High Harmonic XUV Spectral Phase Interferometry for Direct Electric-Field Reconstruction

We demonstrate the first experimental complete temporal characterization of high-harmonic XUV pulses by spectral phase interferometry, with an all-optical setup. This method allows us to perform single-shot measurements of the harmonic temporal profile and phase, revealing a remarkable shot-to-shot stability. We characterize harmonics generated in argon by a 50 fs 800 nm laser pulse. The 11th harmonic is found to be 22 fs long with a negative chirp rate of $-4.8\times {10}^{27}{\mathrm{s}}^{-2}$. This duration can be reduced to 13 fs by modulating the polarization of the generating laser. The technique is easy to implement and could be routinely used in femtosecond XUV pump-probe experiments with harmonics.

Figure 1

Principle of the HHSPIDER measurement (see text).

Figure 2

(a) Harmonic 11 spectrum generated by two identical IR pulses with intensity $I_{\mathrm{IR}}=9\times {10}^{13}\mathrm{W}·{\mathrm{cm}}^{-2}$, delayed by $\tau =150\mathrm{fs}$. (b) Harmonic 11 spectra, generated by an IR pulse at ${\omega }_0$ (solid line), and ${\omega }_0-\delta \omega$ (dashed line). Inset: superposition of the two spectra after translation by $\Omega =4.2\times {10}^{13}\mathrm{rad}·{\mathrm{s}}^{-1}$.

Figure 3

(a) SPIDER spectrum of harmonic 11 generated at $I_{\mathrm{IR}}=9\times {10}^{13}\mathrm{W}·{\mathrm{cm}}^{-2}$, obtained with $\tau =150\mathrm{fs}$ and $\Omega =4.2\times {10}^{13}\mathrm{rad}·{\mathrm{s}}^{-1}$. (b) Harmonic 11 spectrum (solid line) and spectral phase (dashed line) retrieved from the SPIDER measurement. (c) Temporal intensity (gray filled) and phase (dashed line) obtained by Fourier transform of the complex spectrum. The gray solid line shows the generating IR pulse intensity obtained by conventional SPIDER measurement.

Figure 4

Temporal profiles from 10 successive single-shot HHSPIDER measurements of harmonic 11.

Figure 5

Temporal confinement of harmonic emission by polarization gating. The harmonic 11 temporal intensity (solid line) and phase (dashed line) is measured without (gray) and with (black) temporal gating.