Β-Ba B2 O4 (BBO) was found to have huge birefringence in the 0.1-1.1 THz frequency region. Polarized terahertz transmission spectroscopy revealed a Δnn value of about 0.12±0.02. This result could be related to a polarization-dependent absorption that was observed at about 0.65 THz; in close agreement with first principles-calculations, showing this to be due to low-frequency phonon modes of the [B3 O6] 3 -rings. These findings suggest the utilization of the birefringent properties of BBO for optical parametric devices including amplifiers and oscillators, operating in the terahertz regime.
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