  Supercontinuum generation by femtosecond filaments in air is investigated for
different laser wavelengths ranging from ultraviolet to infrared. Particular
attention is paid on the role of third-harmonic generation and temporal
steepening effects, which enlarge the blue part of the spectrum. A
unidirectional pulse propagation model and nonlinear evolution equations are
numerically integrated and their results are compared. Apart from the choice of
the central wavelength, we emphasize the importance of the saturation intensity
reached by self-guided pulses, together with their temporal duration and
propagation length as key players acting on both supercontinuum generation of
the pump wave and emergence of the third harmonics. Maximal broadening is
observed for large wavelengths and long filamentation ranges.
