Signature of superradiance from a nitrogen-gas plasma channel produced by strong-field ionization

Abstract

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${}_{2}{}^{+}$: $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\ensuremath{\Sigma}}_{u}^{+}\left(\ensuremath{\nu}=0\right)\ensuremath{-}X{\phantom{\rule{0.16em}{0ex}}}^{2}{\ensuremath{\Sigma}}_{g}^{+}\left(\ensuremath{\nu}=0\right)$] 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 \ensuremath{\sim}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.