Materials with large optical anisotropy are sought after for polarization control, nonlinear phase matching, the realization of unconventional surface waves, among other applications in classical and quantum optics. Here, we demonstrate a bulk uniaxial crystal with atomic-scale structural modulations, Sr9/8TiS3, has a record birefringence Δn = 2.1 across a broad transparent window in the mid- to far-infrared. The excess Sr atoms, compared to stoichiometric SrTiS3, introduce additional electrons into TiS6 octahedral blocks to form highly polarizable clouds, which selectively boost the extraordinary refractive index. Structural modulation is a new tool for the engineering of refractive-index and anisotropy of quantum materials.
Birefringence is a fundamental optical property of anisotropic materials where the refractive index depends on the polarization of light, and is an essential property for devices such as waveplates and polarizers. In 2018, we reported barium titanium sulfide (BaTiS3) to have a broadband birefringence of 0.76 spanning the mid-to-far-infrared range, exhibiting the largest in-plane birefringence of any known bulk materials. In this talk, we will present the characterization of giant birefringence of two more engineered A1+xBX3 crystals, strontium titanium sulfide (Sr1+xTiS3) and barium titanium selenide (BaTiSe3). Our characterization combines polarization-resolved infrared spectroscopy with generalized ellipsometry to extract the optical properties.
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