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 manuscript 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