Second harmonic generation in the lamellar ferrielectric CuInP2S6

Optical second harmonic generation (SHG) in the lamellar ferrielectric CuInP2S6 is studied for the first time. Samples synthesized by conventional high-temperature (HT) and room temperature solution-chemistry (SC) methods were used. SHG intensity measurements on a single crystal HT sample reveal a transition temperature Tc=313±2 K and a thermal dependence similar to that of the calculated spontaneous polarization; the frequency conversion efficiency is 26 times that of Y-cut quartz. A comparative study of HT and SC pressed-powder pellets suggests that short-range electric dipole order is associated with limited correlation lengths in the latter. Evidence is also given for a phase transition at 309 K in the SC sample.