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 proc
esses, 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.
We show how to adapt a 0-f self-referencing technique to provide a single shot absolute Carrier Envelope Phase (CEP) measurement by using the CEP reference provided by difference frequency generation (DFG) between the spectral wings of the fundamenta
l pulse. Usually, the beat between the input pulse and the DFG signal then provides feedback with which to stabilize the CEP slip in a pulse train. However, with a simple extension we can get a single shot absolute CEP measurement. Success relies on having well characterized input pulses, and the use of accurate propagation models through the nonlinear crystal -- these enable us to construct a mapping between the experimental measurement and the CEP of the optical pulse.
