Sub-wavelength resolution terahertz polarization-sensitive microscopy based on solid immersion effect

Terahertz (THz) technologies have many applications in medical diagnostics and therapy. Most are based on effective medium theory, which assumes that biological tissues are optically isotropic and homogeneous over the scales determined by THz wavelengths. Meanwhile, recent studies have shown the possibility of visualizing mesoscale $(\sim\lambda)$ tissue inhomogeneities using THz microscopy methods, where $\lambda$ is the wavelength. In this regard, the problem arises of studying the corresponding effects of scattering and polarization of THz radiation during interaction with biological tissues, for which there are no suitable tools. To solve this problem, a polarization- sensitive reflection-mode THz microscope based on the solid immersion effect has been developed. It uses a silicon hemispherical immersion lens, a metal wire mesh polarizer and analyzer, a backward wave oscillator as a continuous wave source at 0.6 THz ($\lambda$ = 500 $\mu$m) and a Golay detector. This system makes it possible to study the local polarization-dependent THz response of mesoscale structural elements of tissues with a resolution of up to 0.15$\lambda$. Using the developed method, THz images of test media were obtained for two orthogonal states of polarization of incident THz radiation, which made it possible to identify their THz birefringence (structural optical anisotropy). The structural anisotropy of the THz response of porous biomorphic silicon carbide ceramics is considered. Refractive index distributions of freshly excised rat brain were obtained, where the most pronounced THz birefringence is observed in the Corpus callosum, formed by oriented and densely packed axons connecting the cerebral hemispheres. The obtained results show the perspectives of applying THz polarization-sensitive microscopy in biophotonics and medical imaging.