Single-shot carrier-envelope-phase-tagged ion-momentum imaging of nonsequential double ionization of argon in intense 4-fs laser fields

Single-shot carrier-envelope-phase (CEP) tagging is combined with a reaction mircoscope (REMI) to investigate CEP-dependent processes in atoms. Excellent experimental stability and data acquisition longevity are achieved. Using this approach, we study the CEP effects for nonsequential double ionization of argon in 4-fs laser fields at 750 nm and an intensity of $1.6\times {10}^{14}$ W/cm${}^2$. The Ar${}^{2+}$ ionization yield shows a pronounced CEP dependence which compares well with recent theoretical predictions employing quantitative rescattering theory [S. Micheau et al., Phys. Rev. A 79, 013417 (2009)]. Furthermore, we find strong CEP influences on the Ar${}^{2+}$ momentum spectra along the laser polarization axis.

Figure 1

Schematic of the setup consisting of a single-shot Stereo-ATI phase meter (left chamber) and a REMI spectrometer (right chamber). A beamsplitter (BS) directs light into both chambers. Each arm is equipped with its own wedges for compensating residual chirp. The phase meter contains a layer of μ metal to shield from external electric and magnetic fields. A Xe gas pressure of $6.5\times {10}^{-3}$ mbar fills the inner gas cell. In the Stereo-ATI, xenon atoms are ionized near the focus of the horizontally polarized laser beam and the electrons produced enter the high-vacuum drift tubes through slits. Electrons resulting from the ionization by a single laser shot are detected by MCP detectors to the left and to the right, enabling the determination of the CEP for each REMI event as described in the text. Simultaneously, Ar${}^{2+}$ ions created in the overlap of the laser focus and a skimmed supersonic jet of the REMI are detected by an MCP detector with a delay line anode. The jet stream is efficiently pumped by the catcher. The master trigger for the electronics is provided by a fast photodiode (PD). Note that the ion and electron optics are symmetric and a few REMI electrodes have been removed for visualization of the laser path. Furthermore, the electron detection in the REMI, which is facilitated by additional magnetic fields, was not utilized in these first proof-of-principle experiments, where only ions were detected.

Figure 2

(a) and (b) Time-of-flight traces from the left and right detectors of the Stereo-ATI for two laser shots with different CEPs. Light and dark shaded areas indicate the gated regions for determining $A_1$ and $A_2$, respectively. (c) PAP taken under the same conditions as the Ar${}^{2+}$ data (see text for further details). The color scale indicates the number of counts. (d) The relative CEP, $\phi +{\phi }_0$, as a function of the polar angle in the PAP, $\theta$. The small deviation from the line $\phi +{\phi }_0=\theta$ indicates only minor adjustments are needed for retrieving the correct relative phase.

Figure 3

Comparison of the experimental CEP-dependent Ar${}^{2+}$ yield (solid black squares) with theoretical predictions (colored lines) for three different intensities from Micheau et al. [16]. The error bars indicate the statistical error of the experimental data. The offset phase, ${\phi }_0=0.43\pi$, for the experimental data has been chosen such that the minimum in the experimental data coincides with the theory.

Figure 4

Experimental results for NSDI of argon showing (a) the CEP dependence of the Ar${}^{2+}$ longitudinal momentum along the laser polarization axis ($p_{\left|\right|}$), (b) the longitudinal Ar${}^{2+}$ momentum spectra for the CEP averaged (solid black squares) and four phases φ as indicated in the legend (integrated within $\pm 0.04\pi$ and averaged over five adjacent $p_{\left|\right|}$ values), and (c) the asymmetry parameter, $A$(φ), as defined in the text. The error bars denote the statistical error.