  We demonstrate the generation of high-quality tunable terahertz (THz)
vortices in an eigenmode by employing soft-aperture difference frequency
generation of a vortex output and a Gaussian mode. The generated THz vortex
output exhibits a high-quality orbital angular momentum (OAM) mode with a
topological charge of $\ell_{THz}=\pm1$ in a frequency range of 2-6 THz. The
maximum average power of the THz vortex output obtained was ~3.3 $\mu$W at 4
THz.
