β-Sheet Alignment in Intact Feathers Revealed by Polarization-Resolved Second Harmonic Generation (SHG) and Coherent Raman Imaging

We visualized β-sheet alignment in intact feathers using polarization-resolved second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) imaging. SHG signal intensity was strongly dependent on the polarization of the incident laser, while the amide I and III bands in the CARS spectra exhibited moderate polarization-dependent behavior. These results demonstrate that SHG and CARS enable complementary, label-free assessment of structural anisotropy in β-sheet–rich protein fibers, with SHG being especially sensitive to the orientation of the peptide backbone.