Speedy Nonlinear Optical Analysis

Combining a novel laser-probe method with a fast data-analysis technique enables scientists to rapidly differentiate chemically distinct thin films solely on the basis of subtle differences in the optical properties, according to a research team at Purdue University (Anal. Chem., DOI: 10.1021/ac901832u). Changes in the polarization of light reflected from thin films can serve as a probe of the structure, orientation, and symmetry of molecules at the film’s surface. Yet those measurements can take hours to complete. Purdue’s Nathan J. Begue and Garth J. Simpson report that polarization measurements can now be made in fractions of a second by using a surface-sensitive method they developed known as nonlinear optical Stokes ellipsometry. Time-consuming data analysis based on traditional nonlinear curve fitting likewise can be sped up by orders of magnitude, they say, by combining linear curve fitting with a statistical technique known as principal component analysis. The researchers showed that the new combination can be used to quickly distinguish among monolayer films of four similar dyes. The team plans to apply the technique to probing crystal polymorphism.