  Anisotropic photonic materials with linear dichroism are crucial components
in many sensing, imaging and communication applications. Such materials play an
important role as polarizers, filters and wave-plates in photonic devices and
circuits. Conventional crystalline materials with optical anisotropy typically
show unidirectional linear dichroism over a broad wavelength range. The linear
dichroism conversion phenomenon has not been observed in crystalline materials.
Here, we report the investigation of the unique linear dichroism conversion
phenomenon in quasi-one-dimensional (quasi-1D) hexagonal perovskite
chalcogenide BaTiS3. The material shows record level of optical anisotropy
within the visible wavelength range. In contrast to conventional anisotropic
optical materials, the linear dichroism polarity in BaTiS3 makes an orthogonal
change at an optical wavelength corresponding to the photon energy of 1.78 eV.
First principle calculations reveal that this anomalous linear dichroism
conversion behavior originates from different selection rules of the optical
transitions from the parallel bands in the BaTiS3 material. Wavelength
dependent polarized Raman spectroscopy further confirms this phenomenon. Such
material with linear dichroism conversion property can facilitate new ability
to control and sense the energy and polarization of light, and lead to novel
photonic devices such as polarization-wavelength selective detectors and lasers
for multispectral imaging, sensing and optical communication applications.
