Phase-matched few-cycle high-harmonic generation: ionisation gating and half-cycle cutoffs

For the direct exploration of electron dynamics in molecules, e.g. during a chemical reaction, a short pulsed radiation source is required, delivering flashes of duration less than a femtosecond. Due to their wavelengths conventional laser pulses cannot be shortened enough to reach such pulse durations. High-harmonic generation (HHG) is currently the key to the subfemtosecond regime with wavelengths in the extreme-ultraviolet and soft-X-ray range. HHG is a very inefficient process and, therefore, the radiation produced by every atom involved has to be phase-matched to obtain a macroscopic signal. The intrinsic characteristics of phase matching provide the possibility to produce even single attosecond pulses. A simulation will show, how phase matching acts as temporal gate and allows HHG only at the leading-edge of the driving laser pulse. The behaviour of the leading-edge gating will be analysed for different experimental conditions, such as peak intensity of the laser pulse, density of the gaseous generation medium and the distance between focus and generation region. Half-cycle cutoff (HCO) analysis allows an experimental access to observing the leading-edge gate, that will be compared to the simulation. The HCO-analysis can also be used to estimate the duration of the driving laser pulse. In addition the position- and pressure dependence of the HHG process will be analysed, too