Polarization tagging of two-photon double ionization by elliptically polarized XUV pulses

We explore the influence of elliptical polarization on the (non)sequential two-photon double ionization of atomic helium with ultrashort extreme ultraviolet (XUV) light fields using time-dependent full ab initio simulations. The energy and angular distributions of photoelectrons are found to be strongly dependent on the ellipticity. The correlation minimum in the joint angular distribution becomes more prominently visible with increasing ellipticity. In a pump-probe sequence of two subsequent XUV pulses with varying ellipticities, polarization tagging allows us to discriminate between sequential and nonsequential photoionization. This clear separation demonstrates the potential of elliptically polarized XUV fields for improved control of electronic emission processes.

Figure 1

(a) Joint angular distribution $P({\theta }_1={\theta }_2={90}^{\circ },{\varphi }_1=0^{\circ },{\varphi }_2)$ of the ejected electrons for a pulse duration $T_p=150$ as at $\hslash \omega =70$ eV for different ellipticities $\epsilon$. The innermost line (pink) corresponds to the previously investigated case of linear polarization along $\widehat{x}$ ($\epsilon =0$), whereas the successive outer curves correspond to $\epsilon =0.4$, 0.6, 0.7, 0.8, 0.9, and 1 (circular polarization). All distributions are normalized to a maximum value of 1. (b) Ratio of probability for emission of the two electrons with a relative angle of ${90}^{\circ }$ to the probability for emission into the opposite direction (back-to-back) for the different ellipticities shown in (a). (c) Sketch of a laser field with $\epsilon =1$ [blue (dark grey)] and 0 [pink (light grey)].

Figure 2

(a) Joint angular distributions $P({\theta }_1={\theta }_2={90}^{\circ },{\varphi }_1=0^{\circ },{\varphi }_2)$ of the ejected electrons for different pulse durations at $\hslash \omega =70$ eV. The innermost line (blue) is for $T_p=75$ as, with successive outer lines for $T_p=150$, 300, 500, 1000, 3000 as. The dashed line is the unit circle. All distributions are normalized to the maximum value of the angular distribution at ${\varphi }_2={180}^{\circ }$. (b) Ratio of emission of the two electrons into the same (side-by-side) and into the opposite (back-to-back) direction for the different pulse durations shown in (a).

Figure 3

Double-ionization spectrum $P(E_1,E_2)$ in atomic units (log scale) as a function of $E_1$ and $E_2$ for pulse sequences with different ellipticities. The corresponding total two-photon double-ionization probabilities are (a) $3.4\times {10}^{-8}$, (b) $4.6\times {10}^{-8}$, (c) $2.8\times {10}^{-8}$, and (d) $2.8\times {10}^{-8}$. The central photon energy of both pulses is 65 eV, their duration is $T_p=1$ fs and the pulse delay $\tau =2.5$ fs. The peak intensity in $\widehat{x}$ direction is ${10}^{12}$ W/${\mathrm{cm}}^2$.

Figure 4

Energy-delay time diagram of the interfering pathways in a pump-probe sequence. The shaded areas signify the enclosed interference phases giving rise to the stripes (striped area) and to the checkerboard pattern [red (dark grey), yellow (light grey) shaded area and striped area] in Fig. 3. For details see text.

Figure 5

(a) $P\left(\mathrm{\Delta }E\right)$ for a pump-probe sequence with ${\epsilon }_1=1$ ($L$), ${\epsilon }_2=-$ 1 ($R$) (see inset). $P\left(\mathrm{\Delta }E\right)$ is resolved for the three different $M$ values of the final state. (b) $P\left(\mathrm{\Delta }E\right)$ for a pump-probe sequence with two left-circularly polarized pulses (${\epsilon }_1={\epsilon }_2=1$, see inset) either or one left- and one right-circularly polarized pulse (${\epsilon }_1=1$, ${\epsilon }_2=-$ 1). The dashed-dotted line shows the difference of the two sequences. The central photon energy of the pulses is 65 eV, their duration is $T_p=1$ fs, and their delay is $\tau =2.5$ fs. The peak intensity in $\widehat{x}$ direction is ${10}^{12}$ W/${\mathrm{cm}}^2$.

Figure 6

Joint angular distributions $P({\theta }_1={\theta }_2={90}^{\circ },{\varphi }_1=0^{\circ },{\varphi }_2)$ of the ejected electrons for an XUV pulse with polarization ${\epsilon }_1=1$ followed by an XUV pulse with (a) ${\epsilon }_2=1$ or (b) ${\epsilon }_2=-1$. (c) same as (b) but for ${\epsilon }_1=0.9$ and ${\epsilon }_2=-0.9$. All pulses have photon energy $\hslash \omega =65$ eV and a pulse duration of $T_p=1$ fs. The different lines correspond to different values of $M$ of the final state. All distributions are normalized to the maximum value of the total probability distribution shown in (a) (summed over all $M$).