  We show how to optimize the process of high-harmonic generation (HHG) by
gating the interaction using the field gradient of a driving pulse with a
linear ramp waveform. Since maximized field gradients are efficiently generated
by self-steepening processes, we first present a generalized theory of optical
carrier-wave self-steepened (CSS) pulses. This goes beyond existing treatments,
which only consider third-order nonlinearity, and has the advantage of
describing pulses whose wave forms have a range of symmetry properties.
Although a fertile field for theoretical work, CSS pulses are difficult to
realize experimentally because of the deleterious effect of dispersion. We
therefore consider synthesizing CSS-like profiles using a suitably phased
sub-set of the harmonics present in a true CSS wave form. Using standard
theoretical models of HHG, we show that the presence of gradient-maximized
regions on the wave forms can raise the spectral cut-off and so yield shorter
attosecond pulses. We study how the quality of the attosecond bursts created by
spectral filtering depends on the number of harmonics included in the driving
pulse.
