Resonant third harmonic generation in biological pigments

Third harmonic generation (THG) microscopy provides label-free structural contrast of biological tissues. While the THG process can be enhanced by electronic transitions in absorbing molecules, knowledge of the third-order nonlinear properties of biological pigments is limited, and the potential of THG imaging for chemically selective imaging remains underutilized. In this study, we investigated resonant THG in pigments by performing nonlinear microspectroscopy in situ on three representative absorbers: hemoglobin in red blood cells, pteridines in xanthophores, and melanin in human hair. We measured THG spectra in the 1120–1300 nm excitation range. We observed a wavelength-dependent signal enhancement ranging from 10 to 100 times in these pigmented structures, depending on their respective absorption properties. We developed a numerical model of THG from interfaces, including resonant and non-resonant contributions, to interpret these observations. Our calculations reproduce key experimental trends, including resonance-induced enhancement factors and spectral properties. Finally, we demonstrate that third-order sum-frequency generation (TSFG) imaging provides spectroscopic contrast capable of distinguishing red blood cells from xanthophores in live zebrafish larvae. These results suggest that THG/TSFG is a promising approach to label-free, pigment-specific multiphoton imaging.