Temperature Dependence of Optical Harmonic Generation in KH2PO4Ferroelectrics

Abstract

Optical second-harmonic generation from K${\mathrm{H}}_2$P${\mathrm{O}}_4$ and K${\mathrm{D}}_2$P${\mathrm{O}}_4$ has been measured as a function of temperature above and below the ferroelectric transition, between 77 and 300°K. The relative values of the non-linear constants were obtained over this temperature range. They are independent of temperature in the paraelectric phase, but change by approximately 20% at the ferroelectric transition. Second-harmonic generation from ${\chi }_{33}$ in the ferroelectric transition was not observed. This element is not allowed by symmetry in the paraelectric phase. An extension of the theory of nonlinear optical processes to the ferroelectric phase is discussed. Higher order tensor elements, consistent with the crystal symmetry in the paraelectric phase and the static polarization, are introduced that account for the changes in the linear and nonlinear optical susceptibilities below the transition. Single-domain crystals are necessary for the measurements in the ferro-electric phase. Multiple-domain crystals yield nonreproducible and misleading results because the domain size is comparable to the phase coherence length between the fundamental and harmonic rays. Second-harmonic generation from N${\mathrm{H}}_4$ ${\mathrm{H}}_2$P${\mathrm{O}}_4$ was observed from 300°K to the antiferroelectric transition point. The nonlinear constants are independent of temperature.