Strain-induced second-harmonic generation in pseudocubic Ba0.48Sr0.52TiO3 thin films

We report the observation of a strong second-order nonlinear optical respon se from epitaxial Ba0.48Sr0.52TiO3 thin films grown on MgO(001) by magnetro n sputtering. The result is unexpected since the crystal structure of Ba0.4 8Sr0.52TiO3 is centrosymmetric under ambient conditions. A second-order non linear susceptibility d(33) as large as 11 pm/V at a fundamental wavelength of 1.064 mu m is observed in a 300 Angstrom thick film. By studying a seri es of films with different strains, induced by a film-substrate lattice mis match, we conclude that the resulting tetragonal distortion, presumably acc ompanied by a loss of inversion symmetry, is responsible for the second-har monic generation. (C) 2000 American Institute of Physics. [S0003-6951(00)02 210-5].