Giant Birefringence of Lithium Niobate Crystals in the Terahertz Region

Terahertz time-domain spectroscopy (THz-TDS) is used to study the spectral response of lithium niobate crystals (LiNbO₃) in the far infrared region. The optical constants are derived from the measured complex refractive index. A giant birefringence is observed in this material, and the average refractive-index difference between the ordinary wave and the extraordinary wave, n_o-n_e, can reach up to about 1.6. Such a large birefringence is attributed to the different phonon modes of A₁(z) and E(x,y). This unusual property makes LiNbO₃ a promising material to be used as a functional material in the terahertz region, e.g. employed as wave-plates and polarization separators.