  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.
