Heterogeneity and Birefringence of Soft Tissues Probed by the Polarization-Sensitive Terahertz Solid Immersion Microscopy.

The polarization-sensitive THz solid immersion (SI) microscope is applied to quantify the heterogeneity and birefringence of freshly-excised rat tissues: the abdominal and tongue muscles, tendons of the flexor digitorum and extensor digitorum of the forelimb, Achilles tendon, abdominal aorta, and Corpus callosum of the brain. THz refractive indices of tissues are retrieved for the two orthogonal linear polarizations of the incident beam, which are collinear and transverse toward the fibrous structures. The observed THz birefringence, as high as a few percent, is attributed to tissue morphology relying on the hematoxylin and eosin (H&E)-stained histology. Expectedly, the THz refractive index is higher for the incident field collinear to the fibrous structures. The muscle tissues and Corpus callosum manifest the most pronounced birefringence, while that of tendon is sufficiently lower. Our findings uncover notable heterogeneity and birefringence of tissues; those should be accounted for when developing novel methods of THz biophotonics.