Structured air lasing of N2+

TL;DR

This paper demonstrates the first structured air lasing in N2+ driven by femtosecond laser pulses, generating vortex and cylindrical vector beams with preserved phase and polarization properties, revealing new physical mechanisms.

Contribution

It introduces a novel method for generating structured light via N2+ air lasing, including vortex and cylindrical vector beams, with insights into phase transfer and amplification mechanisms.

Findings

Generated vortex superfluorescent radiation at 391 nm carrying orbital angular momentum.

Amplified Gaussian seed beams into vortex and cylindrical vector beams with preserved polarization states.

Revealed a new physical mechanism for phase transfer in vortex N2+ superfluorescence.

Abstract

Structured light has attracted great interest in scientific and technical fields. Here, we demonstrate the first generation of structured air lasing in N2+ driven by 800 nm femtosecond laser pulses. By focusing a vortex pump beam at 800 nm in N2 gas, we generate a vortex superfluorescent radiation of N2+ at 391 nm, which carries the same photon orbital angular momentum as the pump beam. With the injection of a Gaussian seed beam at 391 nm, the coherent radiation is amplified, but the vorticity is unchanged. A new physical mechanism is revealed in the vortex N2+ superfluorescent radiation: the vortex pump beam transfers the spatial spiral phase into the N2+ gain medium, and the Gaussian seed beam picks up the spatial spiral phase and is then amplified into a vortex beam. Moreover, when we employ a pump beam with a cylindrical vector mode, the Gaussian seed beam is correspondingly amplified into a cylindrical vector beam. Surprisingly, the spatial polarization state of the amplified radiation is identical to that of the vector pump beam regardless of whether the Gaussian seed beam is linearly, elliptically, or circularly polarized. Solving three-dimensional coupled wave equations, we show how a Gaussian beam becomes a cylindrical vector beam in a cylindrically symmetric gain medium. This study provides a novel approach to generating structured light via N2+ air lasing.