Fig. 1. Experimental setup. QP1 and QP2 refer to quarter-wave plates for the 1850 nm pulses. L1 and L2 are convex lenses. CL is the cylindrical lens. The half-wave q-plate converts the incident circularly polarized beam into a circularly polarized vortex beam.

Fig. 2. (a) Spatial profile of the Gaussian femtosecond MIR pulses with wavelength λ = 1850 nm. (b) Free induction decay signal of nitrogen ions pumped by the Gaussian MIR pulses. The gas pressure was 1 bar in (b).

Fig. 3. The emission spectrum as a function of the wavelength of the pump laser. The pulse energy of the pump laser was fixed at 380 µJ. The gas pressure was 1 bar.

Fig. 4. (a) Spatial profile of the vortex femtosecond MIR pulses with topological charge l = 1. The wavelength was 1850 nm. (b), (c) Profile of the vortex MIR pulses obtained with the CCD camera at the focal plane of the cylindrical lens f = 400 mm. In (b) and (c), the cylindrical lens was rotated to different angles θ. The stripes with minimum laser intensity in the center in (b) and (c) reflect that the MIR pulses possess a topological charge of l = 1.

Fig. 5. (a) Spatial profile of the FID signal at 391.4 nm from nitrogen ions. (b) Spectrum of the FID emission. (c), (d) Experimental results of the profile of the 391.4 nm emission taken by the CCD on the focal plane of the cylindrical lens f = 400 mm. The cylindrical lens was rotated by 45° from (c) to (d). (e), (f) Numerically simulated profile of a beam with topological charge of l = 5 at the focal plane of a cylindrical lens, which should be compared with the experimental results in (c) and (d).