Optical emission from ultrafast laser filament-produced air plasmas in the multiple filament regime

We perform optical emission spectroscopy of ultrafast laser filament-produced air plasmas in the multiple filament regime at driving wavelengths of 400 nm and 800 nm. The spatiotemporal structure of the emission from the plasmas are observed and the emission spectra are used to estimate plasma temperature and density for a range of laser parameters. Plasma temperatures are determined from the molecular nitrogen fluorescence, while the electron densities are estimated from Stark broadening of the oxygen-I 777.19-nm line. Electron temperatures are determined to be in the range of 5000-5200 K and they do not vary significantly along the length of the filament, nor are they sensitive to incident laser energy or wavelength. Electron densities are on order of 10¹⁶ cm^-3 and show a greater variation with axial position, laser energy, and laser wavelength. We discuss mechanisms responsible for spatial localization of emitting species within the filament. Optical emission spectroscopy offers a simple, non-perturbing method to measure filament properties, that allows the information on the associated molecular transitions and excitation/ionization mechanisms to be extracted.