Measuring complex SFG: Characterizing a phase reference

Reactions and interactions at interfaces play pivotal roles in processes ranging from atmospheric aerosols influencing climate to battery electrodes determining charge–discharge rates to defects in catalysts controlling the fate of reactants to the outcome of biological processes at membrane interfaces. Tools to probe these surfaces at the atomic-molecular level are thus critical. Chief among non-invasive probes is the vibrational spectroscopy sum frequency generation (SFG). The complex signal amplitude generated by SFG requires techniques to interfere the unknown amplitude with a well-characterized one. An interferometric method is described to characterize the signal from any nonresonant reference material. The technique is demonstrated by measuring the phase of polycrystalline GaAs, chosen due to the strong signal and insensitivity to surface contamination. With a 515 nm visible field, the phase of GaAs is 54.5° ± 0.5°. The capability of choosing a reference based solely on its signal intensity enables probing a wide range of interfaces.