  We present the generation of THz radiation by focusing ultrafast laser pulses
with three incommensurate wavelengths to form a plasma. The three colors
include 800 nm and the variable IR signal and idler outputs from an optical
parametric amplifier. Stable THz is generated when all three colors are
present, with a peak-to-peak field strength of ~200 kV/cm and a relatively
broad, smooth spectrum extending out to 6 THz, without any strong dependence on
the selection of signal and idler IR wavelengths (in the range from 1300-2000
nm). We confirm that 3 colors are indeed needed, and comment on the
polarization characteristics of the generated THz, some of which are
challenging to explain with plasma current models that have had success in
describing two-color plasma THz generation.
