Objective Air radiation providing a cavity-free coherent radiation channel in the atmosphere, is of interest for remote spectroscopy, nonlinear light-matter interaction, and atmosphere-based coherent detection. Femtosecond vortex pulses differ from Gaussian pulses in three coupled aspects: orbital angular momentum (OAM), annular intensity distribution, and phase singularity. These features make vortex fields suitable for regulating strong-field ionization, seed formation, gain evolution, and emission mode simultaneously. The physical role of vortex-field topology in the air radiation generation, however, has not been sufficiently verified experimentally. This work aimed to clarify how an ultrafast vortex field affected the generation of N₂⁺ air radiation, whether OAM was transferred from the pump to the lasing signal, and how pump pulse energy and gas pressure influenced the lasing yield under vortex pumping. Another objective was to compare vortex and Gaussian excitation under identical conditions and determine whether the vortex field provided practical advantages in output efficiency and control range.

Methods A Ti: sapphire femtosecond laser system delivered pulses at 800 nm with a pulse duration of 35 fs, a repetition rate of 1 kHz, and a maximum pulse energy of 7 mJ. After attenuation, the beam passed through a circular aperture and a spiral phase plate to form a vortex pump beam carrying OAM. The beam was focused by a lens with a focal length of 20 cm into nitrogen gas to generate forward air radiation near 428 nm. The emitted light was collimated by a second lens. Residual 800 nm pump light and part of the supercontinuum background were removed with dichroic mirrors and band-pass filters. The filtered signal was then recorded with a fiber-coupled spectrometer. To identify the spatial mode and topological charge of the signal, a cylindrical-lens method was used. The output beam was projected onto a CCD camera after passing through the cylindrical lens. The number of slit-like intensity features at the focal plane was used to determine the OAM quantum number.

Results and Discussions The experiments showed that a single femtosecond vortex pump pulse generated stable forward N₂⁺ air radiation near 428 nm. The measured spectrum contained two distinct peaks around 428 nm, which were assigned to the P branch and R branch associated with rotational transitions between the B²Σ_u⁺ (υ′ = 0) state and the X²Σ_g⁺ (υ = 1) state of N₂⁺. This result indicates that the observed radiation was a rotationally resolved coherent air radiation signal rather than ordinary spontaneous fluorescence.

The spatial pattern of the emitted signal is obtained, when the pump beam carried a topological charge of l = 1, the signal produced two slit-like features after the cylindrical lens, which corresponds to a topological charge of l = 2. The emitted 428 nm air radiation signal therefore carried twice the OAM of the pump. This result provides direct experimental evidence of OAM transfer during the air radiation process. It also shows that the emitted field inherited the phase property of the seed that initiated stimulated amplification.

The observed OAM doubling supports a self-seeding mechanism. The 800 nm vortex pump ionized nitrogen molecules, created the population conditions required for gain in N₂⁺, and simultaneously produced a 400 nm second-harmonic seed inside the plasma region. In second-harmonic generation, angular-momentum conservation requires that two pump photons contribute to one harmonic photon. When each pump photon carries one unit of OAM, the generated second-harmonic field carries two units. The 400 nm seed therefore possesses a doubled topological charge and can stimulate the 428 nm transition while preserving the helical phase information. The measured OAM of the lasing signal agrees with this picture.

The yield measurements further showed that the effect of the vortex field depended strongly on the input conditions. At relatively low pulse energy, the annular intensity distribution of the vortex focus reduced the central peak intensity and made population inversion less efficient than in the Gaussian case. Under this condition, Gaussian pumping produced stronger lasing output. As the pump pulse energy increased, the trend reversed. Vortex pumping then generated a higher lasing yield than Gaussian pumping. This behavior is attributed to the different focusing and filamentation characteristics of the vortex beam. The annular profile modifies energy deposition in the plasma and can raise the effective threshold associated with intensity clamping, which allows stronger effective excitation in the gain region. The resulting plasma structure becomes more favorable for stimulated amplification.

Pressure-dependent measurements showed a similar tendency. At high input energy, both l = 1 and l = −1 vortex pumps produced a higher 428 nm lasing yield than the Gaussian beam over a wider pressure interval. The saturation pressure under vortex pumping was also shifted upward by nearly 0.01 MPa relative to the Gaussian case. This shift indicates a broader pressure tolerance and a wider gain-control window. These results demonstrate that vortex-field topology affects not only the output beam mode, but also ionization dynamics, nonlinear frequency conversion, plasma-channel structure, gain build-up, and saturation behavior. The topological degree of freedom therefore acts as an active control parameter in air radiation generation rather than as a passive label of the emitted beam.

Conclusions Ultrafast vortex light fields enable effective generation and control of N₂⁺ air radiation. The experiments confirmed that vortex pumping produced 428 nm air radiation with a clear rotational branch structure, a ring-shaped spatial distribution, and definite OAM inheritance. A pump beam with l = 1 generated a lasing signal with l = 2, which supports the self-seeded amplification mechanism involving second-harmonic generation and demonstrates OAM transfer and conservation in the air radiation process. Comparative measurements also show that vortex pumping has clear advantages over Gaussian pumping in the high-energy regime, including stronger lasing output and higher saturation pressure. These findings establish OAM as a practical control degree of freedom in strong-field ionization, nonlinear conversion, and gain-assisted coherent emission in gases. This work enriches the physical picture of structured-light-controlled air radiation and supports future applications in remote atmospheric sensing, coherent spectroscopy, and quantum-radar-related schemes.