  Recently, Yao et al. demonstrated the creation of coherent emissions in
nitrogen gas with two-color (800 nm + 400 nm) ultrafast laser pulses [New J.
Phys. 15, 023046 (2013)]. Based on this two-color scheme, here we report on
systematic investigation of temporal characteristics of the coherent emission
at 391 nm by experimentally examining its evolution 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 quantum coherence via cooperation of an ensemble of excited N2+
molecules. Our findings shed more light on the mechanisms behind the laser-like
emissions induced by strong-field ionization of molecules.
