Preeti Gahtori, Raiden Speelman, Franz M. Geiger, Julianne Gibbs
Second Harmonic Generation (SHG) is a coherent second-order nonlinear optical technique that provides interface-specific information about molecular orientation, surface structure, and electrostatics across a broad range of interfaces. This Primer covers conventional intensity-based SHG and modern phase-resolved SHG (PR-SHG), with emphasis on the fundamentals and instrumentation, as well as data collection, processing, and analysis. Representative applications in acid-base interfacial chemistry, biological sensing, electrochemistry, and high-resolution imaging illustrate the method's scope. Practical guidance on reproducibility, current limitations, and future directions is also provided.