Large femtosecond third-order optical nonlinearity of Bi3.15Nd0.85Ti3O12 ferroelectric thin films

Both the linear and nonlinear optical properties of Bi3.15Nd0.85Ti3O12 (BNT0.85) ferroelectric thin films deposited on quartz substrates were investigated. The fundamental optical constants were determined as a function of light wavelength by optical transmittance measurements. By performing single-beam Z-scan experiments with femtosecond laser pulses at a wavelength of 800 nm, the two-photon absorption (TPA) coefficient β and third-order nonlinear refraction index γ were measured to be 1.15 × 102 cm/GW and −8.15 × 10−3 cm2/GW, respectively. The large TPA is attributed to an indirect transition process via the intermediate energy levels and the large refractive nonlinearity is the result of the electronic polarization and ferroelectric polarization arisen from the femtosecond midinfrared radiation. The results indicate that the BNT0.85 thin film is a promising candidate for applications in nonlinear photonic devices.