  Attosecond science promises to reveal the most fundamental electronic
dynamics occurring in matter and it can develop further by meeting two linked
technological goals related to high-order harmonic sources: higher photon flux
(permitting to measure low cross-section processes) and improved spectral
tunability (allowing selectivity in addressing specific electronic
transitions). New developments come through parametric waveform synthesis,
which provides control over the shape of high-energy electric field transients,
enabling the creation of highly-tunable isolated attosecond pulses via
high-harmonic generation. Here we show that central energy, spectral
bandwidth/shape and temporal duration of the attosecond pulses can be
controlled by shaping the laser pulse waveform via two key parameters: the
relative-phase between two halves of the multi-octave spanning optical
spectrum, and the overall carrier-envelope phase. These results not only
promise to expand the experimental possibilities in attosecond science, but
also demonstrate coherent strong-field control of free-electron trajectories
using tailored optical waveforms.
