Origins of THz Modes and Their IR Intensities in LiNbO₃

LiNbO₃ crystals have been essential materials for generating freely propagating terahertz (THz) pulses among other materials because of their excellent nonlinear optical properties and high damage thresholds. However, one of their dielectric propertiesthe huge IR intensities of THz modeshas been impeding their practical applications. Such IR intensities produce additional contributions to the refractive index, making the phase-matching between the optical pump and THz radiation difficult. The large IR intensities also lead to strong THz absorptions, reducing the conversion coefficient. Although a couple of techniques have been applied to solve or avoid the relevant problems, a more fundamental question regarding the atomic mechanism of generating the huge IR intensities of THz modes in LiNbO₃ has not been discussed so far. We performed a rigorous analysis of the nature of THz modes in terms of the contributions from the constituent atoms Li, Nb, and O using a quantitative approach originally developed in the previous studies. We further clarified the roles played by the motions of the constituent atoms in generating IR intensities by calculating the associated Born charge vectors. We revealed that the librations of the O octahedra, despite their prominent presence in some THz modes, are IR-silent motions. The translations of the O octahedra and Nb cations are the leading contributors to IR intensities, while the internal vibrations of the O octahedra play a destructive role. We propose two mechanisms for reducing the IR intensities of THz modes in LiNbO₃ based on the analyzed results.