Fig. 1. (a)–(c) The experimentally measured PEMDs for strong-field tunneling ionization of Ar by the parallel two-color (800nm+400nm) field with relative phase φ=0, 0.5π, and 1.5π, respectively. The laser intensities of the FM and SH fields are 1.2×1014 and 0.3×1011W/cm2, respectively. (d)–(f) NDs for the distributions in (a)–(c) (see text for details).

Fig. 2. (a) The ND as a function of φ for the momentum (px,py)=(−0.6,0.1) a.u. The open circles show the experimental data and the green curve shows the fitted results. (b) The fitted ND by Eq. (2) as a function of φ for the momentum (px,py)=(−0.6,0) (blue curve), (−0.6,0.1) (purple curve), and (−0.6,0.2) a.u. (green curve). The data are normalized such that the maximum of each curve is unity. (c) Same as (b) but for (px,py)=(−0.4,0) (black curve), (−0.5,0) (red curve), and (−0.6,0) a.u. (yellow curve). (d) The optimal phase φm in the region of px∈[−1.1,1.1] a.u. and py∈[−0.5,0.5] a.u. (e) Cuts of φm at px=−0.4 a.u. (red crosses), −0.5 a.u. (green squares), and −0.6 a.u. (blue triangles). The error bars show the 95% confidence interval in fitting.

Fig. 3. (a) Illustration of the ionization times of the long and short orbits in strong-field tunneling ionization. The long and short orbits correspond to the ionization events, where the electron is released at the falling and rising edges of electric field, respectively. The blue curve indicates the electric field of the FM field and the red curve shows its vector potential. (b) The ND(p;φ) for the long orbit with transverse momentum py=0, calculated by CCSFA. The solid red curve indicates the optimal phase φmL. (c) The same as (b) but for the short orbit. The black arrow denotes the phase window formed by φmL and φmS. (d) The optimal phase φmL of the long orbit for px∈[−1,−0.2] a.u. and py∈[−0.36,0.36] a.u. (e) The same as (d) but for the short orbit.

Fig. 4. (a) The ND at (px,py)=(−0.5,0) a.u. as a function of φ. The solid green and purple curves show the theoretical results of the signal from the long and short orbits, respectively. The black squares represent the experimental data, where both the long and short orbits contribute. The dashed blue lines indicate the optimal phases φmL and φmS, and the blue arrow denotes their phase window. (b) The same as (a) but for (px,py)=(−0.5,0.2) a.u. (c) The ratios α/β extracted from the experimental data. (d) The cuts of α/β at px=−0.4 (red circles), −0.6 (green squares), and −0.95 a.u. (purple triangles), respectively.