Phase-sensitive surface spectroscopy, simplified

The interface between dissimilar materials—solid and liquid, solid and gas, liquid and gas, or even two different solids—is often the setting for interesting and important physical, chemical, and biological phenomena. Sum frequency generation (SFG) spectroscopy is a versatile tool for probing such interfaces: It reveals the vibrational spectra of molecules there and can be used to deduce the molecules’ identities and orientations. It works by shining a visible laser beam of fixed frequency and an IR laser beam of adjustable frequency on the same spot on the surface. When the IR frequency matches an illuminated molecule’s vibrational resonance, the molecule emits photons at the sum of the two beam frequencies. (See the article by Gabor Somorjai and Jeong Young Park, Physics Today, October 2007, page 48 .)

Most SFG spectroscopy setups, however, detect only the intensity of the surface response; phase and amplitude information—which contains details about dynamics, overlapping resonances, and more—gets lost. Phase measurements are difficult, in large part because they are vulnerable to subtle changes in geometry—due, for instance, to temperature drift. Jing Wang and colleagues in Mary Jane Shultz’s lab at Tufts University have demonstrated a new approach that uses a nonlinear interferometer to stabilize and directly measure the phase response. The interferometer splits both input beams, directing one part of each split beam to the sample and the other to a reference surface (the experimenters used quartz). The SFG responses of the two surfaces get recombined, and the resulting interference pattern reveals the phase information with high sensitivity. Moreover, the interferometer can be used to actively compensate for drift. Without stabilization, the SFG interference can shift from constructive to destructive over minutes; with it, the interference is stable over weeks. (J. Wang et al., J. Chem. Phys. 147, 064201, 2017, doi:10.1063/1.4997736 .)