Subcycle Control of Electron-Electron Correlation in Double Ionization
Li Zhang,
1
Xinhua Xie,
1
Stefan Roither,
1
Yueming Zhou,
2,3
Peixiang Lu,
2,3,*
Daniil Kartashov,
1
Markus Schöffler,
1
Dror Shafir,
5
Paul B. Corkum,
4
Andrius Baltuška,
1
André Staudte,
4
and Markus Kitzler
1,†
1
Photonics Institute, Vienna University of Technology, A-1040 Vienna, Austria
2
School of Physics, Huazhong University of Science and Technology, and
Wuhan National Laboratory for Optoelectronics, Wuhan 430074, China
3
Key Laboratory of Fundamental Physical Quantities Measurement of Ministry of Education, Wuhan 430074, China
4
Joint Laboratory for Attosecond Science of the National Research Council and the University of Ottawa,
Ottawa, Ontario, Canada K1A 0R6
5
Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel
(Received 9 April 2013; revised manusc ript received 26 February 2014; published 14 May 2014)
Double ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using
coincidence momentum imaging. We show that the two-electron emission dynamics in nonsequential
double ionization can be controlled by tuning the subcycle shape of the electric field of the OTC pulses. We
demonstrate experimentally switching from correlated to anticorrelated two-electron emission, and control
over the directionality of the two-electron emission. Simulations based on a semiclassical trajectory model
qualitatively explain the experimental results by a subcycle dependence of the electron recollision time on
the OTC field shape.
DOI: 10.1103/PhysRevLett.112.193002 PACS numbers: 33.20.Xx, 32.80.Rm
Angström and attosecond control of free electron wave
packets is one of the pinnacles of attosecond science.
Orthogonally polarized two-color (OTC) laser fields allow
us to control the motion of field-ionizing electronic wave
packets both in time and space [1,2]. In OTC pulses time
and space are connected and thus an attosecond time scale
is established in the polarization plane for both the emitted
and the recolliding wave packets [3,4]. OTC pulses have
been proposed to increase the efficiency [5] and to allow
control over the polarization state of high-harmonic radi-
ation [6], and they have been used to interrogate atomic and
molecular orbital structure [7–9] via high harmonic radi-
ation. The ability to steer electrons with two-color laser
fields has led to proposals for using them in laser induced
electron diffraction [10] and double ionization [11,12].
Here, we report on experiments and semiclassical sim-
ulations of nonsequential double ionization (NSDI) in
orthogonally polarized 800 and 400 nm laser fields. We
show that these OTC fields provide control over the emission
dynamics of two electrons from neon atoms in the intensity
regime of NSDI. By manipulating the subcycle shape of the
OTC field we demonstrate switching from correlated to
anticorrelated two-electron emission along the polarization
direction of the fundamental field. Simultaneously, the OTC
pulses provide control over the two-electron emission
direction along the second-harmonic field axis, similar to
a single color carrier-envelope phase stabilized few cycle
pulse [13]. Finally, we find that the NSDI rate in OTC pulses
is very sensitive to the initial transverse momentum of the
recolliding electron.
In our experiments the OTC pulses were produced by
combining an 800 nm laser pulse, frequency ω, and its
second harmonic pulse, frequency 2ω, polarized along x and
z, respectively, in a collinear geometry at a rate of 5 kHz. The
laser peak intensity in either color was I
800 nm
¼ I
400 nm
¼
ð2 0.2Þ × 10
14
W=cm
2
. The durations (FWHM) of the
fundamental (46 fs) and second harmonic pulse (48 fs)
were measured by using second-harmonic frequency
resolved optical gating (FROG) and self-diffraction
FROG, respectively. Temporal overlap of the two pulses
was ensured by compensating for their different group
velocities with calcite plates and a pair of fused silica
wedges. The electric field of the OTC pulses can be written
as (atomic units are used unless otherwise stated)
~
EðtÞ¼
f
x
ðtÞ cosðωtÞ
~
e
x
þ f
z
ðtÞ cosð2 ωt þ ΔφÞ
~
e
z
, with Δφ the
relative phase of the two colors. Variations of Δφ by fine
steps of one of the wedges allows us to control the waveform
of the OTC pulse on a subcycle time scale [1,3]. The three-
dimensional momentum vector of electrons and ions emitted
from neon atoms upon interaction with the OTC pulses was
measured as a function of Δφ using cold-target recoil-ion
momentum spectroscopy (COLTRIMS) [14]. The relative
phase was calibrated by the peaks of the Ne
þ
yield
modulation measured at lower intensities [15,16].The
COLTRIMS setup was described in detail previously [15].
In short, electrons and ions created in the laser focus were
guided by weak uniform electric (1.8 V=cm) and magnetic
(10.5 G) fields onto two multihit position- and time-sensitive
detectors with delay line anodes for position readout. The ion
rate was adjusted to ≈0.3 per laser shot. From the measured
time of flight and position of each particle, its three dimen-
sional momentum vector was calculated.
The process of NSDI, i.e., the emission of two electrons
from an atom or molecule due to inelastic scattering of an
PRL 112, 193002 (2014)
PHYSICAL REVIEW LETTERS
week ending
16 MAY 2014
0031-9007=14=112(19)=193002(5) 193002-1 © 2014 American Physical Society
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