Shell-Core Structural Anisotropy in Starch Granules Revealed by Polarization-Dependent Third-Harmonic Generation Imaging

We investigate the polarization-dependent third-harmonic generation (P-THG) response of starch granules, revealing distinct structural organization between their inner and outer regions. By rotating the linear polarization of the excitation beam, we uncover two characteristic P-THG modulation patterns within single granules, corresponding to an outer shell and a structurally distinct inner core. These patterns are analyzed using a theoretical model based on orthorhombic symmetry, allowing us to extract key ratios of third-order nonlinear susceptibility tensor components and determine the average molecular orientation within each region. In particular, we define an anisotropy ratio, AR=\c{hi}_xxxx^((3) )/\c{hi}_yyyy^((3) ) as a quantitative descriptor of local molecular alignment. Our results show that the shell and core exhibit significantly different AR values, underscoring a transition in molecular organization across the granule. This study establishes P-THG as a powerful contrast mechanism for probing sub-micron structural heterogeneity in biological materials beyond conventional THG intensity imaging.