Femtosecond Third-order Optical Nonlinearity Of BNT Ferroelectric Thin Films

The nonlinear optical(NLO) materials have attracted many intensive research efforts because of their potential civilian and military applications in signal processing, ultrafast optical communication, data storage, optical limiting, logic devices, optical switching, image transmission, and microfabrication. Now the research advances in NLO materials field depend critically and primarily on the development of new materials with strong NLO capabilities and effective tailoring of their NLO properties. Ferroelectric thin film is a promising candidate material for applications in nonlinear photonic devices due to their wide optical transmission, large and ultrafast nonlinear responses. In this thesis, Bi3.15Nd0.85Ti3O12(BNT) ferroelectric thin films were fabricated on the quartz substrates by plused laser deposition(PLD) technique and sol-gel method, respectively. The femtosecond third-order NLO properties of BNT thin films were investigated and a dramatic tunability of NLO absorption in BNT thin films was achieved by film thickness variation. The main contents and results are summarized as follows:1. The construction of the single-beam Z-scan systemTo study the NLO properties of the BNT thin films, a single-beam Z-scan system was built based on the femtosecond laser system which consisted of a mode-locked Ti:Sapphire oscillator and a regenerative amplifier(800 nm, 300 fs). And the femtosecond Z-scan experimental system was first calibrated with the carbon disulfide and then confirmed by the 1-mm-thick quartz substrate.2. The femtosecond third-order optical nonlinearity of the BNT ferroelectric thin filmsThe BNT thin films were fabricated on the quartz substrates by the sol-gel method. X-ray diffraction(XRD) and a UV-VIS-near infrared spectrophotometer were used to study the microstructure and linear optical property. The results indicated that the papered BNT films exhibit preferred a-orientation, and have high transmittance and large band-gap. By performing the single-beam Z-scan experiments with femtosecond laser pulses, the two-photon absorption(TPA) coefficient ? and third-order nonlinear refraction index ? were measured to be 1.15×102 cm/GW and-8.15×10-3 cm2/GW, respectively. The large ? and ? demonstrated the good NLO properties of the BNT thin films in the femtosecond range. The TPA is attributed to an indirect transition process via the intermediate energy levels. The large refractive nonlinearity may be the result of the electronic polarization and ferroelectric polarization.3. Nonlinear optical absorption tuning in BNT ferroelectric thin filmsThe BNT thin films with different film thickness(106.8, 139.7 and 312.9 nm) were fabricated on the quartz substrates by PLD. All these films have desired layered perovskite structures and exhibit preferred c-orientation. With the increase of the film thickness, the grain size of the BNT films increase and the band-gap decrease from 4.02 eV to 3.68 eV. The NLO absorption properties of the BNT thin films were determined by the single-beam Z-scan system constructed above. The representative open-aperture Z-scan transmittance spectra of three samples demonstrated that a dramatic tunability of NLO absorption in BNT thin films was achieved by thickness variation, which can not only switch the NLO absorption type from saturable absorption to reverse saturable absorption, but also improve the NLO absorption performance. The competition between the ground-state excitation and the two-photon absorption, which depends critically on the band-gap energy of the films and the density of defect states in the band-gap, is responsible for the observed switching behavior.