  Cavity-free lasing in atmospheric air has stimulated intense research towards
fundamental understanding of underlying physical mechanisms. In this Letter, we
identify a new mechanism -- third harmonic photon mediated resonant energy
transfer pathway leading to population inversion in argon via initial
three-photon excitation of nitrogen molecules irradiated by intense 261 nm
pulses -- that enables bidirectional two-color cascaded lasing in atmospheric
air. By making pump-probe measurements, we conclusively show that such cascaded
lasing results from superfluorescence (SF) rather than amplified spontaneous
emission (ASE). Such cascaded lasing with the capability of producing
bidirectional multicolor coherent pulses opens additional possibilities for
remote sensing applications.
