Influence of tunnel ionization to third-harmonic generation of infrared femtosecond laser pulses in air

Viktorija Tamulienė,Ihar Babushkin,Virgilijus Vaičaitis,Danas Buožius,Uwe Morgner,Greta Juškevičiūtė

doi:10.1038/s41598-020-74263-x

Abstract

Here we present an experimental as well as theoretical study of third-harmonic generation in tightly focused femtosecond filaments in air at the wavelength of 1.5 ,upmu hbox {m}. At low intensities, longitudinal phase matching is dominating in the formation of 3rd harmonics, whereas at higher intensities locked X-waves are formed. We provide the arguments that the X-wave formation is governed mainly by the tunnel-like ionization dynamics rather than by the multiphoton one. Despite of this fact, the impact of the ionization-induced nonlinearity is lower than the one from bound–bound transitions at all intensities.

Highlights

We present an experimental as well as theoretical study of third-harmonic generation in tightly focused femtosecond filaments in air at the wavelength of 1.5 μm
An important part of this filament formation picture is a description of ionization process which is very often made in the terms of multiphoton dynamics
The IR signal wave pulses were focused in air by the lenses of various focal lengths, which resulted in a visible third harmonic generation

Summary

Introduction

We present an experimental as well as theoretical study of third-harmonic generation in tightly focused femtosecond filaments in air at the wavelength of 1.5 μm. We provide the arguments that the X-wave formation is governed mainly by the tunnel-like ionization dynamics rather than by the multiphoton one. Despite of this fact, the impact of the ionization-induced nonlinearity is lower than the one from bound–bound transitions at all intensities. For the filaments obtained from femtosecond Ti:Sapphire laser pulses with the central wavelength of about 800 nm, intensity clamping takes place at intensities of the order of 1013 W/cm , which corresponds to γ ≈ 2 , at the boundary of the multiphoton range. Ionization-based radiation mechanism is related to transitions of electrons which were bounded

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Conclusion