2018-06-11, Kübel, M., Arbeiter, M., Burger, C., Kling, Nora G., Pischke, T., Moshammer, R., Fennel, T., Kling, M.F., Bergues, B.

We investigate the carrier-envelope phase (CEP) and intensity dependence of the longitudinal momentum distribution of photoelectrons resulting from above threshold ionization of argon by few-cycle laser pulses. The intensity of the pulses with a center wavelength of 750 nm is varied in a range between 0.7 × 1014 and . Our measurements reveal a prominent maximum in the CEP-dependent asymmetry at photoelectron energies of 2 U P (U P being the ponderomotive potential), that is persistent over the entire intensity range. Further local maxima are observed around 0.3 and 0.8 U P. The experimental results are in good agreement with theoretical results obtained by solving the three-dimensional time-dependent Schrödinger equation. We show that for few-cycle pulses, the amplitude of the CEP-dependent asymmetry provides a reliable measure for the peak intensity on target. Moreover, the measured asymmetry amplitude exhibits an intensity-dependent interference structure at low photoelectron energy, which could be used to benchmark model potentials for complex atoms.

2018-05-11, Pisanty, Emilio, Hickstein, Daniel D., Galloway, Benjamin R., Durfee, Charles G., Kapteyn, Henry C., Murnane, Margaret M., Ivanov, Misha

The interaction of intense mid-infrared laser fields with atoms and molecules leads to a range of new opportunities, from the production of bright, coherent radiation in the soft x-ray range, to imaging molecular structures and dynamics with attosecond temporal and sub-angstrom spatial resolution. However, all these effects, which rely on laser-driven recollision of an electron removed by the strong laser field and its parent ion, suffer from the rapidly increasing role of the magnetic field component of the driving pulse: the associated Lorentz force pushes the electrons off course in their excursion and suppresses all recollision-based processes, including high harmonic generation as well as elastic and inelastic scattering. Here we show how the use of two non-collinear beams with opposite circular polarizations produces a forwards ellipticity which can be used to monitor, control, and cancel the effect of the Lorentz force. This arrangement can thus be used to re-enable recollision-based phenomena in regimes beyond the long-wavelength breakdown of the dipole approximation, and it can be used to observe this breakdown in high harmonic generation using currently available light sources.

2018-05-30, Ayuso, David, Decleva, Piero, Patchkovskii, Serguei, Smirnova, Olga

The generation of high-order harmonics in a medium of chiral molecules driven by intense bi-elliptical laser fields can lead to strong chiroptical response in a broad range of harmonic numbers and ellipticities (Ayuso et al 2018 J. Phys. B: At. Mol. Opt. Phys. 51 06LT01). Here we present a comprehensive analytical model that can describe the most relevant features arising in the high-order harmonic spectra of chiral molecules driven by strong bi-elliptical fields. Our model recovers the physical picture underlying chiral high-order harmonic generation (HHG) based on ultrafast chiral hole motion and identifies the rotationally invariant molecular pseudoscalars responsible for chiral dynamics. Using the chiral molecule propylene oxide as an example, we show that one can control and enhance the chiral response in bi-elliptical HHG by tailoring the driving field, in particular by tuning its frequency, intensity and ellipticity, exploiting a suppression mechanism of achiral background based on the linear Stark effect.

2017-07-10, Bredtmann, Timm, Patchkovskii, Serguei, Ivanov, Misha Yu

Using ab-initio simulations, we demonstrate amplification of extreme-ultraviolet (XUV) radiation during transient absorption in a high-harmonic generation type process using the example of the hydrogen atom. The strong IR driving field rapidly depletes the initial ground state while populating excited electronic states through frustrated tunnelling, thereby creating a population inversion. Concomitant XUV lasing is demonstrated by explicit inclusion of the XUV seed in our simulations, allowing a thorough analysis in terms of this transient absorption setup. Possibilities for increasing this gain, e.g. through preexcitation of excited states, change of the atomic gain medium or through multi-center effects in molecules, are demonstrated. Our findings should lead to a reinterpretation of recent experiments.

2018-02-28, Ayuso, David, Decleva, Piero, Patchkovskii, Serguei, Smirnova, Olga

The application of strong bi-elliptically polarized laser fields to the generation of high-order harmonics in organic molecules offers exceptional opportunities for chiral recognition and chiral discrimination. These fields are made by combining an elliptically polarized fundamental, typically in the infrared range, with its counter-rotating second harmonic. Here we present a theoretical study of the harmonic emission from the chiral molecule propylene oxide in bi-elliptical fields. Our calculations include, for the first time in such a complex system, accurate photorecomination matrix elements, evaluated using the static-exchange density functional theory method. We show that bi-elliptical light can induce strong chiral dichroism in the harmonic spectra of chiral molecules in a broad range of harmonic numbers and ellipticities.