T3: Strong field excitation and HHG in tailored fields

The full understanding of the physics of strong-field excitation and dissociation of molecules, involving also the formation of excited neutral fragments requires a joint effort from theory and experiment. Apart from being fundamentally significant, we expect real world applications to emerge, for example in the context of backward lasing observed in laser driven filaments.

Fig. 1. By changing the position of the focus with respect to the generating medium, we can control the ellipticity of the generated harmonics. This changes the contrast between the 3N+1 vs 3N+2 harmonics, so that the attosecond pulses change their polarization from close to linear to highly elliptical (e~ 75%). This is a simple and practical path to control the polarization of the generated harmonic fields in a easy and predictable fashion.

We study and aim to shed light on the fundamental processes that drive excitation, and aim to control the relative population of laser-dressed states, thus preparing a medium that is optimized for population inversion. The closely related subjects of atomic strong-field excitation and acceleration also place a special focus on non-dipole effects and on extensions towards ions and dense gases, with the prospect of unraveling the prominence of Kramers-Henneberger (laser-dressed) states in the dynamics. Non-dipole and propagation effects will be added to the available theoretical tool-set.

Another important goal pursued within this topic is to perform HHG spectroscopy experiments of chiral molecules using tailored light fields, including multi-color schemes and a non-collinear geometry. Here we are following a bicircular approach to generate attosecond pulses with a high degree of ellipticity in the form of isolated pulses (IAP) and pulse trains (APT). We use a fundamental theoretical analysis and implement an experimental realization for characterization of attosecond pulses with time-dependent ellipticity. We develop schemes for the generation of highly elliptical XUV radiation using multi-color fields in a collinear as well as in noncollinear geometries. We perform XUV polarimetry experiments based on reflective phase-retarding optics and X-ray Magnetic Circular Dichroism measurements, in connection with investigations of ultrafast processes in magnetic systems (see project 3.2).

Original publication

Extended ellipticity control for attosecond pulses by high harmonic generation

N. Zhavoronkov, M. Ivanov

Optics Letters 42 (2017) 4720-4723

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