Microscopic-Macroscopic Modelling of Dispersion and Optical Anisotropy in LiNbO3 and LiIO3 Using the Point Dipole Approximation

Abstract

This study aims to establish a unified understanding of the wavelength-dependent optical properties of
the uniaxial ferroelectric crystals LiNbO₃ and LiIO3, with the objective of improving material selection
and optimization for electro-optic and nonlinear photonic applications. By integrating the Point Dipole
Approximation (PDA) with Sellmeier dispersion formulations, the ordinary refractive index (no),
extraordinary refractive index (ne ) birefringence (Δn = ne-no), and the corresponding polarizabilities
were systematically determined over the wavelength range of 436-3391 nm. The results show a
monotonic decrease in nₒ and nₑ with wavelength for both materials, while LiNbO₃ consistently
exhibits higher refractive indices and birefringence than LiIO3 due to the smaller and more strongly
polarizing Nb⁵⁺ ion. At λ = 633 nm, the PDA-reconstructed indices show excellent agreement with
experimental data (e.g., LiNbO₃: no = 2.2846, ne = 2.2124; LiIO₃: no = 2.2459, ne = 2.2102), validating
the model. These findings highlight that crystal structure and ionic radius fundamentally govern
optical anisotropy, confirming LiNbO₃ as more suitable for electro-optic modulation, whereas LiIO3 is
better suited for high-power nonlinear optical applications. Overall, this work provides a predictive,
structure-based tool for designing and engineering advanced photonic and optoelectronic devices.