Chirality-sensitive defect localization in helical nanoparticle assemblies via generalized Mie scattering polarimetry

Chiral helical nanoparticle assemblies are promising for photonics and biosensing, but self-assembly defects compromise reliability. Conventional defect detection typically relies on invasive electron microscopy, which falls short of the requirements for rapid, non-destructive quality control. In this work, we establish a generalized-Mie-theory-based quantitative framework that links point defect geometry to far-field polarization responses via scattering polarimetry. For a 15-particle SiO₂ helical chain containing two representative defect prototypes-vacancy (Type A) and relaxed vacancy (Type B)-we find that local symmetry breaking selectively perturbs the collective eigenmodes, yielding pronounced and position-dependent scattering signatures. The backscattering cross section (BCS) reveals that a defect at the chain center induces exceptionally strong resonance peaks at specific incident angles (30°, 70°, and 90°), distinctly different from those of end-located defects or the perfect structure. Multipole decomposition reveals order-selective relative changes in scattering power, confirming that defects induce mode redistribution rather than uniform attenuation. The Mueller matrix element M₁₄ in the forward scattering region (0°-30°) offers discriminative fingerprints via oscillation amplitudes, extrema, and waveform topologies (V, S, M, N, C, and L). Under particle-size polydispersity of ±5%, center-defect fingerprints remain clearly distinguishable and most robust. Our findings move beyond viewing defects as mere imperfections, establish a direct link from defect configuration to polarization response, and provide a theoretical foundation for non-invasive inspection of self-assembled photonic devices.