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.

中文翻译：

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水蒸气的太赫兹非线性光谱

水分子在红外和太赫兹区域有数百万次旋转跃迁，由此产生的吸收特征主导了大气太赫兹测量的透射光谱，即使传播距离短至几十厘米。值得注意的是，我们观察到这些共振位置的吸收随着太赫兹强度的增加而增加。我们通过水分子中的逐步多光子跃迁来解释这一观察结果。有效三阶磁化率的估计值非常大：χ ⁽³⁾ = (0.4 + 6 i ) × 10 ² m ² /V ²。我们注意到为了比较，熔融石英在光学频率下的 χ ⁽³⁾值约为 10^–22 m ² /V ²。此外，我们的研究结果为水分子的各种跃迁及其对电磁辐射的非线性响应提供了新的见解，为更精确的太赫兹光谱、成像和传感系统铺平了道路，从而促进了未来新兴的太赫兹技术。