Disentangling Intracycle Interferences in Photoelectron Momentum Distributions Using Orthogonal Two-Color Laser Fields

We use orthogonally polarized two-color (OTC) laser pulses to separate quantum paths in the multiphoton ionization of Ar atoms. Our OTC pulses consist of 400 and 800 nm light at a relative intensity ratio of $10:1$. We find a hitherto unobserved interference in the photoelectron momentum distribution, which exhibits a strong dependence on the relative phase of the OTC pulse. Analysis of model calculations reveals that the interference is caused by quantum pathways from nonadjacent quarter cycles.

Figure 1

(a) Vector potential of an OTC field for phase delays $\phi =1.5\pi$ (left) and $\phi =\pi$ (right) over one optical cycle of the 800 nm field. The eight quarter cycles of the stronger 400 nm field are color coded and labeled by $A1–A4$ and $B1–B4$. The time window in each panel is adjusted to the reference quarter cycle $A1$. (b) Measured photoelectron momentum distributions of Ar ionized in a 45 fs OTC pulse, for the phase delays in (a). Superimposed on the momentum distributions is the simple man’s prediction of the electron momentum, $\mathbf{p}=-\mathbf{A}(t_i)$, for $\mathbf{A}(t)$ from panel (a). The arrows mark nodes (experimental artifacts) caused by the spectrometer magnetic field [33].

Figure 2

Angle-resolved energy spectra of photoelectrons for a phase delay $\phi =1.5\pi$ (left) and $\phi =\pi$ (right). (a) Experiment. (b) SFA calculation using Eq. (3). (c) SFA calculation with the OTC pulse restricted to one optical cycle of the 800 nm field. The arrows in (a) mark the experimental artifacts from Fig. 1.

Figure 3

(a) Electric fields ($E_x$, $E_z$) and vector potentials ($A_x$, $A_z$) for $\phi =1.5\pi$ within one cycle of 800 nm. $A1–A4$ and $B1-B4$ denote the different ionization time windows introduced in Fig. 1. The top axis shows the asymptotic emission angle $\theta$ of the photoelectron according to the simple man’s model $\mathbf{p}=-\mathbf{A}(t)$. (b)–(g) SFA calculations for $\phi =1.5\pi$ [(b)–(e)] and $\phi =\pi$ [(f),(g)] using Eq. (3) with quantum orbits from the different ionization time windows as indicated in the figures.