Revisiting self-seeding mechanism by generating vector ultraviolet N$_2^{+}$ lasing

Abstract

An intense femtosecond laser pulse can generate ultraviolet air lasing, offering a promising remote light source. A long-standing hypothesis is whether it is seeded by a self-generated spectral component, such as the second harmonic that is inevitably produced by the plasma gradient. Here, we report the generation of both radially and azimuthally polarized N$_2^+$ lasing driven by a single 800-nm cylindrical vector beam. Meanwhile, the same vector pump was applied to drive the generation of vector second harmonics in plasma. The radially polarized pump produces radially polarized second harmonics while the azimuthally polarized pump yields no second harmonic generation owing to the radial direction of plasma gradient. The absence of the azimuthally polarized second harmonic rules out the hypothesis of self-seeding by second harmonics, as both radially and azimuthally polarized N$_2^+$ lasing are observed with comparable intensities. By characterizing the spatial phase distribution of vector 391-nm lasing, we concluded that its phase is synchronized with the pump. These results suggest that amplified spontaneous emissions are the origin of N$_2^+$ lasing under the most common condition of low gas pressure, which was effectively demonstrated by theoretical simulations. Our work provides a promising method for remotely generating vector ultraviolet light sources.

Figures (5)

• Figure 1: Generation of vector ultraviolet N$_2^+$ lasing. (a) Schematic of the experiment. Red and blue arrows represent electric field polarization distributions of CVB pump and vector air lasing, respectively. Grey arrows in q-plate represent fast axis locally. L1, L2 are the lenses with the focal length of 30 cm. The filter is a band-pass filter with a central wavelength of 390 and a bandwidth of 10 nm. (b) Measured 391-nm N$_2^{+}$ lasing spectrum (blue solid line) and the second harmonic spectrum from argon (red dashed line). (c, d) Measured beam profiles of the generated radially (c) and azimuthally (d) polarized 391-nm lasing. (e) Measured radially polarized 391-nm lasing beam profiles after passing a polarizer oriented along different angles marked with black arrows. (f) Corresponding experimental results for azimuthally polarized 391-nm lasing. The color bar applies to all panels c-f with the same normalized standard.
• Figure 2: Second harmonic generation in argon plasma driven by cylindrical vector beams. The first column shows the beam profile of second harmonics. Columns two through four show the beam profile after passing a polarizer with the orientation marked in the upper-right corner. (a-d) Calculated beam intensity profile driven by radially polarized light fields. The arrows in (a) indicate the polarization state locally. (e-h) Calculated beam intensity profile driven by azimuthally polarized light fields. The intensity is rigorously zero in Eq. (1) for the case of azimuthal polarization. The intensities in the simulation share the color bar with the same standard in (e). (i-l) Measured beam intensity profile driven by radially polarized light fields. (m-p) Measured beam intensity profile driven by azimuthally polarized light fields. The intensities in (m-p) are normalized to those in (i-l).
• Figure 3: Phase distribution measurement of the radially polarized 391-nm N$_2^{+}$ lasing by a cylindrical lens. (a) Calculated results when the spatial phase of 391-nm lasing is synchronized with the driving field. After passing through a polarizer, the two lobes have a phase difference of $\pi$. Blue and white arrows represent local polarization direction of air lasing. P is polarizer, CL is cylindrical lens. (b) Calculated results when the spatial phase of 391-nm lasing is double that of the driving field. After passing through a polarizer, the two lobes have a phase difference of $2\pi$. Blue and white arrows represent local polarization direction of air lasing. P is polarizer, CL is cylindrical lens. (c, d) Calculated beam profiles in the focus plane in the scenarios of (a) and (b), respectively. (e) Experimental result.
• Figure 4: Simulated amplified spontaneous emissions in a radially anisotropic gain medium. (a-d) Beam profiles of ASE at different propagation distances in the radial gain, each amplified from an independent random spontaneous-emission source shown in the first column. Each spontaneous-emission source has some random noise light spots with random amplitudes, random phases, and random locations. Columns two through five correspond to propagation distances of 2 mm, 4 mm, 8 mm, and 16 mm, respectively. Black ellipses and lines represent directions and states of polarization locally. A shared color bar is used for all panels.
• Figure 5: Simulated amplified spontaneous emissions in an azimuthally anisotropic gain medium. (a-d) Beam profiles of ASE at different propagation distances in the azimuthal gain, each amplified from an independent random spontaneous-emission source shown in the first column. Each spontaneous-emission source has some random noise light spots with random amplitudes, random phases, and random locations. Columns two through five correspond to propagation distances of 2 mm, 4 mm, 8 mm, and 16 mm, respectively. Black ellipses and lines represent directions and states of polarization locally. A shared color bar is used for all panels.