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Two-photon fluorescence and second harmonic generation phenomena in the ordered tissue

The knowledge of the peculiarities of nonlinear optical phenomena in different types of tissue is important for the development of new techniques for optical biomedical diagnostics. In this work, two-photon fluorescence (TPF) and second harmonic generation (SHG) have been studied simultaneously in ordered native tissue under the picosecond laser irradiation. For excitation of TFP with signal level in order of SHG the samples of tendon tissue were colored by Rhodamine 6G. The colored samples were irradiated by the laser beam in the direction, which was perpendicular to biofibers. SHG and TPF exhibited the different polarization dependence. Magnitude of SHG signal was about 2.5 times larger in case when polarization of the laser beam was parallel to the collagen fibers than in the perpendicular case. In contrast to SHG, the TPF signal was not show the polarization dependence. Since TPF and SHG both quadratically depend on intensity, the changing of irradiation intensity due to scattering would have the same impact on these phenomena. Influence of the light scattering was revealed using the weak linear polarized beam. The strong polarization dependence of SHG can be explained by the low propagation of the linear polarized light in the ordered tissue.