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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 in great detail. It is shown that the SFG spectrum is heavily modified by linear optical effects.
Barmentlo et al. (Fri,) studied this question.
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