Control of photoelectron momentum distributions by bichromatic polarization-shaped laser fields

Abstract

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 ...

Time-resolved 3D imaging of ultrafast spin–orbit wave packet dynamics

“…Additional pathways to a p-type continuum (via s-and d-type intermediates) are not shown but included in the numerical simulations presented in section 4 (see figures 5 and 6). Even though these pathways are typically less efficient due to propensity rules [36,37], the photoelectron wave packet is, in general, described by a coherent superposition of a p-and an f-wave [20,21,38].…”

“…In addition, when both colors overlap in time (  t D | | t ), intra-pulse frequency mixing by simultaneous absorption of two blue and one red photon contributes at ε 2 =ÿ(2ω b +ω r )−ε ip . However, only the photoelectron wave packet at ε 1 is sensitive to the time evolution of the SOWP, while the other three contributions are either τ-independent (ε 0 and ε 3 ) or vanish for time-delays exceeding the pulse duration (ε 2 ) [20]. Thus, by energetic disentanglement of the different photoionization channels, the bichromatic pump-probe technique allows for background-free detection of the SOWP dynamics mapped into the ε 1 -channel.…”

“…In the experiment, the SOWP dynamics  ( ) r t , is mapped into the photoelectron momentum distribution  e q f t ( ) , , ; . The two-photon ionization probe step is described using second order time-dependent perturbation theory [20,39,40]. Incorporating the electron spin in the treatment increases the number of involved ionization pathways considerably.…”

“…In this contribution, we present a technique for time-resolved 3D reconstruction of ultrafast quantum dynamics by combining the aforementioned advantages of polarization-shaped pulses and highly differential photoelectron detection. The technique is based on the combination of bichromatic polarization pulse shaping, for the generation of polarization-shaped two-color pump-probe pulse sequences, with photoelectron imaging tomography to reconstruct the full energy and angular distribution of photoelectron wave packets released by the probe [19][20][21]. The experimental scheme has been introduced in [20] and applied to the coherent control of photoelectron momentum distributions aiming at the design of unusual angular momentum superposition states.…”

“…The technique is based on the combination of bichromatic polarization pulse shaping, for the generation of polarization-shaped two-color pump-probe pulse sequences, with photoelectron imaging tomography to reconstruct the full energy and angular distribution of photoelectron wave packets released by the probe [19][20][21]. The experimental scheme has been introduced in [20] and applied to the coherent control of photoelectron momentum distributions aiming at the design of unusual angular momentum superposition states. Here, we demonstrate the power of the method to observe ultrafast dynamics and determine atomic and molecular properties.…”