  We report on the experimental investigation of the efficiency of some
nonlinear crystals to generate microwave (RF) radiation as a result of optical
rectification (OR) when irradiated with intense pulse trains delivered by a
mode-locked laser at $1064\,$nm. We have investigated lithium triborate (LBO),
lithium niobate (LiNbO$_3$), zinc selenide (ZnSe), and also potassium titanyl
orthophosphate (KTP) for comparison with previous measurements. The results are
in good agreement with the theoretical predictions based on the form of the
second-order nonlinear susceptibility tensor. For some crystals we investigated
also the second harmonic generation (SHG) to cross check the theoretical model.
We confirm the theoretical prediction that OR leads to the production of higher
order RF harmonics that are overtones of the laser repetition rate.
