  Terahertz pulse generation by ultra-intense two-color laser fields ionizing
gases with near- to far-infrared carrier wavelength is studied from
particle-in-cell (PIC) simulations. For long wavelength ($10.6\ \mu$m)
promoting a large ratio of electron density over critical, photoionization is
shown to catastrophically enhance the plasma wakefield, causing a net downshift
in the optical spectrum and exciting THz fields with tens of GV/m amplitude in
the laser direction. This emission is accompanied by coherent transition
radiation (CTR) of comparable amplitude due to wakefield-driven electron
acceleration. We analytically evaluate the fraction of CTR energy up to 30 % of
the total radiated emission including the particle self-field and numerically
calibrate the efficiency of the matched blowout regime for electron densities
varied over three orders of magnitude.

Plasma Physics