Enhancement of high harmonics generated by field steering of electrons in a two-color orthogonally polarized laser field

Brugnera, Leonardo; Frank, F.; Hoffmann, David; Torres, Ricardo; Siegel, Thomas; Underwood, Jonathan G.; Springate, E.; Froud, C. A.; Turcu, E.; Tisch, J. W. G.; Marangos, Jonathan P.

doi:10.1364/ol.35.003994

Cited by 51 publications

(34 citation statements)

References 13 publications

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“…Other applications include the field-free orientation of molecules [16][17][18], the generation of high-harmonic spectra [19][20][21][22] and probing atomic and molecular orbital symmetry [23][24][25].…”

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confidence: 99%

Controlling electron-electron correlation in frustrated double ionization of triatomic molecules with orthogonally polarized two-color laser fields

2017

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We demonstrate the control of electron-electron correlation in frustrated double ionization (FDI) of the two-electron triatomic molecule D + 3 when driven by two orthogonally polarized two-color laser fields. We employ a three-dimensional semi-classical model that fully accounts for the electron and nuclear motion in strong fields. We analyze the FDI probability and the distribution of the momentum of the escaping electron along the polarization direction of the longer wavelength and more intense laser field. These observables when considered in conjunction bear clear signatures of the prevalence or absence of electron-electron correlation in FDI, depending on the time-delay between the two laser pulses. We find that D + 3 is a better candidate compared to H 2 for demonstrating also experimentally that electron-electron correlation indeed underlies FDI.PACS numbers: 33.80. Rv, 34.80.Gs, 42.50.Hz Frustrated double ionization (FDI) is a major process in the nonlinear response of multi-center molecules when driven by intense laser fields, accounting roughly for 10% of all ionization events [1,2]. In frustrated ionization an electron first tunnel-ionizes in the driving laser field. Then, due to the electric field of the laser pulse, it is recaptured by the parent ion in a Rydberg state [3]. This process is a candidate for the inversion of N 2 in free-space air lasing [4]. In FDI an electron escapes and another one occupies a Rydberg state at the end of the laser pulse. In theoretical studies of strongly-driven two-electron diatomic and triatomic molecules, two pathways of FDI have been identified [2,8]. Electron-electron correlation is important, primarily, for one of the two pathways. It is well accepted that electron-electron correlation underlies a significant part of double ionization in strongly-driven molecules-a mechanism known as non-sequential double ionization [9,10]. However, electron-electron correlation in FDI has yet to be accessed experimentally.Here, we propose a road for future experiments to identify the important role of electron-electron correlation in FDI. We identify the parameters of orthogonally polarized two-color (OTC) laser fields that best control the relevant pathway for electron-electron correlation in FDI. We demonstrate traces of attosecond control of electron motion in space and time in two observables of FDI as a function of the time-delay between the fundamental 800 nm and the second harmonic 400 nm laser field. We show that, together, the FDI probability and the momentum of the escaping electron along the fundamental laser field bear clear signatures of the turning on and off of electronelectron correlation.Two-color laser fields are an efficient tool for controlling electron motion [11,12] and for steering the outcome of chemical reactions [13][14][15]. Other applications include the field-free orientation of molecules [16][17][18], the generation of high-harmonic spectra [19][20][21][22] and probing atomic and molecular orbital symmetry [23][24][25]. The strongly-driven dynam...

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Controlling electron-electron correlation in frustrated double ionization of triatomic molecules with orthogonally polarized two-color laser fields

2017

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“…This scheme, involving only a frequency doubling crystal, was extensively investigated over the past years. In particular, it was established that harmonic pulses, containing both odd and even orders, can be generated with improved efficiency [22][23][24][25][26] compared with single-colour harmonic generation. The polarization of the harmonics, generated in such a specific case of orthogonally polarized o and 2o laser fields in a centrally symmetric gas medium, was not measured so far and was expected to be linear 27 .…”

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Towards enabling femtosecond helicity-dependent spectroscopy with high-harmonic sources

et al. 2015

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Recent advances in high-harmonic generation gave rise to soft X-ray pulses with higher intensity, shorter duration and higher photon energy. One of the remaining shortages of this source is its restriction to linear polarization, since the yield of generation of elliptically polarized high harmonics has been low so far. We here show how this limitation is overcome by using a cross-polarized two-colour laser field. With this simple technique, we reach high degrees of ellipticity (up to 75%) with efficiencies similar to classically generated linearly polarized harmonics. To demonstrate these features and to prove the capacity of our source for applications, we measure the X-ray magnetic circular dichroism (XMCD) effect of nickel at the M 2,3 absorption edge around 67 eV. There results open up the way towards femtosecond time-resolved experiments using high harmonics exploiting the powerful element-sensitive XMCD effect and resolving the ultrafast magnetization dynamics of individual components in complex materials.

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“…In addition, a new set of parameters becomes available to control the HHG emission through the twodimensional steering of the continuum electron by the bi-chromatic field [7]. It has been shown that two orthogonally polarized fields can be successfully used for imaging atomic wavefunctions [8] or the generation of shorter attosecond pulses [9,10]. It has been proposed that by controlling the relative phase of the two pulses, long and short trajectory selection can be achieved [11].…”

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

et al. 2011

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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...

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Enhancement of high harmonics generated by field steering of electrons in a two-color orthogonally polarized laser field

Cited by 51 publications

References 13 publications

Controlling electron-electron correlation in frustrated double ionization of triatomic molecules with orthogonally polarized two-color laser fields

Controlling electron-electron correlation in frustrated double ionization of triatomic molecules with orthogonally polarized two-color laser fields

Towards enabling femtosecond helicity-dependent spectroscopy with high-harmonic sources

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

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