Phase-sensitive sum-frequency spectroscopy.

Phase-sensitive sum-frequency spectroscopy (SFS) allows the complete measurement of the complex spectra of surface nonlinear response coefficients. Similar to linear spectroscopy, the spectrum of the imaginary part of a surface response coefficient directly characterizes surface resonances without complication. This newly developed technique has greatly enhanced the capability of surface SFS and provides many new research opportunities for surface science. This article describes the experimental schemes and underlying theory for the technique and briefly reviews works that have clearly demonstrated its power.