Strain-induced nonlinear susceptibility enhancement in thin film Ba_1-xSr_xTiO₃

The search for improved optoelectronic devices and ferroelectric field effect transistors demands the ability to manipulate optical properties in ferroelectric thin films. Ferroelectrics are materials with built-in electric polarization P, which breaks inversion symmetry and creates a strong electro-optic coefficient. Second harmonic generation (SHG) is used as a non-invasive optical method to characterize second order nonlinear susceptibility χ⁽²⁾, a property closely related to P, in thin film Ba_1-xSr_xTiO₃ (BSTO). Through lattice mismatch with the SrTiO₃(001) substrate, one is able to tune compressive strain induced at the substrate-film interface by changing the Sr molar ratio x in the BSTO alloy. An enhancement in χ⁽²⁾ is observed at x=0.3, corresponding to an approximate strain of 1.5% at the interface, which is indicative of the SHG sensitivity to strain via elongation of P, as well as a fine tuning in the strain relaxation mechanism resulting from misfit dislocations.