Subcycle Dynamics of Coulomb Asymmetry in Strong Elliptical Laser Fields

We measure photoelectron angular distributions of noble gases in intense elliptically polarized laser fields, which indicate strong structure-dependent Coulomb asymmetry. Using a dedicated semiclassical model, we have disentangled the contribution of direct ionization and multiple forward scattering on Coulomb asymmetry in elliptical laser fields. Our theory quantifies the roles of the ionic potential and initial transverse momentum on Coulomb asymmetry, proving that the small lobes of asymmetry are induced by direct ionization and the strong asymmetry is induced by multiple forward scattering in the ionic potential. Both processes are distorted by the Coulomb force acting on the electrons after tunneling. Lowering the ionization potential, the relative contribution of direct ionization on Coulomb asymmetry substantially decreases and Coulomb focusing on multiple rescattering is more important. We do not observe evident initial longitudinal momentum spread at the tunnel exit according to our simulation.

Figure 1

The two-dimensional experimental (a) and simulated (b) photoelectron momentum distributions of Ne in an elliptically polarized laser field at an intensity of $3\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

(a) The momentum-integrated angular distribution of Ne in an elliptical laser field (the ellipticity $\sim 0.25$) at an intensity of $3\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ of experiment (dotted) and simulation (solid). (b) The momentum-integrated angular distribution with respect to the tunneling time. The red solid line shows the field component along the major axis. Two groups of electrons marked $A$ and $B$ are mainly induced by direct ionization and multiple forward scattering, which are separated by the white dashed line. (c) and (d) show two-dimensional photoelectron momentum distributions of direct ionization and multiple forward scattering, respectively.

Figure 3

(a) and (b) show the momentum-integrated angular distributions with respect to the initial transverse velocities of along the minor axis and the laser propagation direction. (c) and (d) show the electron tunneling time with respect to the final transverse momentum along the laser propagation direction (c) for Ne $3\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and (d) for Xe $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ (the ellipticity $\sim 0.25$). The solid lines and dashed lines in (c) and (d) show the position of half maximum of $p_y{}^{\mathrm{final}}$ and the tunneling time when rescattering begins to occur, respectively, and the color scale is normalized to the maximum rate for each tunneling time.

Figure 4

The experimental (a) and simulated (b) momentum-integrated angular distributions of Ar, Kr, and Xe (ellipticity $\sim 0.25$) at an intensity of $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 5

(a) The simulated ellipticity-dependent momentum-integrated angular distribution for He. (b) The simulated angular distributions for two ellipticity values. In the simulation, there is no initial longitudinal momentum included.