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: | d ₃₃| = 15.1 ± 2.1, |d₃₁| = 8.5 ± 1.2, |d ₁₅| = 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.