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Study of attosecond electronic wavepacket dynamics in molecules using High Harmonic Generation

Final Report Summary - MOLWAVE (Study of attosecond electronic wavepacket dynamics in molecules using High Harmonic Generation.)

MolWave: Study of attosecond electronic wavepacket dynamics in molecules using High Harmonic Generation.
Intra-European Fellowship of Dr. Emmanouil SKANTZAKIS
Performed at CEA-Saclay, France, from 10/04/2012 till 09/04/2014
Scientist in charge: Dr. Pascal SALIERES

In the MOLWAVE project, we studied the process of high harmonic generation (HHG) from different molecules in order to probe electronic and nuclear rearrangements on an attosecond timescale (1 attosecond=10-18 second). In the past few years HHG from aligned small molecules has been extensively studied (Refs. 1-11) as a unique means to access both structural and dynamical information on the molecules in strong fields. According to the three-step model for HHG where an Electronic Wave Packet (EWP) first tunnels out the molecular potential, then acquires energy in the continuum and finally recombines with the ionic core resulting in attosecond emission in the XUV domain, the Recombination Dipole Moment (RDM) of the molecule is imprinted into the harmonic emission. Measuring the harmonic intensity, phase and polarization dependence on the angle between the recombining EWP and the molecular axis provides knowledge on the RDM and reveals both attosecond dynamics occurring during HHG (Refs. 1-3) and the structure of the molecular orbital(s) involved in HHG with a precision at the angstrom scale (Refs. 4-6).

First, we have developed a new experimental setup in order to characterize completely the amplitude and phase of the harmonic emission from small linear molecules that are aligned with respect to the polarization of the driving laser. The full mapping of the phase as a function of both photon energy and alignment angle required combining two very complex techniques (RABBIT and 2-source interferometry (TSI)), which had never been done before. This novel approach for the first full mapping of the attosecond emission from aligned molecules was named CHASSEUR (Combined Harmonic Spectroscopy by two-Source EUv interferometry and Rabbit). This technique combines optical (two-source) interferometry (TSI) and quantum interferometry (RABBIT) measurements in the exact same experimental conditions in order to access to both the angular (TSI) and spectral (RABBIT) variations of the phase of the emission. The experimental setup was developed on the LUCA laser of the Service des Photons, Atomes et Molécules (SPAM). The 800nm, 50fs, 20mJ Ti-Sapphire laser is split into three beams: alignment, generation and dressing beam (the latter being necessary for the RABBIT technique to access the spectral phase). The RABBIT scheme used for this work is based on a generating annular beam that is filtered out after generation by an iris while the central part is used as dressing. To make sure that the experimental conditions are the same for both measurements, we developed a two-source interferometry scheme based on the use of a 0-pi phase plate that is placed into the generating beam in order to produce the two laser foci. For TSI, the aligning beam is overlapped spatially with one of the two foci in order to produce harmonics in aligned molecules, the harmonic beam generated in the “unaligned” focus serving as a reference. The shift of the harmonic far-field fringe pattern as a function of the alignment angle of the molecules gives direct access to the angular variation of the harmonic phase. For the RABBIT measurements, the aligning beam is removed and the measurement is performed with both unaligned sources. This provides the spectral variation of the unaligned sample.
We applied this very delicate technique to nitrogen molecules. The reproducibility of the results was high as well as the accuracy of the measurement. In contrast to previous studies using two tilted glass plates inserted in the focused laser beam to produce the two sources that reported no evolution of the phase with the alignment angle in nitrogen, we were able to resolve these subtle phase effects (Figure 1). This state of the art technique can be extended to the investigation of the EWP dynamics using self-probing in small Hydrocarbons, for instance ethylene (C2H4) or acetylene (C2H2). Our preliminary measurements in these two gases revealed two difficulties: the long acquisition time and the restricted spectral extent of the harmonic emission due to the low ionization potential. The continuation of this work thus involved the development of a similar setup on the PLFA laser system of the Service des Photons, Atomes et Molécules (SPAM), which provides pulses with similar pulse durations but at 1 kHz repetition rate instead of 20 Hz, thus improving greatly the statistics of our experiment and, in conjunction with the recently installed TOPAS OPA system, will greatly extend the effective HHG spectrum for investigation.

The present work opens the road to direct comparison with theory and thus combination of attosecond and ångström resolutions in molecular dynamics. This work also opens a wide range of possibilities such as recovering the single molecular response from fully experimental measurements, performing precise quantum orbital tomography without assumptions, or accessing multi-orbital effects by comparing phase maps for different generation conditions.

Additionally, we reported for the first time quantum path-resolved two-source interferometry phase measurements of the HHG from aligned nitrogen and carbon dioxide. In the HHG process are involved two main quantum paths corresponding to two different electron excursion times. Gabor analysis of the fringe interference pattern was performed and revealed a different behaviour for these short and long quantum paths. This provided new insight into the electronic dynamics occurring during the HHG process when several orbitals are involved in the generation: indeed, this simultaneous characterization of the different quantum paths allows probing the dynamics of the ion in strong field with subcycle resolution.

Finally, towards the development of a sub-cycle waveform synthesizer, a collinear variation of an interferometric polarization gating device was designed for the Carrier-Envelope Phase (CEP) absolute measurement and tagging of the many-cycle laser sources which are operating at SPAM. This device consists in 1 zero-order half waveplate, 1 multi-order quarter waveplate, 1 zero-order quarter waveplate and finally 1 silicon plate for the fine adjustment of the ratio between the electric field components in the output of the device. Special design was needed for the adaption of this technique in the laser system. Ray tracings, SFA calculations and analytical calculations for the dispersion introduced by the optics that will be used for the implementation of this setup were performed.

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