High harmonic generation in gases is a tried and tested approach to produce coherent XUV beams for spectroscopic applications. The world’s first multi-terawatt
few-cycle light source LWS-20 provides more than two orders of magnitude higher peak power than used in previous HHG experiments, suggesting a corresponding enhancement in harmonic photon yield per pulse. By applying quasi-phase matching techniques this enhancement factor may increase even further.
An alternative route to generate high-order harmonics of the incident few-cycle laser pulse is offered by reflecting the laser on a relativistically oscillating plasma surface (Fig. A.3.2). Driven by the incident laser field, it acts as a mirror for the laser radiation. The reflected pulse contains high order harmonics due to the relativistic Doppler shift whose pulse duration can be as short as only a few attoseconds.
Since the conversion efficiency is much higher, much more photons than in gas-HHG are expected, but laser pulses with very high temporal contrast are needed to drive the process. This application will benefit from the superior contrast of LWS-20 and, in particular, from the inherently high contrast of the sub-ps-pulse-pumped OPA system PFS-pro which can be further improved by plasma mirrors if required.
Furthermore, as a high-risk approach we shall study the feasibility of Thomson-backscattering off electron bunches accelerated from few-nanometer DLC foils. The energetic attosecond pulses that become available from these sources will allow attosecond pump / attosecond probe spectroscopy to be carried out for the first time. In reaching these goals we want to continue our fruitful collaboration with the group of Matt Zepf at Belfast University that is among the world leaders in this field.