Giant optical anisotropy in a quasi-one-dimensional crystal

Optical anisotropy is a fundamental building block for linear and nonlinear optical components such as polarizers, wave plates, and phase-matching elements(1-4). In solid homogeneous materials, the strongest optical anisotropy is found in crystals such as calcite and rutile(5,6). Attempts to enhance anisotropic light-matter interaction often rely on artificial anisotropic micro/nanostructures (form birefringence)(7-11). Here, we demonstrate rationally designed, giant optical anisotropy in single crystals of barium titanium sulfide (BaTiS3). This material shows an unprecedented, broadband birefringence of up to 0.76 in the mid-to long-wave infrared, as well as a large dichroism window with absorption edges at 1.6 mu m and 4.5 mu m for light with polarization along two crystallographic axes on an easily accessible cleavage plane. The unusually large anisotropy is a result of the quasi-one-dimensional structure, combined with rational selection of the constituent ions to maximize the polarizability difference along different axes.