With a classical ensemble model that including electron correlations during the whole ionization process, we investigated strong-field sequential double ionization of Ar by elliptically polarized pulses at the quantitative level. The experimentally observed intensity-dependent three-band or four-band structures in the ion momentum distributions are well reproduced with this classical model. More importantly, the experimentally measured ionization time of the second electrons [A. N. Pfeiffer et al., Nature Phys. 7, 428 (2011)], which can not be predicted by the standard independent-electron model, is quantitatively reproduced by this fully classical correlated model. The success of our work encourages classical description and interpretation of the complex multi-electron effects in strong field ionization where nonperturbative quantum approaches are currently not feasible.PACS numbers: 32.80. Rm, 31.90.+s, 32.80.Fb Among various intense laser-induced phenomena, strong field double ionization (DI) is one of the most important and fundamental processes. During the past decades, a great number of experimental as well as theoretical studies have been performed on this area. It has been known that DI proceeds either sequentially or nonsequentially. In nonsequential double ionization (NSDI), the second electron is ionized by the recollision of the first tunneled electron [1]. Because of this recollision, the two electrons from NSDI exhibit a highly correlated behavior [2][3][4][5][6]. In sequential double ionization (SDI), it is usually assumed that no correlation exists between the two electrons, and thus the ionization of the electrons can be treated as two independent tunneling-ionization steps. However, this assumption has been called into doubt by recent experiments [7,8]. In Ref. [7], it has been shown that there is a clear angular correlation between the two electrons from SDI, which implies that the successive ionization steps are not independent in SDI. In Ref.[8], the authors found that the ionization time of the second electron from SDI is much earlier than the prediction of the independent-electron model. These observations declare that the electron correlations in SDI should be reexamined carefully.Theoretically, an accurate description of the electron correlations in DI needs full quantum theory. However, due to the enormous computational demand, the solution of the time-dependent schrödinger equation of the two-electron system in the strong field, especially in the case of elliptically polarized laser fields, is not feasible currently. Instead, a fully classical treatment of the twoand multi-electron systems proposed by Eberly et al. has been well established [9][10][11][12]. During the past decade, this model has been successful in exploring the strong-field ionization processes at the qualitative level. However, * Corresponding author: lupeixiang@mail.hust.edu.cn it fails when made a quantitative comparison with experiments. For example, in ref. [13] it has been shown that the saturation intensity ...

Double ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using coincidence momentum imaging. We show that the two-electron emission dynamics in nonsequential double ionization can be controlled by tuning the subcycle shape of the electric field of the OTC pulses. We demonstrate experimentally switching from correlated to anticorrelated two-electron emission, and control over the directionality of the two-electron emission. Simulations based on a semiclassical trajectory model qualitatively explain the experimental results by a subcycle dependence of the electron recollision time on the OTC field shape.