Shedding light on surface effects: Nonlinear probes of complex materials

The refinement of materials to facilitate their use in a broad range of applications is dependent on a detailed characterization and understanding of their interaction with light. This is especially true for the properties of materials' surfaces and interfacial regions where deviations from the bulk structure significantly impact the flow of energy. Adding to the complexity of this problem is the fact that these regions contain an overall small number of reporters resulting in undetectable signal buried under the massive bulk response. To directly overcome these challenges, electronic sum frequency generation (eSFG) can selectively probe interfacial species, defects, and ordering. The sensitivity of this technique arises from the requirement that second order nonlinear signals originate from noncentrosymmetry that is inherent at surfaces and interfaces. Further, the enhancement of eSFG signal due to resonance of material transitions with any one of the three electric fields involved generates a spectrum analogous to linear absorption but originating solely from these regions of interest. Here we present our instrumental implementation of this technique which centers around the use of supercontinuum from a photonic crystal fiber for broadband spectral analysis and a microscopic apparatus to limit, and eventually probe, sample heterogeneity. Finally our application of this instrument to multiple crystalline materials provides new information to inform future design directions.