High photon flux table-top coherent extreme-ultraviolet source

Abstract

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)

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

“…Basically, they relate important quantities, such as the HHG medium length and gas density, for different focusing geometries with the assumption that the same peak intensity, pulse duration, and central wavelength are used. Following these guidelines and using the recently demonstrated efficient HHG of post-compressed fiber lasers as a basis 21 , a 65 mm nozzle size and xenon gas is chosen. Experimentally, the position of the gas nozzle relative to the focus and the backing pressure are optimized for maximum signal.…”

“…Additionally, we consider the residual absorption of the harmonics signal on its way to the detector as described in the supplementary information of the Ref. 21. The required ionization rates are calculated with the established model of Ammosov-DeloneKrainov (ADK) 45 .…”

“…21. This includes the diffraction efficiency of the spectrometer grating, the characteristics of the detector (CCD, characterized by the manufacturer) and the measured filter transmission (see Supplementary material).…”

“…Additionally, the use of fiber-chirped pulse amplifiers in combination with nonlinear compression progressively increases the photon flux obtained via HHG. More than 3 3 10 13 photons s -1 per harmonic (140 mW) at 30 eV 21 and more than 3 3 10 9 photons s -1 within a 1% bandwidth at 120 eV 22 have been generated recently, which can now be considered the highest photon flux HHG sources in this spectral range. The repetition rate in those experiments was 600 kHz and 100 kHz, respectively, but a further increase to the multi-MHz level is required to address the previously mentioned applications.…”

Generation of Bright, Spatially Coherent Soft X-Ray High Harmonics in a Hollow Waveguide Using Two-Color Synthesized Laser Pulses

“…We identify the optimal conditions for generating best-quality harmonics and uncover the underlying mechanism of dynamic phase matching. We show that the best phase-matched harmonics from the extreme ultraviolet (XUV) to soft x rays have low divergences (smaller than 1 mrad), which can be compared to vacuum ultraviolet (VUV) or XUV harmonics generated by traditional 800-nm lasers [31][32][33][34][35][36]. In comparison, soft x-ray harmonics generated with a midinfrared laser alone reported the full divergence angle of about 7 mrad [37] or 4 to 8 mrad [38] (half divergence angle should be compared to 1 mrad).…”