  High-harmonic generation is one of the most extreme nonlinear-optical
processes observed to date. By focusing an intense laser pulse into a gas, the
light-atom interaction that occurs during the process of ionising the atoms
results in the generation of harmonics of the driving laser frequency, that
extend up to order ~300 (corresponding to photon energies from 4 to >500eV).
Because this technique is simple to implement and generates coherent,
laser-like, soft-x-ray beams, it is currently being developed for applications
in science and technology including probing of dynamics in chemical and
materials systems and for imaging. In this work we demonstrate that by
carefully controlling the shape of intense light pulses of 6-8 optical cycles,
we can control the interaction of light with an atom as it is being ionised, in
a way that improves the efficiency of x-ray generation by an order of
magnitude. Furthermore, we demonstrate that it is possible to control the
spectral characteristics of the emitted radiation and to channel the
interaction between different-order nonlinear processes. The result is an
increased utility of harmonic generation as a light source, as well as the
first demonstration of optical pulse-shaping techniques to control high-order
nonlinear processes.
