Optical Interference Enhances Nonlinear Spectroscopic Sensitivity: When Light Gives You Lemons, Model Lemonade

Optical interference effects can be a nuisance in spectroscopy, especially in nonlinear experiments in which multiple incoming and outgoing beams are present. Vibrational sum frequency generation is particularly susceptible to interference effects because it is often applied to planar, layered materials, driving many of its practitioners to great lengths to avoid signal generation from multiple interfaces. In this perspective, we take a positive view of this metaphorical lemon and demonstrate how optical interference can be used as a tool to extract subtle changes in interfacial vibrational spectra. Specifically, we use small frequency shifts at a buried interface in an organic field-effect transistor to determine the fractional charge per molecule during device operation. The transfer matrix approach to nonlinear signal modeling is general and readily applied to complex layered samples that are increasingly popular in modern studies. More importantly, we show that a failure to consider interference effects can lead to erroneous interpretations of nonlinear data.