Studies of Interfacial Regions by Sum-Frequency Generation with a Free-Electron Laser

The use of a Free-Electron Laser (FEL) allows the study of (non)linear optical properties of materials over unsurpassed large spectral intervals. As an example, we report on the use of a FEL as the infrared source in spectroscopic infrared-visible Sum-Frequency Generation (SFG). Employing the extremely wide tunability of the Free-Electron Laser for Infrared eXperiments (FELIX) at Rijnhuizen, we have studied the frequency dependence of the nonlinear susceptibility for sum-frequency generation in gallium phosphide between 20 and 32 mu m in great detail. We have developed a short-pulse visible laser system that is highly synchronous with FELIX thereby creating a two-color setup that can be broadly applied. Resonantly enhanced SFG in alpha-quartz has been used to study the relative timing stability of FELIX and the synchronized picosecond-laser system.