Enhanced optical anisotropy via dimensional control in alkali-metal chalcogenides

Abstract

Crystals with both large birefringence and wide transparent range are suitable for broad applications in the areas of optical communications, the laser industry and modulation of the light polarization requirement. In this work, to assist the design of urgently needed crystals with large birefringence in the infrared (IR) region, typical alkali-metal chalcogenides, KPSe₆, Na₂Ge₂Se₅, and Li₂In₂GeSe₆ have been studied. They exhibit a hierarchical characteristic in the calculated birefringence by about 0.21, 0.11, and 0.04, respectively. To explore the origin of the birefringence difference, the polarizability anisotropy and the effect of electron distribution anisotropy are analyzed. The alkali-metal chalcogenides KPSe₆, Na₂Ge₂Se₅, and Li₂In₂GeSe₆ feature infinite one-dimensional (1D) chains of [PSe₆], 2D anionic framework of [Ge₂Se₅] layers and 3D [In₂GeSe₉] networks, respectively. It is found that the anionic group with low-dimensional configuration could enhance polarizability anisotropy and render large birefringence for the macroscopic structure. This provides evidence that a low-dimensionality configuration in the structure would be beneficial for the enhancement of optical anisotropy, which can motivate the exploration and design of novel IR birefringent materials.