Enhancement of birefringence and refractive index dispersion optimization from iodates to fluorooxoiodates

Abstract

Nonlinear optical (NLO) materials with lone electron pairs have excellent optical properties and the introduction of F atoms is a new way to obtain NLO materials with more balanced properties. However, the research of NLO materials still faces great challenges and difficulties, especially regarding the phase-matching (PM) wavelength, which is an important parameter of NLO materials and is influenced by birefringence and refractive index dispersion. Iodates have been widely noticed as excellent NLO materials. In this work, we have selected the typical alkali metal iodates MIO₃ and fluorooxoiodates MIO₂F₂ (M = Na, K, Rb, and Cs), and systematically explored the functional modules as well as the mechanisms regulating birefringence in iodates and fluorooxoiodates. Meanwhile, the refractive index dispersion of the above iodates and fluorooxoiodates was also analyzed. The results show that the introduction of F atoms will adjust the birefringence and refractive index dispersion. At the same time, in general, the birefringence from iodates to fluorooxoiodates is decreased, but the birefringence of NaIO₂F₂ is increased compared with that of NaIO₃, because the anion groups in NaIO₂F₂ are arranged in the direction beneficial to enhancing optical anisotropy, and the direction of lone pair electrons is consistent with the optical principal axis. In terms of the refractive index dispersion, the refractive index dispersion from iodates to fluorooxoiodates is decreased because of the regulation of F atoms. This could further facilitate the design of new NLO materials with excellent properties.