Giant Modulation of Refractive Index from Picoscale Atomic Displacements

Abstract

Structural disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical, and mechanical properties of materials. However, the possibility of obtaining novel optical and optoelectronic properties from structural disorder remains an open question. Here, we show unambiguous evidence of disorder in the form of anisotropic, picoscale atomic displacements modulating the refractive index tensor and resulting in the giant optical anisotropy observed in BaTiS${}_3$, a quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS${}_6$ chains along the c-axis, and three-fold degenerate Ti displacements in the a-b plane. ${}^{47/49}$Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. We used scanning transmission electron microscopy to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS${}_3$, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity.

arXiv comment24 pages, 3 figures

References in corpus (8)

• All-electron magnetic response with pseudopotentials: NMR chemical shifts
• Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics
• Hidden structural order controls Li-ion transport in cation-disordered oxides for rechargeable lithium batteries
• Anomalous Localization in Low-Dimensional Systems with Correlated Disorder
• Emergence of long-range order in BaTiO3 from local symmetry-breaking distortions
• A first-principles approach to nonlinear lattice dynamics: Anomalous spectra in PbTe
• Orientation Controlled Anisotropy in Single Crystals of Quasi-1D BaTiS3
• Tuning electronic and phononic states with hidden order in disordered crystals

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