Sum frequency generation (SFG) spectroscopy at surfaces and interfaces: Adsorbate structure and molecular bond orientation

Infrared (IR)-visible (Vis) sum frequency generation (SFG) is a second-order nonlinear optical process which is forbidden in centrosymmetric bulk media or isotropic phases, but allowed at (open) surfaces or (buried) interfaces where the inversion symmetry is broken. SFG spectroscopy is thus inherently surface- or interfacespecific, providing information about the structure, orientation, surface number density, chirality, and dynamics of molecules, provided the system of interest is accessible by light. This review illustrates basic SFG concepts, theory, operation modes (e.g., frequency-domain, broadband, homodyne/heterodyne, timeresolved), and recent extensions and developments of SFG (e.g., doubly resonant, plasmon-enhanced, chiral, microscopy). To illustrate the wide range of SFG applications, selected case studies discuss the characterization of molecular structure and bond orientation at solid-gas, solid-liquid, liquid-air, liquid-liquid, and solid-solid interfaces.