Recently, Yao et al. demonstrated the creation of coherent emissions in nitrogen gas with two-color (800 nm + 400 nm) ultrafast laser pulses [J. Yao, G. Li, C. Jing, B. Zeng, W. Chu, J. Ni, H. Zhang, H. Xie, C. Zhang, H. Li, H. Xu, S. L. Chin, Y. Cheng, and Z. Xu, New J. Phys. 15, 023046 (2013)]. Based on this two-color scheme, here we report on systematic investigation of temporal characteristics of the radiation emitted at 391 nm [N${}_2{}^{+}$: $B{}^2{\Sigma }_u^{+}\left(\nu =0\right)-X{}^2{\Sigma }_g^{+}\left(\nu =0\right)$] by experimentally examining its temporal profiles with the increase of the plasma channel induced by the intense 800-nm femtosecond laser pulses at a nitrogen-gas pressure of ∼25 mbar. We reveal unexpected temporal profiles of the coherent emissions, which show significant superradiance signatures owing to the cooperation of an ensemble of excited N${}_2$${}^{+}$ molecules that are coherently radiating in phase. Our findings shed more light on the mechanisms behind the coherent laserlike emissions induced by strong-field ionization of molecules.

• Received 9 December 2013

DOI:https://doi.org/10.1103/PhysRevA.89.033833