Research ArticlePHYSICS

High-flux soft x-ray harmonic generation from ionization-shaped few-cycle laser pulses

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Science Advances  11 May 2018:
Vol. 4, no. 5, eaar3761
DOI: 10.1126/sciadv.aar3761
  • Fig. 1 Generation and characterization of water window harmonics.

    Pulses of 1.8-μm wavelength, 12-fs duration, and 550-μJ energy are focused into an 820-μm outer diameter needle filled with multiple-atmosphere pressures of gas, with a spot size of 40-μm FWHM (A) and good spatiotemporal quality (B). Differential pumping keeps the chamber pressure below 10−2 mbar. The harmonics pass through optional diagnostic optics: a 45° annular mirror (C) for reflecting the IR pulses for analysis or a movable slit (D) for selecting a slice of the harmonics for spatial phase measurement. The harmonics then pass through metallic filters and a spectrometer slit before being detected with a flat-field grating and photon-counting x-ray charge-coupled device (CCD) camera. VLS, variable line spacing.

  • Fig. 2 Tunable CEP-dependent harmonic generation across the water window.

    (A to F) Pressure dependence (A and D), CEP dependence (B and E), and spectra at two phases separated by π/2 (C and F) of harmonics emitted in neon (A to C) and helium (D to F). The pressure-dependent plots are shown averaged over all values of CEP. The CEP dependence is shown at the carbon and oxygen K-edges, using spectra generated at pressures indicated by the gray dashed lines in (A) and (D). Colored dashed lines correspond to the lineouts shown in (C) and (F), whereas the black dashed lines indicate the carbon and oxygen K-edges, respectively. All fluxes are shown in arbitrary units.

  • Fig. 3 Ionization-induced phase shift (log scale) as a function of backing pressure in the target for helium (blue) and neon (red).

    The vacuum focus is 1.4 mm behind the gas cell; experimental data (x) are compared to predictions from the full-dimensional model (solid lines) and neglecting the effect of plasma upon the propagation (dashed lines). Error bars reflect the extreme values obtained over three measurements.

  • Fig. 4 Wavefront of harmonics generated in 2 bar of helium, measured with SWORD, with spatial intensity shown as solid regions and spatial phase shown as dashed lines for seven photon energies between 300 and 450 eV.

    The 10-μm slit is positioned 128.5 mm upstream from the gas target.

  • Fig. 5 Propagation of fundamental laser.

    (A) On-axis peak intensity (red line), peak intensity neglecting ionization (red dashed line), gas density (shaded area), on-axis ionization fraction (green line), and final electron density (purple line) along the propagation direction for a backing pressure of 4 bar of helium. (B) Spatiotemporal structure of the fundamental laser field, on axis at the center of the gas target.

  • Fig. 6 Phase matching of harmonic radiation.

    (A) Theoretical harmonic spectrum as a function of backing pressure, averaged over three values of the CEP. The plot is partially saturated to reveal high pressure and energy features. (B) Buildup of on-axis harmonic radiation as a function of longitudinal position inside the gas cell considering a pulse filtered for energies about 284 eV. A backing pressure of 4 bar is used, with the vacuum focus 1.4 mm behind the target and a CEP offset of 0.3π. The inner diameter of the target spans from −0.257 to 0.257 mm.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/5/eaar3761/DC1

    section S1. Role of filamentation

    section S2. Focal position scan

    section S3. Harmonic flux

    section S4. Oxygen K-edge XANES

    section S5. Spectral phase interferometry of plasma density

    section S6. Thirty-centimeter focusing results

    section S7. Spatial wavefront characterization

    section S8. Simulation methods

    section S9. Harmonic buildup

    section S10. Overdriven limit

    fig. S1. Gas flow from thin needle target.

    fig. S2. Harmonic focal scans.

    fig. S3. Oxygen K-edge spectroscopy.

    fig. S4. Spectral interferometer for plasma characterization.

    fig. S5. Plasma-induced phase shifts.

    fig. S6. Linear-scale plasma phase shift.

    fig. S7. Harmonics generated with looser focusing.

    fig. S8. Buildup of harmonic flux.

    fig. S9. Buildup of harmonic flux.

    table S1. Harmonic flux in the water window.

    References (4154)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Role of filamentation
    • section S2. Focal position scan
    • section S3. Harmonic flux
    • section S4. Oxygen K-edge XANES
    • section S5. Spectral phase interferometry of plasma density
    • section S6. Thirty-centimeter focusing results
    • section S7. Spatial wavefront characterization
    • section S8. Simulation methods
    • section S9. Harmonic buildup
    • section S10. Overdriven limit
    • fig. S1. Gas flow from thin needle target.
    • fig. S2. Harmonic focal scans.
    • fig. S3. Oxygen K-edge spectroscopy.
    • fig. S4. Spectral interferometer for plasma characterization.
    • fig. S5. Plasma-induced phase shifts.
    • fig. S6. Linear-scale plasma phase shift in neon (red) and helium (blue).
    • fig. S7. Harmonics generated with looser focusing.
    • fig. S8. Buildup of harmonic flux.
    • fig. S9. Buildup of harmonic flux.
    • table S1. Harmonic flux in the water window.
    • References (41–54)

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