Optimizing the 391-nm lasing intensity from ionized nitrogen molecules in 800-nm femtosecond laser fields

We investigate the 391-nm lasing dynamics from ionized nitrogen molecules in 800-nm femtosecond laser fields. By comparing the radiation intensity, spectrum shape, and temporal profile of the 391-nm lasing at various experimental conditions, we conclude that the lasing dynamics contains not only the generation and the decay of ionized nitrogen molecules, but also the seed-built coherence among emitters as well as the propagation effect in the plasma filamentation. These results provide reliable guidance for optimizing the 391-nm lasing from ionized nitrogen molecules in 800-nm femtosecond laser fields, which have potential applications for remote sensing in the atmosphere.

Figure 1

Forward emission spectra of pure nitrogen molecules at various gas pressures irradiated by intense 800-nm laser pulses (a) without and (b) with the presence of an external seed. The integral time is 200 ms in the absence of the external seed and 2 ms in the presence of the external seed.

Figure 2

Forward 391-nm lasing intensity at various gas pressures under the condition of self-seed and external seed. The integral time is 200 ms in the former case and 2 ms in the latter case.

Figure 3

Temporal profile of the 391-nm lasing signal recorded by cross-correlation measurement of sum frequency. The sum frequency signal at 263 nm is generated by the 391-nm lasing signal and another 800-nm probe pulse.

Figure 4

Forward emission spectra of gas mixture of nitrogen molecules and argon atoms at various gas pressures irradiated by intense 800-nm laser fields in the presence of an external seed.

Figure 5

Forward emission spectra around 391 nm in the presence of an external seed with optimized time delay for (a) gas mixture of nitrogen gas and argon atoms and (b) pure nitrogen gas. Optical gain for (c) gas mixture of nitrogen gas and argon atoms and (d) pure nitrogen gas. The total gas pressure is 10 mbar.

Figure 6

The forward 391-nm lasing intensity as a function of time delay from pure nitrogen molecules at the pressure of (a) 4, (b) 12, and (c) 20 mbar.

Figure 7

Zoomed-in forward emission spectra of pure nitrogen molecules irradiated by intense 800-nm laser pulses (a) without and (b) with the presence of an external seed. $J$ is the rotational quantum number of the lower electronic state. The spectrum of the $R$ branch shifts to a small rotational quantum number and the $P$ branch becomes narrower when the gas pressure changes from 4 to 20 mbar.

Figure 8

Theoretical predicted position and width of spectral lines consisting of the 391-nm lasing. The adjacent emission lines of the $R$ branch are well separated and those of the $P$ branch are overlapped when the spectral broadening is included.