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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...

The emerging techniques of molecular spectroscopy by high order harmonic generation have hitherto been conducted only with Ti:Sapphire lasers which are restricted to molecules with high ionization potentials. In order to gain information on the molecular structure, a broad enough range of harmonics is required. This implies using high laser intensities which would saturate the ionization of most molecular systems of interest, e.g. organic molecules. Using a laser at 1300 nm, we are able to extend the technique to molecules with relatively low ionization potentials (approximately 11 eV), observing wide harmonic spectra reaching up to 60 eV. This energy range improves spatial resolution of the high harmonic spectroscopy to the point where interference minima in harmonic spectra of N(2)O and C(2)H(2) can be observed.

We demonstrate enhancement by 1 order of magnitude of the high-order harmonics generated in argon by combining a fundamental field at 1300 nm (10(14) W cm(-2)) and its orthogonally polarized second harmonic at 650 nm (2 × 10(13) W cm(-2)) and by controlling the relative phase between them. This extends earlier work by ensuring that the main effect is the combined field steering the electron trajectory with negligible contribution from multiphoton effects compared to the previous schemes with 800/400 nm fields. We access a broad energy range of harmonics (from 20 eV to 80 eV) at a low laser intensity (far below the ionization saturation limit) and observe deep modulation of the harmonic yield with a period of π in the relative phase. Strong field theoretical analysis reveals that this is principally due to the steering of the recolliding electron wave packet by the two-color field. Our modeling also shows that the atto chirp can be controlled, leading to production of shorter pulses.

We investigate how short and long electron trajectory contributions to high harmonic emission and their interferences give access to intra-molecular dynamics. In the case of unaligned molecules, we show experimental evidences that the long trajectory signature is more dependent upon the molecule than the short one, providing a high sensitivity to cation nuclear dynamics within 100's of as to few fs. Using theoretical approaches based on Strong Field Approximation and Time Dependent Schödinger Equation, we examine how quantum path interferences encode electronic motion whilst molecules are aligned. We show that the interferences are dependent on channels superposition and upon which ionisation channel is involved. In particular, quantum path interferences encodes electronic migration signature while coupling between channels is allowed by the laser field. Hence, molecular quantum path interferences is a promising method for Attosecond Spectroscopy, allowing the resolution of ultra-fast charge migration in molecules after ionisation in a self-referenced manner.