Measurement of wavelength-dependent second order susceptibility tensor elements in types I and II collagens

Abstract In this work, we measured the second order susceptibility (SOS) tensor ratios of χ zzz /χ zxx and χ xzx /χ zxx between 725–875 nm in type I (rat tail tendon) and II (rat trachea) collagen. In this wavelength range, χ zzz /χ zxx and χ xzx /χ zxx were found to be frequency-independent. Specifically, χ zzz /χ zxx and χ xzx /χ zxx are around 1.4 and 0.7 for collagen I; 1.2 and 0.4 for collagen II, respectively. Therefore, the near-infrared wavelength range of 700–800 nm is sufficiently far from the collagen resonance at 247 nm in order for the SOS tensor ratios to be wavelength independent. Our results suggest that microscopy imaging based on the use of SOS as a contract mechanism can use the simultaneous illumination of different excitation wavelengths with different polarizations for faster implementation of second order susceptibility microscopy.