Low-frequency measurements of electro-optic coefficients of Na:KTP, and   RTA
  We report the measurement of the r23 and r33 electro-optic coefficients and of the z-axis electrical conductivity of the nonlinear optical crystals Na:KTP and RTA. We observed a marked decrease of the electro-optic effect in the Na:KTP crystals for frequencies below 100 Hz. This effect is absent in RTA. 
Daruo Xie and Olivier Pfister