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

We report the observation of a strong second-order nonlinear optical response from epitaxial Ba0.48Sr0.52TiO3 thin films grown on MgO(001) by magnetron sputtering. The result is unexpected since the crystal structure of Ba0.48Sr0.52TiO3 is centrosymmetric under ambient conditions. A second-order nonlinear 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 series of films with different strains, induced by a film-substrate lattice mismatch, we conclude that the resulting tetragonal distortion, presumably accompanied by a loss of inversion symmetry, is responsible for the second-harmonic generation. (C) 2000 American Institute of Physics. [S0003-6951(00)02210-5].