Terahertz Nonlinear Spectroscopy of Water Vapor

Abstract

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) × 10 2 m 2 /V 2 . We note for comparison that the value of χ (3) for fused silica at optical frequencies is on the order of 10 −22 m 2 / V 2 . 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.

THz Response of Charge Carriers in Nanoparticles: Microscopic Master Equations Reveal an Unexplored Equilibration Current and Nonlinear Mobility Regimes

“…Likewise, the corresponding and are sign‐inverted and asymmetric as well accounting for the strong alterations of the imaginary parts in Figure 3h. We remark that recent nonlinear terahertz absorption experiments of Rasekh et al [ 68 ] on (gaseous) water molecules resulted in extremely large nonlinear absorption of rotational transitions, corresponding to real and imaginary of the order of 10 2 m 2 V −2 . Further, similar to our observations made in Figure 3g, they report a—in the two‐level approximation not explained—nonlinear growth of a specific absorption line.…”

Recent Advances in THz Detection of Water

“…Rasekh et al [ 77 ] used terahertz techniques to study the nonlinear absorption spectra of water vapor and attributed this nonlinear response to step multiphoton jumps in water molecules. Wang et al [ 78 ] characterized the water-involved hydrogen bonding in oil-paper insulating materials via terahertz dielectric spectroscopy.…”

“…was found that the THz spectra of the hydrogen bonding network would be red-shifted when the temperature increased, indicating a strong correlation between its center frequency and the strength of the network. Rasekh et al [77] used terahertz techniques to study the nonlinear absorption spectra of water vapor and attributed this nonlinear response to step multiphoton jumps in water molecules. Wang et al [78] characterized the water-involved hydrogen bonding in oilpaper insulating materials via terahertz dielectric spectroscopy.…”

Observation of an extremely large nonlinear response in crystalline quartz in THz regime

“…[10] Nonlinear effects such as THz-induced impact ionisation and inter-valley scattering in semiconductors, [11,12,13,14,15] THz high-harmonic generation by hot carriers, [16,17,18,19] and THz-induced ferroelectricity and collective coherence control have been demonstrated. [20,21] A very large third-order nonlinearity has been reported for water vapor, [22] where the stepwise multiphoton transitions in water molecules lead to a third-order susceptibility of χ (3) = (0.4 + 6i) × 10 2 m 2 V −2 . Extreme THz-induced Kerr effects have been reported for different liquids, [23,24,25,26] where the nonlinear refractive indices can be several orders of magnitude larger than their values in the optical regime.…”