Quantitative Polarization‐Resolved Second‐Harmonic‐Generation Microscopy of Glycine Microneedles

Abstract Second‐harmonic generation (SHG) is a nonlinear optical process that can provide disease diagnosis through characterization of biological building blocks such as amino acids, peptides, and proteins. The second‐order nonlinear susceptibility tensor χ⁽²⁾ of a material characterizes its tendency to cause SHG. Here, a method for finding the χ⁽²⁾ elements from polarization‐resolved SHG microscopy in transmission mode is presented. The quantitative framework and analytical approach that corrects for micrometer‐scale morphology and birefringence enable the determination and comparison of the SHG susceptibility tensors of β‐ and γ‐phase glycine microneedles. The maximum nonlinear susceptibility coefficients are d₃₃ = 15 pm V⁻¹ for the β and d₃₃ = 5.9 pm V⁻¹ for the γ phase. The results demonstrate glycine as a useful biocompatible nonlinear material. This combination of the analytical model and polarization‐resolved SHG transmission microscopy is broadly applicable for quantitative SHG material characterization and diagnostic imaging.