Controlling Nonsequential Double Ionization in Two-Color Circularly Polarized Femtosecond Laser Fields

Atoms undergoing strong-field ionization in two-color circularly polarized femtosecond laser fields exhibit unique two-dimensional photoelectron trajectories and can emit bright circularly polarized extreme ultraviolet and soft-x-ray beams. In this Letter, we present the first experimental observation of nonsequential double ionization in these tailored laser fields. Moreover, we can enhance or suppress nonsequential double ionization by changing the intensity ratio and helicity of the two driving laser fields to maximize or minimize high-energy electron-ion rescattering. Our experimental results are explained through classical simulations, which also provide insight into how to optimize the generation of circularly polarized high harmonic beams.

Figure 1

(a) The experimental scheme used to study nonsequential double ionization in two-color ($\omega$, $2\omega$) circularly polarized fields. The apparatus consists of a femtosecond laser system, a Mach-Zehnder interferometer, and a time-of-flight spectrometer. (b) The electric fields for two-color counter-rotating fields for $I_B/I_R$ ratios of 2 (left-hand thick red curve) and 4 (middle thick purple curve) and for a co-rotating field at an $I_B/I_R$ ratio of 2 (right-hand thick cyan curve). Counter-rotating fields can efficiently drive electrons (small blue spheres) back to the parent ion (large green spheres) in 2D trajectories, whereas it is much harder for an electron to return to the parent ion in co-rotating fields.

Figure 2

Experimentally measured single- and double-ionization yields in argon for two-color circularly polarized fields. The fields are combined as either counter-rotating (filled symbols) or co-rotating (open symbols) fields and with three different ratios of the second harmonic intensity to the intensity of the fundamental $I_B/I_R$: 0.2 (circles), 1 (squares), and 6 (diamonds). NSDI (shaded region) is observed in counter-rotating fields when the $I_B/I_R$ ratio is 1 and 6. However, NSDI is not observed for co-rotating fields or for the counter-rotating case of $I_B/I_R=0.2$. Note: The intensities were slightly rescaled relative to their estimated values in order to provide comparable single-ionization yields (see the SM).

Figure 3

Simulated single- and double-ionization yields in argon using a classical ensemble model. (a) A significant amount of NSDI (shaded region) occurs in counter-rotating fields when the $I_B/I_R$ ratio is 1 and 6 and is suppressed in the co-rotating cases and the counter-rotating case when $I_B/I_R=0.2$. (b) The double-ionization yield of argon at $5\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$ is comprised entirely of NSDI and shows that NSDI occurs over a broad range of $I_B/I_R$ ratios. The ${\mathrm{Ar}}^{2+}$ yield is optimized at $I_B/I_R\approx 2$.

Figure 4

Classical trajectory Monte Carlo simulations show the yield of electrons that return within 0.05 nm of the parent ion, and the energy of the electrons at the moment of return. (a) The percent of electrons that return to the parent ion with a kinetic energy (KE) that is greater than the second ionization energy of argon (27.63 eV) gives an estimate of NSDI and agrees well with the classical ensemble model [Fig. 3]. (b) The cutoff energy of the returning electrons strongly depends on the $I_B/I_R$ ratio. (c) The returning energy spectrum of electrons in two-color counter-rotating fields exhibits a narrower bandwidth than that of a linearly polarized field at the fundamental frequency, allowing for the generation of a quasimonochromatic EUV light source with a tunable photon energy.