Electric-Field-Induced Second Harmonic Generation at Charged Interfaces: From Inception to the Eisenthal 𝜒(3) Method and Towards Interfacial Electrostatics. A Mini Review.

Electric-field-induced second harmonic generation (EFISH) has evolved from a fundamental nonlinear optical phenomenon into a powerful approach for probing the electrostatic and structural properties of charged interfaces. This Mini Review traces the historical development of EFISH from its origins in nonlinear optical physics to its modern implementation for studying physical and chemical phenomena at planar and non-planar interfaces. Particular emphasis is placed on the advances in scientific frameworks and subsequent efforts to obtain increasingly rigorous descriptions of charged interfaces. Current challenges and opportunities for quantitative nonlinear optical measurements of interfacial electrostatics are discussed.