Elliptical Polarization Favors Long Quantum Orbits in High-Order Above-Threshold Ionization of Noble Gases

We demonstrate the significant role of long quantum orbits in strong-field atomic processes by investigating experimentally and theoretically the above-threshold ionization spectra of noble gases in intense elliptically polarized laser pulses. With increasing laser ellipticity, the yields of different energy regions of the measured electron spectrum in high-order above-threshold ionization drop at different rates. The experimental features can be reproduced by a theoretical simulation based on quantum-orbit theory, revealing that increasing ellipticity favors the contributions of the long quantum orbits in the high-order above-threshold ionization process.

Figure 1

Experimental [(a), (b), and (c)] photoelectron spectra of Ar, Kr, and Xe along the major polarization axis for elliptically polarized laser pulses with wavelength of 800 nm, intensity of $7.0\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, and ellipticities $\xi =0$, 0.07, 0.14, 0.21, 0.29, and 0.36 from top to bottom. The spectra are normalized to unity at zero energy. (d), (e), (f) Ratios of the electron yields for various low-energy intervals over one high-energy interval as functions of the ellipticity. For details, see the text.

Figure 2

(a) The calculated focal-averaged photoelectron spectra of Ar along the major polarization axis for the ellipticities $\xi =0$, 0.07, 0.14, 0.21, 0.29, and 0.36 from top to bottom. (b) Similar to Fig. 1d but for the theoretical calculation, in which the first twenty pairs of quantum orbits are included. (c) Similar to (b), but only the first pair of orbits is considered in the calculation. (d) The ratio of the two ratios displayed in (b) and (c). The laser parameters are chosen as the wavelength of 800 nm and the intensity of $6.2\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, somewhat below the peak intensity of the experiment [30].

Figure 3

Photoelectron spectra along the major polarization axis with only the rescattering electrons, as well as the individual contributions of the first, second, and third pair of the rescattering electrons for (a) linear polarization $\xi =0$ and (b) elliptical polarization $\xi =0.36$. The parameters are the same as in Fig. 2. For details, see the text.