Stimulated-Raman-scattering-assisted superfluorescence enhancement from ionized nitrogen molecules in 800-nm femtosecond laser fields

We have observed collective spontaneous emission (superfluorescence) from ionized nitrogen molecules generated by 800-nm laser pulses. The superfluorescence intensity around 391 nm was found to be greatly enhanced when injecting an external seed with various wavelengths. Our finding provides insights into controlling the coherent emission from air filamentation induced by intense femtosecond laser fields, which has great potential application in remote atmospheric sensing.

Figure 1

(a) Forward emission of pure nitrogen molecules irradiated by intense 800-nm pump laser. In the inset the broad emission around 400 nm is generated in argon atoms and attributed to the second harmonic of the 800-nm pump laser. (b) Spectrum of the 357-nm external seed. (c) Forward emission in the presence of an external 357-nm seed with optimized time delay. The region around 357 nm is magnified by a factor of 10 for visual convenience.

Figure 2

(a) The intensity of the enhanced 391-nm emission as a function of time delay between the 800-nm pump laser and the 357-nm external seed; (b) the sum frequency signal at 263 nm generated by the enhanced 391-nm emission and the weak 800-nm laser as a function of time delay between them. The injection time of the external 357-nm seed is marked by a red arrow.

Figure 3

(a) Forward emission in the presence of an external 391-nm seed with optimized time delay, the seed laser around 391 nm is magnified by a factor of 20 for visual convenience; (b) the intensity of the enhanced 391-nm emission as a function of time delay between the 800-nm pump laser and the 391-nm external seed; (c) the sum frequency signal at 263 nm generated by the enhanced 391-nm emission and the 800-nm laser as a function of time delay between them. The injection time of the external 391-nm seed is marked by a red arrow.

Figure 4

Resonance enhanced Raman scattering related with vibrational Raman process from (a) ${{\mathrm{N}}_2}^{+}\left(X^2{{\mathrm{\Sigma }}_g}^{+},v=0\to v=1\right)$, and (b) ${{\mathrm{N}}_2}^{+}\left(B^2{{\mathrm{\Sigma }}_u}^{+},v^{\prime }=0\to v^{\prime }=1\right).$

Figure 5

(a) Forward emission in the presence of an external 357-nm seed with polarization perpendicular to the 800-nm pump laser polarization, (b) polarization properties of the 800-nm pump laser and 357-nm seed, (c) polarization properties of the 391-nm coherent emission with and without the 357-nm seed.