Complete Dielectric Tensor and Giant Optical Anisotropy in Quasi-One-Dimensional ZrTe₅

Anisotropy in low-dimensional materials offers an extra degree of freedom to understand and tune the peculiar or potential properties to design novel electronic, optical, thermal, and optoelectronic devices. However, most research attention has been paid to qualitatively observing anisotropic phenomena, lacking quantitatively revealing optical anisotropy, in particular, and deeply digging for the physical mechanism. In this work, for the first time to our knowledge, the complete dielectric tensor of quasi-one-dimensional ZrTe5 is determined by Mueller matrix spectroscopic ellipsometry (MMSE) to quantitatively and comprehensively investigate the giant optical anisotropy, and the underlying physical mechanism is revealed by combination with the first-principle calculations. The ZrTe5 demonstrates giant dispersive birefringence and dichroism, and the birefringence (Δn = 0.58) outperforms many other low-dimensional and conventional birefringent materials, showing great potential and advantages in fabricating miniature and integrated polarization-sensitive devices. By combining the critical point (CP) analysis and first-principle calculations, the specific interband transitions related to the CPs in dielectric function spectra along three crystal axes of ZrTe5 are identified, revealing the physical essence of the optical anisotropy from the perspective of quantum mechanics. The proposed method is general and can be easily extended to quantitatively investigate the optical anisotropies in other novel low-symmetry materials.