Foundations of Second Harmonic Generation Circular Dichroism (SHG-CD) Microscopy

Abstract Recent advances in chiral microscopy have expanded the scope of SHG-CD well beyond biological tissue-imaging, including analysis of Moiré superlattices, twisted crystals, and other emergent quantum materials. The exceptional sensitivity of nonlinear optical processes to chirality combined with the experimental simplicity of second harmonic generation has established SHG-CD as a powerful platform for probing symmetry-breaking spanning molecular to microscopic length scales. In this work, recent physical models proposed for driving the most common SHG-CD observations are reviewed and cast within a unified conceptual and mathematical framework. Mechanisms include reciprocal SHG-CD from splayed stacking, nonreciprocal SHG-CD arising from the interplay of birefringence and chiral-specific SHG, resonance-enhanced chiral-specific SHG relevant to strongly coupled and structurally twisted materials, and chiral isotropic hyper-Rayleigh scattering arising from magnetic/electric dipole coupling.