Multiphoton ionization of potassium atoms with a sequence of two counter-rotating circularly polarized femtosecond laser pulses produces vortex-shaped photoelectron momentum distributions in the polarization plane describing Archimedean spirals. The pulse sequences are produced by polarization shaping and the three-dimensional photoelectron distributions are tomographically reconstructed from velocity map imaging measurements. We show that perturbative ionization leads to electron vortices with c_{6} rotational symmetry. A change from c_{6} to c_{4} rotational symmetry of the vortices is demonstrated for nonperturbative interaction.

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Bichromatic polarization-shaped femtosecond laser pulses are used to control three-dimensional photoelectron momentum distributions (3D-EDs) from resonance enhanced multi-photon ionization of potassium atoms. The light fields consisting of two spectral bands with different ellipticity are produced using an ultrafast polarization pulse shaper equipped with a custom polarizer in the Fourier plane. The tomographically reconstructed 3D-EDs from ionization with counterrotating circularly or orthogonal linearly polarized bichromatic laser pulses show different angular momentum superposition states at four distinct photoelectron energies. The analysis of the measured 3D-EDs reveals that the underlying physical mechanism is based on the interplay of ionization pathway selection via quantum mechanical selection rules for optical transitions and intrapulse frequency mixing of the spectral bands with different ellipticity. multiple ionization pathways by energetic separation of different angular momentum target states. Control of the ionization pathway along with energy-and angle-resolved detection allows us, on the one hand, to generate individual angular momentum free electron wave packets using CNR-CP bichromatic pulses and, on the other hand, to create unusual angular momentum superposition states using O-LP bichromatic pulses. In the experiment, bichromatic polarization-tailored fields are produced using a f 4 polarization pulse shaper [35] equipped with a custom polarizer in the Fourier plane [36,37]. Multiple photoelectron momentum images are recorded employing a velocity map imaging (VMI) spectrometer [38] and subsequently combined to reconstruct the 3D-EDs using a tomography algorithm [39][40][41]. Positive and negative time delays between the two colors are introduced to discriminate resonant from non-resonant ionization and to observe signatures from spin-orbit wave packets (SOWPs) in the 3D-EDs. A similar setup was recently used to produce electron vortices with a sequence of two one-color CNR-CP femtosecond laser pulses [42]. In that experiment, the minimal time delay in the sequence was given by the pulse length in order to avoid the creation of linear polarization in the overlapping interval. However, CNR-CP bichromatic fields, consisting of temporally overlapping left-(L-CP) and right-handed circularly (R-CP) polarized disjoined spectral bands do not create linearly polarized light. This feature allows us to observe photoelectrons from REMPI by overlapping bichromatic CNR-CP pulses such that frequency mixing between photons of opposite helicity can occur. For example, we demonstrate that 3D-EDs from three-photon ionization with two red L-CP photons and one blue R-CP photon are observed in a specific kinetic energy window.We start in section 2 with a theoretical discussion of the 3D-EDs from 1+2 REMPI with bichromatic polarization-shaped laser pulses. Section 3 introduces the experimental strategy based on the combination of polarization pulse shaping and tomographic photoelectron imaging. The experimental ...

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We combine bichromatic polarization pulse shaping with photoelectron imaging tomography for time-resolved spatial imaging of ultrafast spin-orbit wave packet (SOWP) dynamics in atoms. Polarization-shaped two-color pump-probe sequences are generated by spectral amplitude and phase modulation of a femtosecond input pulse and used to excite SOWPs in the potassium p 4 finestructure doublet. By selecting different spectral bands for pump and probe pulse, we achieve interference-free detection of the spatiotemporal SOWP dynamics. Using tomographic techniques, we reconstruct the three-dimensional photoelectron momentum distribution (3D-ED) created by the probe pulse. Time-resolved measurement of the 3D-ED reveals the orbital realignment dynamics induced by spin-orbit interaction in the neutral atom.