Colossal Optical Anisotropy from Atomic-Scale Modulations.

Materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light-matter coupling, such as hyperbolic phonon polaritons. Layered 2D materials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure-Sr TiS -is transparent and positive-uniaxial, with extraordinary index n = 4.5 and ordinary index n = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS , results in the formation of TiS trigonal-prismatic units that break the chains of face-sharing TiS octahedra in SrTiS into periodic blocks of five TiS octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary index n and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation.