Optimizing imaging depth of anisotropic scattering tissues with polarization engineered second harmonic generation microscopy

Connective tissues in vertebrates consist of many anisotropic structures formed by collagen and muscle fibers, which could also generate intense second harmonic (SH). In SHG based tissue imaging, the incident light, when subjected to birefringence and scattering, would lead to a rapid decrement in imaging depth. The work simulating polarized light propagating through a thick and highly-scattering semi-infinite medium using a polarization-sensitive Monte Carlo model find that circular polarization would achieve deeper penetration depth. Henceforth, we use polarization engineered SHG imaging to investigate fish scales and pig tendon/dermis of various thickness, as well as the corresponding depolarization effect as a function of the imaging depth in this work. Critically, we have verified quantitatively the previous simulation results and presented the possibility to greatly improve the imaging of thick anisotropic and scattering tissues through engineering polarization. In parallel to wavefront shaping that uses a spatial light modulator or a wavefront sensor based deformable mirror to increase the signal-to-background (SBR) ratio in imaging, our approach is simple, effective, and sensitive to tissue anisotropy