  We present an antipodal Vivaldi antenna for broadband double metal waveguide
terahertz quantum cascade lasers and frequency combs. Its exponentially curved
flare profile results in an adiabatic in-plane mode expansion, producing an
improved far-field with a single-lobed beam of (23{\deg}x19{\deg}) full width
half maximum with an octave-spanning bandwidth. The antenna also acts as a wave
retarder, rotating the polarization from vertical toward horizontal
polarization by a frequency-dependent angle. The laser's emission spectrum and
current-voltage characteristics are not affected, as well as frequency comb
operation. Measurements agree well with numerical simulations, and the proposed
antenna covers a broad spectral range (1.5-4.5 THz).
