Stress-induced enhancement of second-order nonlinear optical susceptibilities of barium titanate films

A method introducing stress is suggested to enhance optical nonlinearity. Three stress induced samples of BaTiO3 thin film, BaTiO3/SrTiO3 superlattice, and cerium doped BaTiO3/SrTiO3 superlattice were epitaxially grown on SrTiO3 (100) substrates by laser molecular beam epitaxy. The measurement of the dependence of the effective second-harmonic generation (SHG) coefficients on the fundamental polarization angle and incident angle indicates that SHG coefficients d(33) of the three samples are greatly enhanced, with the maximum value being one order of magnitude larger than that of the bulk BaTiO3 crystal. Our results clearly show that the introduction of stress is a very powerful means to enhance nonlinear optical susceptibility. (C) 2000 American Institute of Physics. [S0021-8979(00)03910-4].