  Ultrashort long-wave infrared (LWIR) laser pulses can resonantly excite
vibrations in N2 and O2 through a two-photon transition. The absorptive,
vibrational component of the ultrafast optical nonlinearity grows in time,
starting smaller than, but quickly surpassing, the electronic, rotational, and
vibrational refractive components. The growth of the vibrational component
results in a novel mechanism of 3rd harmonic generation, providing an
additional two-photon excitation channel, fundamental + 3rd harmonic. The
original and emergent two-photon excitations drive the resonance exactly out of
phase, causing spatial decay of the absorptive, vibrational nonlinearity. This
nearly eliminates two-photon vibrational absorption. Here we present
simulations and analytical calculations demonstrating how these processes
modify the ultrafast optical nonlinearity in air. The results reveal nonlinear
optical phenomena unique to the LWIR regime of ultrashort pulse propagation in
atmosphere.
