Generation of terahertz radiation from ionizing two-color laser pulses in Ar filled metallic hollow waveguides.

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. More precisely, after several millimeters of propagation the spectrum is extended from hundreds of GHz up to approximately 150 THz. For longer propagation distances, nearly single-cycle near-infrared pulses with wavelengths around 4.5 microm are obtained by appropriate spectral filtering, with an efficiency of 0.1-1%.