  Polarimetric infrared 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 was discovered in quasi-1D narrow-bandgap hexagonal perovskite
sulfides, A1+xTiS3, specifically BaTiS3[1,2] and Sr9/8TiS3[3,4]. In these
materials, the critical role of atomic-scale structure modulations[4,5] in the
unconventional electrical[5,6], optical[7,8], and thermal[7,9] properties
raises the broader question of other materials that belong to this family. To
address this issue, for the first time, we synthesized high-quality single
crystals of a largely unexplored member of the A1+xTiX3 (X = S, Se) family,
BaTiSe3. Single-crystal X-ray diffraction determined the room-temperature
structure with the P31c space group, which is a superstructure of the earlier
reported[10] P63/mmc structure. The crystal structure of BaTiSe3 features
antiparallel c-axis displacements similar to BaTiS3,[2] but is of lower
symmetry. Polarization-resolved Raman and Fourier transform infrared (FTIR)
spectroscopy were used to characterize the optical anisotropy of BaTiSe3, whose
refractive index along the ordinary (perpendicular to c) and extraordinary
(parallel to c) optical axes was quantitatively determined by combining
ellipsometry studies with FTIR. With a giant birefringence {\Delta}n~0.9,
BaTiSe3 emerges as a new candidate for miniaturized birefringent optics for
mid-wave infrared to long-wave infrared imaging.
