The piezo-induced second-order nonlinear optical effect in polyvinyl alcohol/YAB:Cr nanocomposites

Piezo-induced picosecond nonlinear optical effects are investigated in polyvinyl alcohol photopolymer doped by YAB:Cr nanocrystallites of 10-40 nm size. The second harmonic generation (SHG) is studied versus the applied uniaxial mechanical stress up to 20 GPa. The observed phenomena are explained within a framework of fifth-order nonlinear optical susceptibilities with the inclusion of piezo-induced phonons, which favour the observed non-centrosymmetric electron-phonon anharmonicity. The substantial role of the nanocrystalline confinement effect in the observed phenomenon is demonstrated through the size-dependent behaviour of the effective second-order susceptibility. Quantum chemical and molecular dynamics simulations of the observed phenomenon are carried out.