Phys.org  September 27, 2023
Materials with large birefringence are sought after for polarization control, 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. A team of researchers in the US (University of Wisconsin, Washington University, University of Southern California, Oak Ridge National Laboratory, Lawrence Berkeley National Laboratory) demonstrated that a bulk crystal with subtle periodic modulations in its structure—Sr9/8TiS3—is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3, resulted in the formation of TiS6 trigonal-prismatic units that break the chains of face-sharing TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr formed highly oriented electron clouds, which selectively boosted the extraordinary negative index and resulted in record birefringence. The connection between subtle structural modulations and large changes in refractive index suggested new categories of anisotropic materials and tunable optical materials with large refractive-index modulation… read more. Open Access TECHNICAL ARTICLEÂ
Structural modulation… Credit: Advanced Materials, 02 August 2023Â