Above-threshold ionization in neon produced by combining optical and bichromatic XUV femtosecond laser pulses

We consider the ionization of neon induced by a femtosecond laser pulse composed of overlapping, linearly polarized bichromatic extreme ultraviolet and infrared fields. In particular, we study the effects of infrared light on a two-pathway ionization scheme for which Ne $2s^{2}2p^{5}3s{}^{1}P$ is used as the intermediate state. Using time-dependent calculations, supported by a theoretical approach based on the strong-field approximation, we analyze the ionization probability and the photoelectron angular distributions associated with the different sidebands of the ionization spectrum. Complex oscillations of the angular distribution anisotropy parameters as a function of the infrared light intensity are revealed. Finally, we demonstrate that coherent control of the asymmetry is achievable by tuning the infrared frequency to a nearby electronic transition.

Figure 1

Scheme of the $\omega +2\omega +{\mathrm{\Omega }}_0$ process in neon in the dipole approximation. The ionization is caused by the fundamental and second harmonics (solid blue arrows) of an XUV pulse whose fundamental frequency $\omega$ is tuned near the $2p\to 3s$ transition. The overlapping IR field induces ATI (small red arrows) transitions leading to sidebands in the spectrum. Only $s, p$, and $d$ waves are displayed, although higher partial waves can contribute. Dashed arrows represent additional paths created when the IR frequency is tuned near the $3s\to 3p$ transition (see text).

Figure 2

Example of a photoelectron spectrum showing the different sidebands associated with the minimum number of absorbed or emitted photons.

Figure 3

TDSE results for the ionization probability (in units of ${10}^{-3} {\text{eV}}^{-1}$) as a function of the photoelectron energy for different IR-field amplitude ratios ${\eta }_0$. The fundamental XUV intensity is $I={10}^{12}{\mathrm{W}/\mathrm{cm}}^2, {\eta }_X=0.1$, and the infrared frequency is ${\mathrm{\Omega }}_0=0.55$ eV.

Figure 4

Ionization probability associated with the different bands calculated in the TDSE (solid lines) and the SFA (dashed lines) approaches as a function of the IR-field amplitude ratio ${\eta }_0$.

Figure 5

Anisotropy parameters ${\beta }_k (1\le k\le 6)$, for ${\phi }_X=0$, as a function of the IR-field amplitude ratio ${\eta }_0$. Labels are the same as in Fig. 4.

Figure 6

(a) Subtracted and (b) summed forward and backward ionization probability signals (in units of ${10}^{-4}$), as well as (c) left-right asymmetry, as a function of the IR-field amplitude ratio ${\eta }_0$, as calculated in the TDSE and SFA approaches. Note the constant value of $A\left(0^{\circ }\right)$ in the SFA.

Figure 7

(a) Left-right asymmetry $A\left(0^{\circ }\right)$ for ML (solid black line), ${\text{SB}}_1$ (dashed red line), and ${\text{SB}}_2$ (dotted blue line), as a function of ${\phi }_X$ at ${\mathrm{\Omega }}_0=0.55$ eV and ${\eta }_0=0.2$. (b) Left-right asymmetry $A\left(0^{\circ }\right)$ as a function of the IR frequency ${\mathrm{\Omega }}_0$ at ${\eta }_0=0.2$ (dashed lines) and ${\eta }_0=0.3$ (solid lines). Inset: Corresponding ionization probability spectrum (in units of ${10}^{-3} {\text{eV}}^{-1}$) at ${\eta }_0=0.3$.