Birefringence is a fundamental property of materials that enables optical components such as wave plates and polarizers, and is quantified by the difference between extraordinary and ordinary refractive indices. Solid homogeneous crystals like calcite and rutile are some of the most birefringent materials at visible and near-infrared wavelengths. However, at longer wavelengths (i.e., mid to far infrared) these materials become highly lossy. In the mid infrared, the most birefringent materials that are transparent are significantly less birefringent than their visible counterparts. While structured materials with strong optical anisotropy exist at these wavelengths (i.e., with form birefringence), their utility is limited by fabrication constraints.
In the talk, we will report on a rationally designed and synthesized material, barium titanium sulfide (BaTiS3), which has broadband and giant birefringence surpassing that of any known transparent anisotropic crystal throughout the infrared. We will detail our extensive optical characterization to extract the anisotropic complex refractive index spanning the ultraviolet to the mid infrared by combining generalized spectroscopic ellipsometery and polarized reflection and transmission measurements. We report a difference between the ordinary and extraordinary refractive index of up to 0.76 in a mid-infrared region of transparency, more than twice that of rutile in the visible, and show that the unprecedented optical anisotropy extends to the limit of our detection capabilities (16.7 μm). This material also features highly anisotropic Raman scattering, and we are currently working on measuring polarized infrared photoluminescence measurements to provide further insight into the anisotropy of this unique material.