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.
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.
Engineered mid-infrared absorbers and thermal emitters have recently enabled a variety of applications including passive cooling and acceleration of water condensation. We demonstrate a large-area absorber in the 8-10 μm range based on a lossy medium with an ultra-low refractive index. Our low-index material is a large-area inverse opal film (air holes, silica matrix), which is crack-free on the centimeter scale. Due to the large fraction of air in these structures (more than 70%), their effective refractive index is close to that of air, facilitating an impedance match for a broad range of incidence angles, and requiring no top-down patterning.
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