Abstract

A thus far unexplained nonlinear optical process was observed in KDP. A cw argonion laser (1 W) was used as a pump, and a He-Ne laser (100 µW) was used as a probe. The pump beam was chopped, and the two beams were combined and focused into the KDP. The probe light was detected after the KDP by a photodiode and lock-in detection. A modulation (of the order of 10⁻⁴) of the probe transmission was observed. This modulation was linear in the pump power and quadratic in the probe power. The modulation was seen only close to normal incidence with the KDP crystal, thus implying that the backreflected wave is involved in the interaction. The response time of the modulation was about the same as that calculated using the thermal properties of KDP and the beam size in the crystal. However, the quadratic dependence on the weak probe argues against a purely thermal model. A model involving cascaded X⁽²⁾⁾ processes explains the effect qualitatively but is orders of magnitude too weak for quantitative agreement. The effect was seen in two different KDP crystals with different thicknesses and cut at different angles.

© 1989 Optical Society of America