Bright high-repetition-rate source of narrowband extreme-ultraviolet harmonics beyond 22 eV

Abstract

Novel table-top sources of extreme-ultraviolet light based on high-harmonic generation yield unique insight into the fundamental properties of molecules, nanomaterials or correlated solids, and enable advanced applications in imaging or metrology. Extending high-harmonic generation to high repetition rates portends great experimental benefits, yet efficient extreme-ultraviolet conversion of correspondingly weak driving pulses is challenging. Here, we demonstrate a highly-efficient source of femtosecond extreme-ultraviolet pulses at 50-kHz repetition rate, utilizing the ultraviolet second-harmonic focused tightly into Kr gas. In this cascaded scheme, a photon flux beyond ≈3 × 10(13) s(-1) is generated at 22.3 eV, with 5 × 10(-5) conversion efficiency that surpasses similar harmonics directly driven by the fundamental by two orders-of-magnitude. The enhancement arises from both wavelength scaling of the atomic dipole and improved spatio-temporal phase matching, confirmed by simulations. Spectral isolation of a single 72-meV-wide harmonic renders this bright, 50-kHz extreme-ultraviolet source a powerful tool for ultrafast photoemission, nanoscale imaging and other applications.

Figure 1. Efficient high-repetition-rate source of extreme-ultraviolet (XUV) pulses.

(a) Scheme for two-stage high-harmonic generation, starting from 120 μJ near-infrared pulses at 50-kHz repetition rate, which in the first step are frequency doubled to 390 nm wavelength in BBO. These ultraviolet (UV) pulses are subsequently focused sharply onto a thin column of Krypton gas to initiate high-harmonic generation. The resulting XUV light is filtered with thin metal foils, followed by photon flux and spectral characterization with a calibrated XUV photodiode and grating spectrometer. (b) Intensity-normalized CCD images of the spectrally dispersed q-th XUV harmonics, generated by either the UV pulses or the near-infrared fundamental at their respective driving wavelength λ₀. (c) Scaling of XUV intensity with Kr gas pressure, for the brightest UV-driven harmonic at 22.3 eV.

Figure 2. XUV spectra and isolation of a single harmonic.

(a) Spectra of the XUV harmonics (vertical CCD lineouts) driven either by the 48-μJ ultraviolet (UV) pulses or by the 120-μJ pulses from the near-infrared (IR) laser fundamental. Spectra are measured for a Kr gas pressure of 60 Torr, and are shown for the same integration time of 4 s and with CCD background noise subtracted. For the ultraviolet-driven harmonics, the spectrum was corrected for the effect of one additional Al filter, inserted to avoid CCD saturation. (b) Emission profile of the 7th harmonic at 22.25 eV with corresponding line width (FWHM). (c) Isolated single harmonics (solid lines) after absorptive spectral filtering with thin metal foils. The 7th-harmonic is isolated via combined Sn and Al filters of 300-nm thickness each, whereas the 5th-harmonic is selected by an In foil in combination with Brewster reflection from two Si plates to suppress the residual laser beam. For comparison, the theoretical transmission is shown for Sn (dashed line) and In (dotted line) of 300-nm thickness.

Figure 3. HHG phase matching simulations in the tight focus geometry.

The calculations are for a 75-fs FWHM driving pulse with 1.8·10¹⁴ W cm⁻² peak intensity, Rayleigh length of z_R=1 mm and with the gas volume centred at z=z_R/2 after the focus. (a) Driving pulse intensity. (b,c) Coherence length π/|Δk|, mapped as function of time and Kr gas pressure, comparing the phase-matching at similar XUV photon energies of the near-infrared (NIR)-driven 15th and ultraviolet (UV)-driven 7th harmonic. Plots are shown at 200 μm before the gas volume exit, comparable to the XUV absorption length at 50 Torr. The Kr ionization level is shown for comparison (blue line) as calculated with the YI model. (d,e) Resulting XUV photon flux enhancement, obtained by spatially integrating the HHG emission across the interaction volume (see Supplementary Methods) for the two cases, with the single-atom efficiency β_S omitted. The flux enhancement is mapped as a function of gas pressure and emission time within the driving pulse. (f) Pressure dependence of the UV-driven XUV emission (dotted line), obtained by integrating the data in e across the pulse. The ratio of the time-integrated phase-matching enhancements ξ_UV/ξ_IR is shown for the above model (dashed line), and for better comparison with experiment using a 50-fs NIR driving pulse (solid line).