and the generated XUV field9. The phase-mismatch is determined by the dispersion of the neutral atoms and free electrons, the intrinsic phase due to the propagation of the free electron wave-packet as well as a geometrical term, which is caused by the Gouy phase-shift10,11. The dispersion terms (neutral atoms, free electrons) have opposite signs and are density dependent. Therefore, phase-matching can be transiently achieved (i.e., for a certain instance of time of the laser pulse) by adjusting the pressure of the generation gas to balance the terms. However, this approach only works when the ionization of the gas medium is maintained below a so-called critical ionization, which is a few percent depending on the gas species and harmonic order1,9. This critical ionization level restricts the intensity that can be used for HHG to a maximum value that significantly increases with decreasing pulse duration. Consequently, ultrashort pulses (,10 cycles) will allow the use of a higher intensity, which in turn increases the single atom response while still achieving phase-matching and high efficiencies (.1026)1. Therefore, the Ti:Sapphire-based laser systems, due to their ultrashort pulse duration, are still considered the work horse in this field1. However, due to the thermo-optical constraints, the repetition rate does not exceed a few kilohertz (a few Watt of average power). 1 Friedrich-Schiller-Universität Jena, Institute of Applied Physics, Abbe Center of Photonics, Albert-Einstein-Straße 15, 07745 Jena, Germany; 2Helmholtz-Institute Jena, Fröbelstieg 3, 07743 Jena, Germany and 3Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Straße 7, 07745 Jena, Germany Correspondence: Steffen Hädrich, Email: Received 28 October 2014; revised 7 April 2015; accepted 8 April 2015; accepted article preview online 13 April 2015 Towards turn-key coherent XUV sources S Hädrich et al 2 Light: Science & Applications improvement of four orders of magnitude for a directly driven multiMHz repetition rate HHG28,29, and it provides a photon flux that is higher than that delivered by standard kHz laser systems30 and intracavity HHG within that wavelength range8,24,31. The use of a lowenergy driving laser with a compact nonlinear compression setup not only has the potential to significantly reduce the complexity of high repetition rate HHG systems but will also enable efficient HHG at 10 MHz repetition rate, a regime so far only addressable with enhancement cavities. MATERIALS AND METHODS The experimental setup of the nonlinear compression stage followed by a HHG experiment is shown in Figure 1. The front end is a fiber chirped pulse amplification system that optionally allows for the coherent combination of up to four main amplifier channels (CCFCPA)32. This system is chosen because of its short pulse duration of 250 fs, which also enables shorter pulses after the nonlinear compression. It is important to emphasize that only 8–9 mJ of pulse energy is used at up to 10.7 MHz repetition rate (90 W average power). Therefore, this system, which was used due to its availability, can be easily replaced by compact, turn-key fiber laser systems33,34 or with stateof-the-art thin-disk oscilla (...truncated)