  Broadband laser sources based on supercontinuum generation by femtosecond
laser filamentation have enabled applications from stand-off sensing and
spectroscopy to the generation and self-compression of high-energy few-cycle
pulses. Filamentation relies on the dynamic balance between self-focusing and
plasma defocusing, mediated by the Kerr nonlinearity and multiphoton ionization
respectively. The filament properties, including the supercontinuum generation,
are therefore highly sensitive to the properties of both the laser source and
the propagation medium. Here, we report the anomalous spectral broadening of
the supercontinuum for filamentation in molecular gases, which is observed for
specific elliptical polarization states of the input laser pulse. The resulting
spectrum is accompanied by a modification of the supercontinuum polarization
state and a lengthening of the filament plasma column. Numerical simulations
confirm that the anomalous behavior originates from the delayed rotational
response of the medium, demonstrating a new parameter to control supercontinuum
generation.
