Terahertz Nonlinear Optical Response of Water Vapor

TL;DR

This study investigates the nonlinear optical response of water vapor at THz frequencies, revealing strong resonances and a large nonlinear refractive index, which enhances understanding for THz spectroscopy and sensing applications.

Contribution

The paper provides the first detailed measurement of water vapor's nonlinear response at THz frequencies, including estimates of its nonlinear refractive index, advancing THz spectroscopy techniques.

Findings

Large nonlinear response at water vapor resonances

Estimated nonlinear refractive index around 10^2 m^2/W

Enhanced understanding of water molecule energy levels

Abstract

We report on the nonlinear spectroscopy of water vapor at THz frequencies. Atmospheric water vapor has a rich spectrum with several strong resonances at frequencies below 3 THz, falling within the range of operation of most existing THz sources. We observe an extremely large nonlinear response to THz radiation at the positions of these resonances. Using the optical Kerr model for the nonlinear response, we estimate a minimum nonlinear refractive index of the order of 10^2 m^2/W. Our results provide insight into the energy levels of the water molecule and give a more accurate picture of its response to electromagnetic radiation, paving the way to more accurate THz spectroscopy, imaging and sensing systems, and thereby facilitating future emerging THz technologies.