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Abstract Materials with large birefringence (Δn, wherenis 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—Sr9/8TiS3—is transparent and positive‐uniaxial, with extraordinary indexne= 4.5 and ordinary indexno= 2.4 in the mid‐ to far‐infrared. The excess Sr, compared to stoichiometric SrTiS3, results in the formation of TiS6trigonal‐prismatic units that break the chains of face‐sharing TiS6octahedra in SrTiS3into periodic blocks of five TiS6octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary indexneand 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.
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Infrared Optical Anisotropy in Quasi‐1D Hexagonal Chalcogenide BaTiSe 3
Abstract Polarimetric infrared (IR) detection bolsters IR thermography by leveraging the polarization of light. Optical anisotropy, i.e., birefringence and dichroism, can be leveraged to achieve polarimetric detection. Recently, giant optical anisotropy is discovered in quasi‐1D narrow‐bandgap hexagonal perovskite sulfides, A1+xTiS3, specifically BaTiS3and Sr9/8TiS3. In these materials, the critical role of atomic‐scale structure modulations in the unconventional electrical, optical, and thermal properties raises the broader question of the nature of other materials that belong to this family. To address this issue, for the first time, high‐quality single crystals of a largely unexplored member of the A1+xTiX3(X = S, Se) family, BaTiSe3are synthesized. Single‐crystal X‐ray diffraction determined the room‐temperature structure with theP31cspace group, which is a superstructure of the earlier reportedP63/mmcstructure. The crystal structure of BaTiSe3features antiparallelc‐axis displacements similar to but of lower symmetry than BaTiS3, verified by the polarization dependent Raman spectroscopy. Fourier transform infrared (FTIR) spectroscopy is used to characterize the optical anisotropy of BaTiSe3, whose refractive index along the ordinary (E⊥c) and extraordinary (E‖c) optical axes is quantitatively determined by combining ellipsometry studies with FTIR. With a giant birefringence Δn∼ 0.9, BaTiSe3emerges as a new candidate for miniaturized birefringent optics for mid‐wave infrared to long‐wave infrared imaging.
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- PAR ID:
- 10539585
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Optical Materials
- Volume:
- 12
- Issue:
- 29
- ISSN:
- 2195-1071
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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