Strong-field atomic ionization in elliptically polarized laser fields

We experimentally investigate strong-field ionization of noble-gas atoms in elliptically polarized laser fields with the Keldysh parameter $\gamma \sim 1$, which is typical for most current strong-field experiments. Our data show that the single-ionization yields exhibit a rapid decrease with increasing elipticity and, moreover, the decrease becomes more dramatic for lower laser intensity. The experimental results are compared with the simulations of the Ammosov-Delone-Krainov (ADK) theory based on the quasistatic tunneling ionization picture and the strong-field approximation (SFA) theory, which involves the nonadiabatic effect in the tunneling ionization process. Our results show that although both the ADK simulations and the SFA calculations can qualitatively reproduce the experimental results, the SFA calculations are in a better agreement with the experimental data than the ADK simulations, which implies a nonadiabatic effect during strong-field atomic ionization for elliptical polarization with $\gamma \sim 1$.

Figure 1

Single-ionization yields of argon, krypton, and xenon as a function of ellipticity at laser intensities ranging from $5.0\times {10}^{13}$ to $2.4\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$. The yield is normalized at each intensity to its value for linear polarization. (a)–(c) Experimental data, (d)–(f) ADK simulations, and (g)–(i) SFA calculations.

Figure 2

Normalized ionization yields as a function of ellipticity. (a) Argon at a laser intensity of $8.0\times {10}^{13}{\mathrm{W}/\mathrm{cm}}^2$, (b) krypton at $8.0\times {10}^{13}{\mathrm{W}/\mathrm{cm}}^2$, and (c) xenon at $5.0\times {10}^{13}{\mathrm{W}/\mathrm{cm}}^2$. Experimental data, black squares; ADK simulations, red dashed lines; and SFA calculations, blue solid lines.

Figure 3

Normalized ionization yields of (a) argon, (b) krypton, and (c) xenon for circular polarization as a function of the laser intensity. The top axis denotes the value of the Keldysh parameter $\gamma$. Experimental data, black squares; ADK simulations, red circles; and SFA calculations, green triangles. For a better comparison with the simulations, the data are also shifted vertically (open squares).