Temperature Dependence of Optical Harmonic Generation in KDP and ADP Crystals

The temperature dependence of optical harmonics excited by a ruby laser in potassium and ammonium dihydrogen phosphate was investigated from 370^∘{}C down to the Curie temperatures. Both focused and unfocused laser beams were employed. The second harmonic intensity decreases slowly with decreasing temperature when a focused laser beam is employed. For an unfocused laser beam the second-harmonic intensity suffers a sharp decrease due to the destruction of the phase matching condition with changes in temperature. For KDP the phase matching angle was found to be a linear function of the temperature and no significant temperature dependence of the second-harmonic intensity was noted when the phase matching condition was satisfied. The temperature dependence of the second-harmonic generation may be explained in terms of the temperature dependence of the optical indices of refraction.