A New Type of Two-photon Forward Radiation in Pure Liquids

TL;DR

This paper reports unexpected two-photon spectral features in pure liquids like water at low laser intensities, revealing new nonlinear optical phenomena not explained by known Raman processes.

Contribution

It introduces a novel type of forward radiation in pure liquids observed at low intensities, expanding understanding of nonlinear optical responses.

Findings

Spectral features at 2700cm-1 and 5000cm-1 observed in water

Features are quadratic with laser intensity

Similar features found in other liquids

Abstract

Unexpected spectral features are observed in the two photon spectrum of the pure water in the forward direction when an 80 femtosecond laser pulse is focused at 10^10Wcm-2 or less. Such intensity is much lower than the breakdown or stimulated threshold of the liquid water. The two broad features are about 2700cm-1 and 5000cm-1 red shifted from the hyper-Rayleigh wavelength, respectively, and they are quadratic with the laser intensity. They do not match the known Raman or hyper-Raman frequencies of water, and they are both centered at a narrow angle in the forward direction. Several other liquids also exhibited similar but molecular specific spectral features.