3 Bichromatic Control of Free Electron Wave Packets
71
sequences, were observed at the ionization threshold and in the above threshold ionization (ATI) [21]. The symmetry properties of the vortices depend on the excitation
and ionization dynamics: In the perturbative regime, vortices with c 6 (threshold) and
c 8 (ATI) rotational symmetry were observed. In contrast, non-perturbative excitation
with a π -pulse led to the formation of an electron vortex with c 4 rotational symmetry. These studies demonstrated the potential of the vortex method for quantum state
holography and spectroscopic applications [50].
The use of circularly polarized BiCEPS pulses to create and manipulate photoelectron wave packets with odd-numbered rotational symmetry was reviewed in
Sect. 3.3.4. In those experiments, CEP-stable (3ω:4ω) CRCP and COCP laser pulses
were employed to generate 7-fold rotationally symmetric and asymmetric crescentshaped PMDs by MPI of Na atoms [24]. In addition, the creation odd-numbered
photoelectron vortices from time-delayed (3ω:4ω) pulse sequences were demonstrated. The interplay between the symmetry properties of the driving field and the
resulting electron wave packets was examined by changing optical field parameters
such as the relative phases and the CEP. It was demonstrated that the symmetry of
the generated wave packets is not fully determined by the symmetry of the laser
field, but completely described by multipath quantum interference of states with
different angular momenta. In contrast to the even-numbered photoelectron vortices
from MPI with CRCP single-color pulse sequences, the odd-numbered photoelectron wave packets from bichromatic MPI were shown to exhibit a non-zero azimuthal
probability current and a helical phase front.
To observe ultrafast neutral dynamics in K atoms, we used our pulse shaper to generate continuously tunable and phase-locked two-color pump-probe pulse sequences
with variable time delay. In Sects. 3.3.5 and 3.3.6, we demonstrate applications of
the shaper-based bichromatic pump-probe scheme on Rydberg wave packet dynamics and on spin-orbit wave packet (SOWP) dynamics. For this purpose, the variable
time delay between the two colors of BiCEPS pulse sequence was introduced via
linear spectral phase modulation. Due to the energetic disentanglement of the twocolor signal from both single-color signals of the pump and the probe pulse, resonant
bichromatic frequency mixing was shown to be ideally suited for background-free
pump-probe studies of ultrafast quantum dynamics. In the first shaper-based pumpprobe scheme presented in Sect. 3.3.5, fourth-order frequency mixing was used for
background-free measurement of Rydberg wave packet dynamics [49] with angular and energy resolution. The PMDs from ionization with linearly polarized pump
and probe pulses showed pronounced time-, energy- and angle-dependent variations
resulting from the dynamics of the bound Rydberg wave packets. In Sect. 3.3.6,
we applied the bichromatic pump-probe scheme for time-resolved background-free
spatial imaging of ultrafast SOWP dynamics. The analysis revealed a counterintuitive angular precession of the SOWP: As a result of spin-orbit coupling, the initial
electron orbital alignment in the laser polarization plane evolved into an orthogonal
alignment parallel or coplanar to the laser propagation direction.
Building on our results in coherent control with bichromatic fields, we will extend
our studies on the interplay between the symmetry properties of the light field and
the observed matter waves towards even more refined control schemes and more
71
sequences, were observed at the ionization threshold and in the above threshold ionization (ATI) [21]. The symmetry properties of the vortices depend on the excitation
and ionization dynamics: In the perturbative regime, vortices with c 6 (threshold) and
c 8 (ATI) rotational symmetry were observed. In contrast, non-perturbative excitation
with a π -pulse led to the formation of an electron vortex with c 4 rotational symmetry. These studies demonstrated the potential of the vortex method for quantum state
holography and spectroscopic applications [50].
The use of circularly polarized BiCEPS pulses to create and manipulate photoelectron wave packets with odd-numbered rotational symmetry was reviewed in
Sect. 3.3.4. In those experiments, CEP-stable (3ω:4ω) CRCP and COCP laser pulses
were employed to generate 7-fold rotationally symmetric and asymmetric crescentshaped PMDs by MPI of Na atoms [24]. In addition, the creation odd-numbered
photoelectron vortices from time-delayed (3ω:4ω) pulse sequences were demonstrated. The interplay between the symmetry properties of the driving field and the
resulting electron wave packets was examined by changing optical field parameters
such as the relative phases and the CEP. It was demonstrated that the symmetry of
the generated wave packets is not fully determined by the symmetry of the laser
field, but completely described by multipath quantum interference of states with
different angular momenta. In contrast to the even-numbered photoelectron vortices
from MPI with CRCP single-color pulse sequences, the odd-numbered photoelectron wave packets from bichromatic MPI were shown to exhibit a non-zero azimuthal
probability current and a helical phase front.
To observe ultrafast neutral dynamics in K atoms, we used our pulse shaper to generate continuously tunable and phase-locked two-color pump-probe pulse sequences
with variable time delay. In Sects. 3.3.5 and 3.3.6, we demonstrate applications of
the shaper-based bichromatic pump-probe scheme on Rydberg wave packet dynamics and on spin-orbit wave packet (SOWP) dynamics. For this purpose, the variable
time delay between the two colors of BiCEPS pulse sequence was introduced via
linear spectral phase modulation. Due to the energetic disentanglement of the twocolor signal from both single-color signals of the pump and the probe pulse, resonant
bichromatic frequency mixing was shown to be ideally suited for background-free
pump-probe studies of ultrafast quantum dynamics. In the first shaper-based pumpprobe scheme presented in Sect. 3.3.5, fourth-order frequency mixing was used for
background-free measurement of Rydberg wave packet dynamics [49] with angular and energy resolution. The PMDs from ionization with linearly polarized pump
and probe pulses showed pronounced time-, energy- and angle-dependent variations
resulting from the dynamics of the bound Rydberg wave packets. In Sect. 3.3.6,
we applied the bichromatic pump-probe scheme for time-resolved background-free
spatial imaging of ultrafast SOWP dynamics. The analysis revealed a counterintuitive angular precession of the SOWP: As a result of spin-orbit coupling, the initial
electron orbital alignment in the laser polarization plane evolved into an orthogonal
alignment parallel or coplanar to the laser propagation direction.
Building on our results in coherent control with bichromatic fields, we will extend
our studies on the interplay between the symmetry properties of the light field and
the observed matter waves towards even more refined control schemes and more