  The quasi-static anisotropic permittivity parameters of electrically
insulating gallium oxide (beta-Ga2O3) were determined by terahertz
spectroscopy. Polarization-resolved frequency domain spectroscopy in the
spectral range from 200 GHz to 1 THz was carried out on bulk crystals along
different orientations. Principal directions for permittivity were determined
along crystallographic axes c, and b, and reciprocal lattice direction a*. No
significant frequency dispersion in the real part of dielectric permittivity
was observed in the measured spectral range. Our results are in excellent
agreement with recent radio-frequency capacitance measurements as well as with
extrapolations from recent infrared measurements of phonon mode and high
frequency contributions, and close the knowledge gap for these parameters in
the terahertz spectral range. Our results are important for applications of
beta-Ga2O3 in high-frequency electronic devices
