Trajectory Selection in High Harmonic Generation by Controlling the Phase between Orthogonal Two-Color Fields

Abstract

We demonstrate control of short and long quantum trajectories in high harmonic emission through the use of an orthogonally polarized two color field. By controlling the relative phase φ between the two fields we show via classical and quantum calculations that we can steer the 2-dimensional trajectories to return, or not, to the core and so control the relative strength of the short or long quantum trajectory contribution. In experiments we demonstrate that this leads to robust control over the trajectory contributions using a drive field from a femtosecond laser composed of the fundamental ω at 800 nm (intensity ∼ 1.2 × 10 14 W cm −2 ) and its weaker orthogonally polarized second harmonic 2ω (intensity ∼ 0.3 × 10 14 W cm −2 ) with the relative phase between the ω and 2ω fields varied simply by tilting a fused silica plate. This is the first demonstration of short and long quantum trajectory control at the single-atom level.High harmonic generation (HHG) has been extensively studied in the last decade as, for instance, a tool to generate attosecond pulses [1] and to measure both fast nuclear dynamics [2] and hole migration in molecular cations [3,4]. Measuring nuclear and electron dynamics from the emitted harmonics is termed HHG spectroscopy and has hitherto concentrated upon isolating the contribution of the electron trajectories that return most quickly to the core [5] (the short trajectories) to provide a well defined temporal mapping for the emission of different frequency harmonics in the spectrum [2]. To extend these measurement concepts we would like to harness the second set of quantum trajectories, the long trajectories, which return after a longer time in the continuum and so increase the temporal range available in the measurement. Ideally this should be done without changing any other aspect of the experimental conditions e.g. the intensity. Hitherto long trajectories could only be optimised by adjusting the macroscopic phase-matching, a procedure that inevitably alters the experimental intensity. We demonstrate a simple experimental technique that provides a powerful tool to achieve the direct selection of the quantum trajectories for a single atom without changing the field intensity. We find that the phase between two orthogonally polarized fields at ω and 2ω determines whether the momentum transfer from the 2ω field permits or frustrates the recollision. We observed that the phases of the second harmonic field that optimise the recollision differ by π/2 for the short and long trajectory. This provides robust control over the single atom quantum trajectories and allows to efficiently switch between trajectories, shifting the emission time for some harmonics by more than 0.3 of an optical period.In the simplified picture, commonly applied to describe HHG, an electron is ionised near the peak of the laser electric field and is driven away from its parent ion. When the field changes direction, the electron is driven back and may recombine [5], emitting a photon with fre- quency that is an odd multip...

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