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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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Giant Modulation of Refractive Index from Picoscale Atomic Displacements
Abstract It is shown that structural disorder—in the form of anisotropic, picoscale atomic displacements—modulates the refractive index tensor and results in the giant optical anisotropy observed in BaTiS3, a quasi‐1D hexagonal chalcogenide. Single‐crystal X‐ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS6chains along thec‐axis, and threefold degenerate Ti displacements in thea–bplane.47/49Ti solid‐state NMR provides additional evidence for those Ti displacements in the form of a three‐horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. Scanning transmission electron microscopy is used to directly observe the globally disordered Tia–bplane displacements and find them to be ordered locally over a few unit cells. First‐principles calculations show that the Tia–bplane displacements selectively reduce the refractive index along theab‐plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS3, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity.
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- PAR ID:
- 10500109
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials
- Volume:
- 36
- Issue:
- 24
- ISSN:
- 0935-9648
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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