Fig. 2: Intensity and wavelength dependence of the second plateau in HHG from liquids.

a, Intensity dependence: experimental HHG spectra measured in D₂O liquid at varying driving intensity at 1,200 nm. b, Wavelength dependence: experimental liquid D₂O HHG spectra measured at 1,800 nm and 1,200 nm at similar driving peak intensity. c, Intensity dependence: ab initio simulations of intensity-dependent HHG spectra driven at 900 nm in liquid H₂O. d, Wavelength dependence: ab initio HHG simulations for 900 nm and 1,800 nm wavelength for liquid H₂O at the same driving intensity. This data establishes a very weak wavelength and intensity dependence of the second-plateau cut-off energy both in experiments and theory. All spectra have been normalized with respect to the maximum within the 18–35 eV range. Normalized spectra recorded with lower driving intensities or lower wavelengths, respectively, have been shifted upwards for visualization purposes. For additional data, see Supplementary Fig. 8.