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New Journal of Physics

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

Tim Bayer, Daniela Gräfing, Stefanie Kerbstadt, Dominik Pengel, Kevin Eickhoff, Lars Englert, M. Wollenhaupt

Published Jan 2, 2019• 21 citations
Publisher

Abstract

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 fine-structure 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.

DOI
10.1088/1367-2630/aafb87

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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 fine-structure 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.
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Electrons detached from atoms by photoionization carry valuable information about light-atom interactions. Characterizing and shaping the electron wave function on its natural timescale is of paramount importance for understanding and controlling ultrafast electron dynamics in atoms, molecules and condensed matter. Here we propose a novel attoclock interferometry to shape and image the electron wave function in atomic photoionization. Using a combination of a strong circularly polarized second harmonic and a weak linearly polarized fundamental field, we spatiotemporally modulate the atomic potential barrier and shape the electron wave functions, which are mapped into a temporal interferometry. By analyzing the two-color phase-resolved and angle-resolved photoelectron interference, we are able to reconstruct the spatiotemporal evolution of the shaping on the amplitude and phase of electron wave function in momentum space within the optical cycle, from which we identify the quantum nature of strong-field ionization and reveal the effect of the spatiotemporal properties of atomic potential on the departing electron. This study provides a new approach for spatiotemporal shaping and imaging of electron wave function in intense light-matter interactions and holds great potential for resolving ultrafast electronic dynamics in molecules, solids, and liquids.
In this work, we study the coherent dynamics of an atomic Zeeman wave packet using a continuous pump--probe scheme. A polarized wave packet is generated via few-photon excitation by a femtosecond laser pulse, creating a state with a magnetic moment tilted relative to an external magnetic field. The subsequent Larmor precession of the atoms is probed by continuous ionization in the field of an optical dipole trap (ODT) laser. Photoelectrons and photoions are detected in coincidence using a cold target recoil ion momentum spectrometer (COLTRIMS). While the addition of the ODT enables further cooling of the atomic ensemble, it removes the pulsed timing reference typically used to extract photoelectron momentum distributions in COLTRIMS. Here, we present a method that extends the standard COLTRIMS technique by exploiting redundancy in the measured data to reconstruct the time of ionization. The resulting time-dependent ionization signal reflects the coherent precession of the atomic magnetic moment, enabling real-time access to atomic dynamics on nanosecond timescales.
Ultrafast atomic processes, such as excitation and ionization occurring on the femtosecond or shorter time scale, were explored by employing attosecond high-harmonic pulses. With the absorption of a suitable high-harmonic photon a He atom was ionized, or resonantly excited with further ionization by absorbing a number of infrared photons. The electron wave packets liberated by the two processes generated an interference containing the information on ultrafast atomic dynamics. The attosecond electron wave packet, including the phase, from the ground state was reconstructed first and, subsequently, that from the 1s3p state was retrieved by applying the holographic technique to the photoelectron spectra comprising the interference between the two ionization paths. The reconstructed electron wave packet revealed details of the ultrafast photoionization dynamics, such as the instantaneous two-photon ionization rate.
Polarization-tailored bichromatic femtosecond laser fields with cycloidal polarization profiles have emerged as a powerful tool for coherent control of quantum processes. We present an optical scheme to create and manipulate three-dimensional free electron wave packets with arbitrary rotational symmetry by combining advanced supercontinuum pulse shaping with high resolution photoelectron tomography. Here we use carrier-envelope phase-stable polarization-tailored bichromatic (3ω:4ω) counter- and corotating femtosecond laser pulses to generate 7-fold rotational symmetric and asymmetric photoelectron momentum distributions by multiphoton ionization of sodium atoms. To elucidate the physical mechanisms, we investigate the interplay between the symmetry properties of the driving field and the resulting electron wave packets by varying the optical field parameters. Our results show that the symmetry properties of electron wave packets are not fully determined by the field symmetry, but completely described by multipath quantum interference of states with different angular momenta.
Using time-resolved photoelectron imaging, a time-dependent electron orbital alignment is observed during the recurrences of a Rydberg rotational wave packet in krypton. The time-dependent alignment is created via excitation of two ac-Stark shifted states (5d'[5/2] 3 and 8d[1/2] 1 ) using a three-photon femtosecond pulse excitation. A straightforward analysis of the measured photoelectron distributions using angular momentum algebra shows that the observed periodic change in the electron orbital alignment is accompanied by a time-dependent change of the associated Bell states. These Bell states describe the entangled system of excited electron and ionic core. Hence, these experiments present a relatively simple excitation scheme for creating a decoherence-free entangled superposition.