  In modern terahertz (THz) sensing and imaging spectroscopy, water is
considered a nemesis to be avoided due to strong absorption in the THz
frequency range. Here we report the first experimental demonstration and
theoretical implications of using femtosecond laser pulses to generate intense
broadband THz emission from water vapor. When we focused an intense laser pulse
in water vapor contained in a gas cell or injected from a gas jet nozzle, an
extraordinarily strong THz field from optically excited water vapor is
observed. Water vapor has more than 50% greater THz generation efficiency than
dry nitrogen. It had previously been assumed that the nonlinear generation of
THz waves in this manner primarily involves a free-electron plasma, but we show
that the molecular structure plays an essential role in the process. In
particular, we found that THz wave generation from H2O vapor is significantly
stronger than that from D2O vapor. Vibronic activities of water cluster ions,
occurring naturally in water vapor, may possibly contribute to the observed
isotope effect along with rovibrational contributions from the predominant
monomers.
