Gold as a standard phase reference in complex sum frequency generation measurements.

Complex, soft interfaces abound in the environment, biological systems, and technological applications. Probing these interfaces, particularly those buried between two condensed phases presents many challenges. The only current method capable of probing such interfaces with molecular specificity is the vibrational spectroscopy, sum frequency generation (SFG). SFG is a nonlinear method, which often results both in small signals from minority species being lost in the noise and nonunique separation of resonances. Both issues can be addressed if the complex amplitude rather than the intensity-the square of the amplitude-spectrum is measured. Thus, several methods have been developed to determine the complex spectrum by measuring the sample of interest with respect to a nonresonant material. Incorrect assumptions about the phase of the nonresonant material can result in ambiguity about the sample complex components. This ambiguity can be removed if a phase standard is identified and the phase of the chosen reference material is measured against the standard. This paper reports both verification of a phase standard-Z-cut quartz-and measurement of the phase of gold against this standard. Using this phase standard, the standard phase of Au is determined to be -222° with 532 nm excitation and ppp polarization.