Development of HAUP-based method for measuring simultaneously linear birefringence and dichroism near diagonal...

Optical anisotropy is a fundamental spectroscopic property of condensed matters and is crucial for understanding their electronic structures, molecular orientations, and symmetry. Among its aspects, linear birefringence (LB) and linear dichroism (LD) are important because they reflect differences in refractive indices and absorption coefficients for orthogonally polarized light components. The high-accuracy universal polarimeter (HAUP) method is widely used for measuring LB and LD because of its high sensitivity. The conventional HAUP measurement principle is based on the extinction position, where the transmitted light intensity is minimized. However, such measurement becomes challenging for strongly absorbing samples or in short-wavelength regions with weak light intensity. To overcome these limitations, we have developed a HAUP measurement principle based on the diagonal position, where the transmitted light intensity is maximized. The measurement equations are expanded around the diagonal position, giving a different equation structure from the extinction-position method. This configuration simplifies the measurement procedure, eliminates two-dimensional measurements and eliminates systematic errors such as parasitic ellipticities of the polarizer and analyzer. We have applied this measurement principle to MgF₂, and a strongly absorbing high-temperature cuprate superconductor, a Bi₂Sr₂CaCu₂O_8+δ crystals. The measured LB and LD values agreed well with those obtained using the conventional extinction-position method.