Role of the ionic potential in high harmonic generation.

Recollision processes provide direct insight into the structure and dynamics of electronic wave functions. However, the strength of the process sets its basic limitations--the interaction couples numerous degrees of freedom. In this Letter we decouple the basic steps of the process and resolve the role of the ionic potential which is at the heart of a broad range of strong field phenomena.

Beam distortion detection and deflectometry measurements of gigahertz surface acoustic waves.

Gigahertz acoustic waves propagating on the surface of a metal halfspace are detected using different all-optical detection schemes, namely, deflectometry and beam distortion detection techniques. Both techniques are implemented by slightly modifying a conventional reflectometric setup. They are then based on the measurement of the reflectivity change but unlike reflectometric measurements, they give access to the sample surface displacement.

Conical intersection dynamics in NO2 probed by homodyne high-harmonic spectroscopy.

Conical intersections play a crucial role in the chemistry of most polyatomic molecules, ranging from the simplest bimolecular reactions to the photostability of DNA. The real-time study of the associated electronic dynamics poses a major challenge to the latest techniques of ultrafast measurement. We show that high-harmonic spectroscopy reveals oscillations in the electronic character that occur in nitrogen dioxide when a photoexcited wave packet crosses a conical intersection.

High harmonic spectroscopy of multichannel dynamics in strong-field ionization.

We perform high harmonic generation spectroscopy of aligned nitrogen molecules to characterize the attosecond dynamics of multielectron rearrangement during strong-field ionization. We use the spectrum and ellipticity of the harmonic light to reconstruct the relative phase between different ionization continua participating in the ionization, and thus determine the shape and location of the hole left in the molecule by strong-field ionization. Our interferometric technique uses transitions between the ionic states, induced by the laser field on the subcycle time scale.

High-order harmonic generation at a megahertz-level repetition rate directly driven by an ytterbium-doped-fiber chirped-pulse amplification system.

We report the first experimental demonstration (to our knowledge) of high-order harmonic generation in rare gases driven by a state-of-the-art high-power Yb-doped-fiber chirped-pulse amplification system. The fiber laser delivers 270 fs pulses in the 30-100 microJ energy range at repetition rates varying from 100 kHz to 1 MHz. A proper focalization allows reaching several 10(13) W/cm2 in a gas jet.