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We report the terahertz optical properties of vanadium doped (100) β-Ga2O3 using terahertz time-domain spectroscopy (THz-TDS). The V-doped β-Ga2O3 crystal shows strong birefringence in the 0.2-2.4 THz range. Further, phase retardation by the V-doped β-Ga2O3 has been measured over the whole THz range by terahertz time-domain polarimetry (THz-TDP). It is observed that the V-doped β-Ga2O3 crystal behaves both as a quarter waveplate (QWP) at 0.38, 1.08, 1.71, 2.28 THz, and a half waveplate (HWP) at 0.74 and 1.94 THz, respectively.

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We report the growth of electrically-resistive vanadium-doped β -Ga 2 O 3 single crystals via the optical floating zone technique. By carefully controlling the growth parameters V-doped crystals with very high electrical resistivity compared to the usual ntype V-doped β -Ga 2 O 3 (n e ≈ 10 18 /cm −3 ) can be synthesized. The optical properties of such high resistive V-doped β -Ga 2 O 3 are significantly different compared to the undoped and n-doped crystals. We study the polarization-dependent Raman spectra, polarization-dependent transmission, temperature-dependent photoluminescence in the optical wavelength range and the THz transmission properties in the 0.2 -2.6 THz range. The V-doped insulating Ga 2 O 3 crystals show strong birefringence with refractive index contrast ∆n of 0.3± 0.02 at 1 THz, suggesting it to be an ideal material for optical applications in the THz region.