Terahertz birefringence of β-BaB2O4 (BBO) crystal

Birefringence was observed for a beta-BaB₂O₄ crystal in the 0.1 to 1.1 terahertz region. The calculated Deltan value was ~0.296. Moreover, an angle-dependent absorption band observed at ~0.65 terahertz is attributed to low-frequency phonon modes.