Sum-Frequency Generation with a Free-Electron Laser - a Study of Gallium-Phosphide

The use of a free-electron laser (FEL) allows the study of (non)linear optical properties of materials over unsurpassed large spectral intervals. Here we report on the application of a FEL as the IR source in spectroscopic infrared-visible sum-frequency generation (SFG). Using the extremely wide tunability and the high peak power of a FEL we have studied the frequency dependence of the nonlinear susceptibility for sum-frequency generation in gallium phosphide between 20 and 32 mum in great detail. It is shown that the SFG spectrum is heavily modified by linear optical effects.