Water Vapor: An Extraordinary Terahertz Wave Source under Optical Excitation

TL;DR

This paper demonstrates that water vapor can be a highly efficient source of broadband terahertz radiation when excited by femtosecond laser pulses, revealing a new method for THz generation with implications for spectroscopy.

Contribution

It introduces the first experimental and theoretical study of intense THz emission from water vapor, highlighting the role of molecular structure and isotope effects in the process.

Findings

Water vapor generates stronger THz emission than dry nitrogen.

THz generation efficiency is over 50% higher in water vapor.

Molecular structure and isotope effects influence THz wave generation.

Abstract

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.