Terahertz Nonlinear Spectroscopy of Water Vapor

The water molecule has millions of rotational transitions in the infrared and terahertz region, and the resulting absorption features dominate the transmission spectrum of atmospheric terahertz measurements, even for propagation distances as short as tens of centimeters. Remarkably, we observe that the absorption at the position of these resonances increases with the THz intensity. We explain this observation by stepwise multiphoton transitions in water molecules. The estimated value of the effective third-order susceptibility is extremely large: χ(3) = (0.4 + 6i) × 102 m2/V2. We note for comparison that the value of χ(3) for fused silica at optical frequencies is on the order of 10–22 m2/V2. Furthermore, our results provide new insight into the various transitions of the water molecule and their nonlinear response to electromagnetic radiation, paving the way to more accurate THz spectroscopy, imaging, and sensing systems, thereby facilitating future emerging THz technologies.