The nonlinear refractive index of water can be as large as 7 × 10–10 cm2 W–1 in the THz frequency range — a million times larger than the value in the visible and near-infrared, according to Russian researchers. The finding confirms earlier theoretical predictions that ionic vibrations in water generate large THz nonlinearities. Anton Tcypkin and co-workers from ITMO University in St. Petersburg used a broadband, pulsed THz beam and the Z-scan method to make the measurements. Terahertz pulses with a pulse energy of 400 nJ, a duration of 1 ps and a spectral width of 0.1–2.5 THz were generated by optical rectification of femtosecond laser pulses by a lithium niobate crystal. A parabolic mirror with a short focal length of 12.5 mm was then used to focus the THz pulses onto a 0.1-mm-thick water jet that traverses the THz beam at right angles. The water jet is scanned a distance of ±4 mm along the direction of the beam (z axis) so that it passes through the beam’s focal point. After passing through the water jet the THz beam is collimated by a second parabolic mirror and detected by a Golay cell.