70
S. Kerbstadt et al.
phase-stable Polarization-tailored Supercontinuum (BiCEPS) fields for atomic multiphoton ionization (MPI). For this purpose, continuously tunable BiCEPS fields were
generated using a novel white light polarization pulse shaping scheme and a custom
composite polarizer [17, 18]. By combining this bichromatic shaping scheme with
high-resolution photoelectron tomography based on Velocity Map Imaging (VMI)
spectrometry [40], we were able to generate and manipulate three-dimensional (3D)
PMDs. To extract the underlying physical mechanisms, we performed our experiments on hydrogen-like atomic model systems (sodium (Na) and potassium (K)
atoms). The common theme in the experiments presented in this chapter was the
intriguing interplay between the symmetry properties of light fields and the observed
PMDs. In six experimental studies, we presented different physical mechanisms for
bichromatic MPI. We studied intra-pulse frequency mixing of spectral bands with
different ellipticities in coherent control scenarios involving ‘N - versus N ’-photon
and ‘N 1 - versus N 2 ’-photon interferences. The analysis of our results showed that
the symmetry properties of free electron wave packets are controlled by multipath
quantum interference of continuum states with different angular momenta. Depending on the number of photons involved and on the polarization state of the driving
fields, different control parameters were active and different symmetries of the free
electron wave packets were observed. In the following, we briefly summarize the
main results obtained in the experiments presented in Sects. 3.3.1–3.3.6 and discuss
some perspectives of our work.
In the first experiment presented in Sect. 3.3.1, we studied bichromatic ‘N - versus N ’-photon MPI by absorption of photons with different color and polarization
leading to high-order intra-pulse frequency mixing [22]. While counter-rotating circularly polarized (CRCP) driving pulses generated four different angular momentum
eigenstates, bichromatic orthogonal linearly polarized (OLP) fields gave rise to quasi
c 6 -rotationally symmetric photoelectron wave packets from third-order intra-pulse
frequency mixing and two orthogonal ‘ f within f ’-type wave packets from 3-photon
single-color ionization.
In the second study presented in Sect. 3.3.2, we focused on coherent control of
the directional photoionization via paths with different numbers of photons (‘N 1
versus N 2 ’). The CEP control of the directionality was achieved by interference
of continuum states with opposite parity. Our results showed that shaper-generated
bichromatic (N 1 ω:N 2 ω) fields, with the center frequency ratio tailored to the MPI
process, confine the CEP-sensitive asymmetries in a predefined kinetic energy window [23, 24]. In the experiment, (3ω:4ω) parallel linearly polarized (PLP) fields were
used to control the MPI of Na atoms. Significant asymmetries in the photoemission
were observed along the two distinct directions parallel to the laser polarization. It
was demonstrated that the attainable degree of control over the multipath interference
in the target state exceeds CEP control by few-cycle ionization scenarios [23].
Full control over the symmetry properties of the 3D PMDs was obtained by
using polarization-tailored pulses. To this end, we studied two control scenarios
using either single-color or bichromatic circularly polarized pulse sequences as
described in Sects. 3.3.3 and 3.3.4. Single color electron vortices, i.e. even-numbered
spiral-shaped free electron wave packets from MPI with CRCP single-color pulse
S. Kerbstadt et al.
phase-stable Polarization-tailored Supercontinuum (BiCEPS) fields for atomic multiphoton ionization (MPI). For this purpose, continuously tunable BiCEPS fields were
generated using a novel white light polarization pulse shaping scheme and a custom
composite polarizer [17, 18]. By combining this bichromatic shaping scheme with
high-resolution photoelectron tomography based on Velocity Map Imaging (VMI)
spectrometry [40], we were able to generate and manipulate three-dimensional (3D)
PMDs. To extract the underlying physical mechanisms, we performed our experiments on hydrogen-like atomic model systems (sodium (Na) and potassium (K)
atoms). The common theme in the experiments presented in this chapter was the
intriguing interplay between the symmetry properties of light fields and the observed
PMDs. In six experimental studies, we presented different physical mechanisms for
bichromatic MPI. We studied intra-pulse frequency mixing of spectral bands with
different ellipticities in coherent control scenarios involving ‘N - versus N ’-photon
and ‘N 1 - versus N 2 ’-photon interferences. The analysis of our results showed that
the symmetry properties of free electron wave packets are controlled by multipath
quantum interference of continuum states with different angular momenta. Depending on the number of photons involved and on the polarization state of the driving
fields, different control parameters were active and different symmetries of the free
electron wave packets were observed. In the following, we briefly summarize the
main results obtained in the experiments presented in Sects. 3.3.1–3.3.6 and discuss
some perspectives of our work.
In the first experiment presented in Sect. 3.3.1, we studied bichromatic ‘N - versus N ’-photon MPI by absorption of photons with different color and polarization
leading to high-order intra-pulse frequency mixing [22]. While counter-rotating circularly polarized (CRCP) driving pulses generated four different angular momentum
eigenstates, bichromatic orthogonal linearly polarized (OLP) fields gave rise to quasi
c 6 -rotationally symmetric photoelectron wave packets from third-order intra-pulse
frequency mixing and two orthogonal ‘ f within f ’-type wave packets from 3-photon
single-color ionization.
In the second study presented in Sect. 3.3.2, we focused on coherent control of
the directional photoionization via paths with different numbers of photons (‘N 1
versus N 2 ’). The CEP control of the directionality was achieved by interference
of continuum states with opposite parity. Our results showed that shaper-generated
bichromatic (N 1 ω:N 2 ω) fields, with the center frequency ratio tailored to the MPI
process, confine the CEP-sensitive asymmetries in a predefined kinetic energy window [23, 24]. In the experiment, (3ω:4ω) parallel linearly polarized (PLP) fields were
used to control the MPI of Na atoms. Significant asymmetries in the photoemission
were observed along the two distinct directions parallel to the laser polarization. It
was demonstrated that the attainable degree of control over the multipath interference
in the target state exceeds CEP control by few-cycle ionization scenarios [23].
Full control over the symmetry properties of the 3D PMDs was obtained by
using polarization-tailored pulses. To this end, we studied two control scenarios
using either single-color or bichromatic circularly polarized pulse sequences as
described in Sects. 3.3.3 and 3.3.4. Single color electron vortices, i.e. even-numbered
spiral-shaped free electron wave packets from MPI with CRCP single-color pulse