The propagation of femtosecond UV laser pulses in air is numerically shown to form intense light filaments over several tenths of Rayleigh lengths. We compare UV filamentation with IR filamentation and show that the balance of the physical processes supporting the filaments is identical in both cases. For IR and UV wavelengths, it is shown that the intensity in the filament and the density of the electron plasma created by ionization of air molecules reach similar values as high as ${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and ${10}^{17}{\mathrm{cm}}^{-3\mathrm{}}$. Spectral data exhibit a large broadening in the IR filament and a limited one for UV, which justifies the white-light generation associated with IR filamentation only.

• Received 16 August 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.135003