Nonlinear optical effects in Bi12TiO20 nanocrystallites embedded within a photopolymer matrix

Photoinduced nonlinear optical effects in large-sized (up to 20 nm) Bi12TiO20 (BTO) nanocrystallites incorporated within olygoether photopolymer matrix have been studied. Photoinduced optical second harmonic generation (PISHG) was measured. Nd:YAG pulsed laser (lambda = 1.06 mum) was used as a source of photoinducing light. As a fundamental light source for the SHG and two-photon absorption, Er:LiYF4 laser (lambda = 2.065 mum) was used. We have found that with increasing IR pump power density, the output doubled frequency SHG signal (lambda = 1.03 mum) increases and achieves its maximum value at the pump power density about 1.4 GW/cm(2) and nanocrystallite (NC) sizes about 15 run. The values of second-order optical susceptibilities were almost 15% smaller compared to d(eff) tensor of the KDP single crystals. With decreasing temperature below 22 K, the SHG signal increases. Pump-probe measurements indicate the occurrence of the SHG maximum at pump-probe delaying time of about 29 ps. This effect is explained within a framework of photoinduced anharmonic electron phonon interaction due to the creation of electrostricted phonons. Two-photon absorption was also investigated. Quantum chemical simulations have shown that the TiO4 structural fragments play a crucial role in the observed NLO effects. The obtained results show that the mentioned nanocomposites can be used as a powerful tool for creation of IR-operated nonlinear optical devices. (C) 2004 Elsevier B.V. All rights reserved.