Terahertz (THz) nonlinear optics is an emerging field enabled by recent developments in the generation of THz pulses with higher intensities. THz radiation can interact with the vibrational/rotational resonances of molecules. For example, vapour in the atmosphere shows a very rich absorption spectrum with hundreds of sharp resonances below 3 THz. We report on the first experimental demonstration of the nonlinear interaction of THz pulses with water vapour. We observed a strong nonlinear response of the vibration/rotation resonances to the THz pulses in the form of reverse saturation of absorption.
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