Strain-induced second-harmonic generation in pseudocubic Ba_0.48Sr_0.52TiO₃ thin films

We report the observation of a strong second-order nonlinear optical response from epitaxial Ba_0.48Sr_0.52TiO₃ thin films grown on MgO(001) by magnetron sputtering. The result is unexpected since the crystal structure of Ba_0.48Sr_0.52TiO₃ is centrosymmetric under ambient conditions. A second-order nonlinear susceptibility d₃₃ as large as 11 pm/V at a fundamental wavelength of 1.064 μm is observed in a 300 Å 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. © 2000 American Institute of Physics