  Quantum interference occurs frequently in the interaction of laser radiation
with materials, leading to a series of fascinating effects such as lasing
without inversion, electromagnetically induced transparency, Fano resonance,
etc. Such quantum interference effects are mostly enabled by single-photon
resonance with transitions in the matter, regardless of how many optical
frequencies are involved. Here, we demonstrate quantum interference driven by
multiple photons in the emission spectroscopy of nitrogen ions that are
resonantly pumped by ultrafast infrared laser pulses. In the spectral domain,
Fano resonance is observed in the emission spectrum, where a laser-assisted
dynamic Stark effect creates the continuum. In the time domain, the
fast-evolving emission is measured, revealing the nature of free-induction
decay (FID) arising from quantum radiation and molecular cooperativity. These
findings clarify the mechanism of coherent emission of nitrogen ions pumped
with MIR pump laser and are likely to be universal. The present work opens a
route to explore the important role of quantum interference during the
interaction of intense laser pulses with materials near multiple photon
resonance.
