Optical properties of barium borate crystal in the THz range revisited

Low-temperature phase (β-form) barium borate (BBO) is one of the most important nonlinear crystals that has been widely used for optical second-harmonic generation (SHG), especially with femtosecond sources. There was growing interest in its applications in the direct generation of terahertz (THz) radiations, but it was hindered by the lack of knowledge of its basic properties in the THz range. In a recent study based on first-principles quantum chemistry calculation, we found that the theoretically calculated refractive indices of β-BBO in the THz frequency range do not agree with the previously reported values. To explore the discrepancy between measured and calculated results, we grew and cut β-BBO crystals and performed independent experimental measurements on the refractive indices of BBO crystals. The new data show that the order of ordinary and extraordinary index of β-BBO in the THz range is opposite, contrary to several papers previously reported. Based on the newly acquired material properties, simple geometries fulfilling phase matching (PM) conditions with n o (THz)= n o g r (800nm) for efficient generation of THz radiation through optical rectification in β-BBO are proposed.