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 ${\mathrm{BaTiO}}_{\mathrm{3}}$ thin film, ${\mathrm{BaTiO}}_{\mathrm{3}}{\mathrm{/SrTiO}}_{\mathrm{3}}$ superlattice, and cerium doped ${\mathrm{BaTiO}}_{\mathrm{3}}{\mathrm{/SrTiO}}_{\mathrm{3}}$ superlattice were epitaxially grown on ${\mathrm{SrTiO}}_{\mathrm{3}}$ (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 ${\mathrm{BaTiO}}_{\mathrm{3}}$ crystal. Our results clearly show that the introduction of stress is a very powerful means to enhance nonlinear optical susceptibility.