  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.
