Fig. 1: Experimental set-up for liquid-phase HHG and observation of second plateaus in experiment and theory.

a, Schematic of the experimental set-up. Tunable-wavelength (here 1,800 nm) laser pulses are focused on a flat liquid jet to generate high harmonics. The harmonics pass through a slit, are dispersed by a grating and detected with an MCP-phosphor-screen-camera system. Inset: a schematic of the first and second solvation shells marked by the dashed black arcs. b, Raw MCP image of high-harmonic spectrum observed from D₂O liquid acquired at 4.8 × 10¹³ W cm⁻², showing the presence of a second-plateau feature. c, Experimentally observed high-harmonic spectrum from D₂O liquid acquired at 4.8 × 10¹³ W cm⁻². In addition to a first plateau, a second plateau is observed from 19 eV upwards, with a cut-off energy (E_c) at ~23 eV. d, Harmonic spectrum of liquid H₂O from ab initio simulations⁵⁹ at 1,800 nm and 5 × 10¹³ W cm⁻² (intensity averaged), showing a second plateau with cut-off E_c at ~20 eV. Red-dashed lines mark cut-off energies, defined at the onset of exponential decay within one-harmonic uncertainty. Norm., normalized.