Fig. 8

Pulse duration dependent spectral shifts of HHG peaks. A bulk ZnO single crystal (c-cut, 300 um thickness) was driven by MIR pulses with a central wavelength of 3.8 \(\upmu\)m, and HHG spectra were recorded for varying second order spectral phases applied to the Dazzler. In the figure we highlight the 9th and 13th harmonic order peaks. The pulse duration is lengthened from 60 fs to 160 fs as the dispersion is increased from – 2.4 \(\times 10^{4}\) fs\(\phantom{0}^{2}\) to – 1.9 \(\times 10^{4}\) fs\(\phantom{0}^{2}\). The optimal, near-FTL pulses are produced with a GDD near – 2.3 \(\times 10^{4}\) fs\(\phantom{0}^{2}\). As the magnitude of the dispersion increases, the temporal duration of the pulse is lengthened and the spectral peaks shift toward higher energies.