Large nonlinear optical coefficients in pseudo-tetragonal BiFeO₃ thin films

Biaxial strain induces a phase transition from a pseudo-rhombohedral (R) to pseudo-tetragonal (T) phase in BiFeO₃ (BFO) thin films. Using optical second harmonic generation, we measure the nonlinear optical d_ij coefficients at a fundamental wavelength of 1550 nm for R and T-BFO thin films. A large increase of the d_ij magnitudes is observed for T-BFO in comparison to R-BFO. The d_ij magnitudes for T-BFO were measured to be: $|d33|=18.1±2.4, |d31|=60.8±8.1$⁠, and $|d15|=47.0±4.2$⁠, and for R-BFO: $|d33|=15.1±2.1, |d31|=8.5±1.2, |d15|=0.9±0.1$⁠, and $|d22|=18.7±2.6 (pm/V)$⁠. The strain-enhanced nonlinear optical properties of T-BFO thin films make them potentially useful for optical applications.