Control of HHG with Sculpted Multi-Color Light Waves

At the heart of strong-field phenoma, of which high harmonic generation (HHG) is one of the most prominent examples, lie electron trajectories which start with tunnel ionization and in most cases end ~1 fs later with recollision of the continuum electron with the ion it had left behind. These trajectories are directly driven by the oscillating electric field of laser pulses, such that complete control over the waveform of the light implies direct control of the ultrafast electron trajectories. Achieving this ground-breaking level of control has recently come within reach when the group of Andrius Baltuška at the Photonics Institute in Vienna have demonstrated an optical parametric amplifier (OPA) which generates signal and idler pulses at 1.5 m and 3.0 m, respectively, which are phase locked to the actively carrier-envelope phase (CEP) stabilized Yb-based pump laser operating at 1.0m wavelength. With these three mutually and absolutely phase locked color components, light waveform synthesis becomes possible. In this project, we propose to develop, based on the technology already demonstrated in Vienna, a multi-color waveform synthesizer which is sufficiently intense to drive HHG. To this end, a CEP-stabilized, <200 fs pump laser with mJ pulse energies at 1kHz will be developed based on the Yb,Na:CaF2 amplifier crystal already well proven in Vienna and a multi-color interferometer with sub-laser-cycle stability will be implemented. We will then demonstrate the enabling nature of the multi-color OPA source by showing how an appropriately designed waveform allows the generation of high harmonics with higher photon energies and significantly superior efficiency as compared to the benchmark set by a single-color laser pulse driving HHG in the saturated regime (i.e. medium ionization limited). To this end, the ionization probability will have to be concentrated at few selected instants during the pulse after each of which an optimized accelerating field half-cycle drives the electrons to very high kinetic energies during an as-short-as-possible excursion time. Such a waveform, dubbed "the perfect wave``, has recently been proposed by the group of Jon Marangos of the Imperial College London, with whom we will work together in the framework of this project. This demonstration will prove the potential of electron trajectory engineering, e.g. for generating intense femtosecond or even attosecond x-ray pulses in the "water-window"or for probing attosecond dynamics in molecules with an engineered recolliding electron wave packet.