Two-photon vibrational excitation of air by long-wave infrared laser pulses

TL;DR

This paper investigates how ultrashort LWIR laser pulses resonantly excite molecular vibrations in air, leading to novel nonlinear optical effects and potential reduction of vibrational absorption.

Contribution

It introduces a new mechanism of two-photon vibrational excitation and its impact on ultrafast optical nonlinearity in air at LWIR wavelengths.

Findings

Vibrational component surpasses electronic nonlinearities in time

Emergence of a third harmonic-driven two-photon excitation channel

Near elimination of vibrational absorption due to phase interactions

Abstract

Ultrashort long-wave infrared (LWIR) laser pulses can resonantly excite vibrations in N2 and O2 through a two-photon transition. The absorptive, vibrational component of the ultrafast optical nonlinearity grows in time, starting smaller than, but quickly surpassing, the electronic, rotational, and vibrational refractive components. The growth of the vibrational component results in a novel mechanism of 3rd harmonic generation, providing an additional two-photon excitation channel, fundamental + 3rd harmonic. The original and emergent two-photon excitations drive the resonance exactly out of phase, causing spatial decay of the absorptive, vibrational nonlinearity. This nearly eliminates two-photon vibrational absorption. Here we present simulations and analytical calculations demonstrating how these processes modify the ultrafast optical nonlinearity in air. The results reveal nonlinear optical phenomena unique to the LWIR regime of ultrashort pulse propagation in atmosphere.