Extended Data Fig. 1: Ellipticity dependence of cutoff is consistent with mean-free-path-limited trajectory model.

(A) Experimental setup for studying the ellipticity dependence of high-harmonic generation from liquids. The rotating half-wave plate and fixed quarter-wave plate geometry is used to keep the axes of the elliptically polarized driving field fixed as the light changes from linear to circular polarization. (B) Harmonic spectra obtained from liquid water at 800 nm for different driver ellipticities. The pink dashed line in each sub-figure represents the cut-off energy for linearly polarized light. The blue dashed line indicates the actual cut-off for each of these spectra. It is observed that as the light changes towards circular polarization (ε = 1), the cut-off shifts to lower energies. (C) Comparison of the experimentally determined change in cut-off and that calculated from the MFP-limited scattering model which considers only recombination with the parent molecule. Details of the theoretical scattering model are given in the supplementary section S3 D. (D) experimental data showing high-harmonic yields vs. ellipticity. (E) Data from cluster calculations showing the high-harmonic yields vs. ellipticity. These calculations were performed using a 900-nm driving field with a peak intensity of 4 × 10¹³W/cm².

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