I'm wondering if there are any major constituents of air which are capable of undergoing two-photon absorption and ultimately fluorescing in the visible spectrum. The original wavelengths of the photons getting absorbed aren't really an issue, nor is the intensity of the light -- just as long as the photon emitted is between 390 and 700 nm. Ideally the two photons would be of different wavelengths (is two-photon absorption even possible with two photons of different wavelength? I've not been able to find much literature on that at all), but it's not too big of a deal if they're the same. I've seen some papers which would suggest that two-photon absorption fluorescence is possible for atomic nitrogen and atomic oxygen in atmospheric plasma, but I've seen nothing that gives any information either way as far as diatomic oxygen or nitrogen, and nothing about anything in regular atmosphere.