Spatiotemporal focusing does not always improve axial intensity localization

TL;DR

This study experimentally compares conventional and spatiotemporal focusing in air, revealing that conventional focusing often provides better axial intensity confinement and delays self-focusing onset.

Contribution

It demonstrates that conventional focusing can outperform spatiotemporal focusing in axial intensity localization under certain conditions.

Findings

Conventional focusing yields higher focal intensity.

High numerical aperture improves axial confinement.

Spatiotemporal focusing does not always enhance intensity localization.

Abstract

We report on an experimental comparison on critical intensities of nonlinear self-focusing in air with conventional focusing and spatiotemporal focusing schemes. Our results show that the conventional focusing with the focal lens completely filled with the incident beam allows for the strongest axial intensity confinement against the self-focusing effect. This is because that in the high-numerical-aperture condition, the focal spot will have a compact size which results in a high focal intensity. Meanwhile, the Rayleigh length of the focused beam will be substantially shortened which efficiently postpones the onset of self-focusing.