Polarization effects in two-photon nonresonant ionization of argon with extreme-ultraviolet and infrared femtosecond pulses

We report the results of experimental and theoretical investigations of the two-color, two-photon ionization of Ar atoms, using femtosecond pulses of infrared laser radiation in combination with its extreme-ultraviolet harmonics. It is shown that the intensities of the photoelectron lines resulting from the absorption of photons from both fields strongly depend both on the respective phases of the fields and on atomic quantities such as the asymmetry parameter. These phases, which are notoriously difficult to measure, can be estimated by changing the polarization state of the laser radiation.

Figure 1

(a) Scheme of the dominant photon absorption and emission processes involved in the generation of the sideband of order $2N$ ($2N\omega$ is the net amount of absorbed energy). (b) Photoelectron spectrum as a function of temporal delay $\Delta \tau$ between the XUV and the IR pulses. (c) Typical spectrum recorded at perfect temporal overlap between XUV and IR pulses $\left(\Delta \tau =0\text{fs}\right)$ with an IR beam intensity of about $5\times {10}^{11}\mathrm{W}∕{\text{cm}}^2$.

Figure 2

(Color online) Schematic of the experimental setup. HHG, high harmonic generation; Al, Aluminum filter; MBES, Magnetic bottle electron spectrometer; WP, Retarding wave plate; L, Adjustable lens; FM, Gold focusing mirror.

Figure 3

Variation of the sideband intensity as a function of the relative angle $\Theta$ between the polarization vectors of the XUV and the IR beams. Experimental data for different sidebands: $(*){\text{SB}}_{14}$, $(◇){\text{SB}}_{16}$, $(\nabla ){\text{SB}}_{18}$, $\left(\Delta \right){\text{SB}}_{20}$, and $(엯){\text{SB}}_{22}$. Theoretical calculations: $\left(\text{−}\text{−}\right){\text{SB}}_{14}$, $\left(\text{−}·\right){\text{SB}}_{16}$, $\left(··\right){\text{SB}}_{18}$, $(-){\text{SB}}_{20}$, and $(-){\text{SB}}_{22}$.

Figure 4

Variation of sideband intensity as a function of the angle of rotation $\gamma$ of the major axis of the ellipse described by the tip of the electric-field vector relative to the polarization direction of the XUV field. [The degree of ellipticity is $\tan \left(\gamma \right)$.] (a) Theoretical calculations for sideband 14 for different $\Delta \varphi$. (b) Experimental data for different sidebands: $(*){\text{SB}}_{14}$, $(◇){\text{SB}}_{16}$, $(\nabla ){\text{SB}}_{18}$ $\left(\Delta \right){\text{SB}}_{20}$, and $(엯){\text{SB}}_{22}$. Broken line: theoretical calculation averaged over all $\Delta \varphi$ for ${\text{SB}}_{14}$.