We present experimental results of birefringence in beta barium borate (β-BaB2O4 or BBO) crystals in the 0.2-1 terahertz (THz) frequency range. Standard polarized THz-transmission spectroscopy of a BBO slab cut under 90° was performed at different azimuthal orientations to determine the ordinary and extraordinary refractive indices. An average Δn/n value of 0.118 was calculated. Moreover, azimuthal angle-dependent absorption was observed at about 0.65 THz, in close agreement with calculations based on first principles. Our calculations showed the absorption feature was due to the low-frequency phonon modes of the [B3O6]3- rings. These results suggest that a BBO-based THz optical-parametric-oscillator design is feasible.
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