Nonlinear Interferometer for Complex Spectral Measurement: a Sensitive Probe of Complex Interfaces

Soft interfaces are common in biology, the environment, and technological applications. Probing these, particularly when the interface is buried between two condensed phases presents many challenges. The only current method available for probing such interfaces with molecular specificity is the vibrational spectroscopy, sum frequency generation (SFG). SFG attains surface sensitivity due to its nonlinear nature. Due to nonlinearity, separating overlapping signals is difficult, usually leading to nonunique separation. This problem has long been recognized by SFG practitioners and several methods for determining the complex components of the signal have been devised. None produce an absolute measurement of the complex signal. This contribution reports a nonlinear interferometer, that not only addresses this issue but also complex measurement issue, but also detects even low concentration interfacial species. The nonlinear interferometer has been demonstrated in both scanning and broadband SFG systems. This contribution reports on results from thin film and other interfaces. Specifically, we show that the conformation of molecules in a hydrophobic interface are significantly altered by water in the film.