  The generation of THz radiation from ionizing two-color femtosecond pulses
propagating in metallic hollow waveguides filled with Ar is numerically
studied. We observe a strong reshaping of the low-frequency part of the
spectrum. Namely, after several millimeters of propagation the spectrum is
extended from hundreds of GHz up to ~150 THz. For longer propagation distances,
nearly single-cycle near-infrared pulses with wavelengths around 4.5 um are
obtained by appropriate spectral filtering, with an efficiency of up to 0.25%.
