Dongyi Guan

Resonant absorption quenching and enhancement of optical nonlinearity in Au:BaTiO3 composite films by adding Fe nanoclusters

We present an approach to enhance the figure of merit χ(3)/α (with χ(3) being the third-order nonlinear susceptibility and α the optical absorption coefficient) in metal-dielectric composites. Nanocomposite thin films containing metal nanoclusters were obtained by embedding gold, or gold+iron in BaTiO3 matrices using the pulsed laser deposition technique. In the optical absorption spectra, the Au:BaTiO3 films showed a strong absorption peak around 562 nm due to surface plasmon resonance of Au particles. However, when a small amount of Fe was added, the resonant absorption was quenched, and χ(3) was enhanced so that χ(3)/α was significantly increased. Our study suggests a method by which to design optical nonlinear materials with large figure of merits for practical application.

Enhancement of optical nonlinearity in periodic gold nanoparticle arrays

Linear and nonlinear optical properties of periodic triangular Au nanoparticle arrays were investigated. We compared the optical nonlinearity of periodic Au nanoparticle arrays with that of the ultra-thin gold film consisting of randomly distributed spheroidal clusters. A pronounced enhancement of the third-order nonlinear optical susceptibility χ((3)) in Au arrays was observed, and the figure of merit, χ((3))/α, of the periodic nanoparticle arrays is one order of magnitude larger than that of the ultra-thin film. Such an enhancement of the optical nonlinearity could be due to the strong local field near the triangular nanoparticles.

Picosecond nonlinear optical responses of Au/PVP composite films

Nonlinear optical properties of colloidal Au and poly(vinylpyrrolidone) (PVP) composite films were investigated using the Z-scan technique with 25 ps pulses at 532 nm near the surface plasmon resonance frequency of Au nanoparticles. Experimental results show large optical nonlinearities of the composite films with the real and imaginary parts of the third-order nonlinear optical susceptibility χ(3) being 4.7 × 10−10 esu and 2.5 × 10−10 esu, respectively. The obtained χ(3) value of Au/PVP composite films is comparable or higher than that of the organometallic complexes reported. Furthermore, a strong optical limiting response was observed and a possible application of the composite films as an optical limiter of picosecond laser pulses was discussed.

Large third-order optical nonlinearity of periodic gold nanoparticle arrays coated with ZnO

The third-order optical nonlinearity of a composite film consisting of periodic triangular gold nanoparticle arrays coated with ZnO was investigated by the z-scan technique. The third-order nonlinear susceptibility, ?(3), measured at a wavelength of 532?nm with a pulse duration of 10?ns, was up to 1.3 ? 10?5?esu. The figure of merit, ?3/? (where ? is the absorption coefficient) was about 1.1 ? 10?9?esu?cm. The large optical nonlinearity is likely due to the strong local electromagnetic field near the triangular gold particles and ZnO matrix. The results show that the composite film has great potential for nonlinear optical devices.

Theoretical simulation and experimental study on resistivity properties of mixed-phase La2∕3Ca1∕3MnO3 thin films

The theoretical simulation and experimental study are reported on the metal-to-insulator transition, thermal hysteresis behavior, magnetic-field-induced reduction, and anisotropic characteristics of resistivity for La2∕3Ca1∕3MnO3 thin films deposited on SrTiO3 (001)-oriented substrates tilted by 10° towards the [010] direction. The simulated results obtained by using a random network model based on phase separation scenario are in quantitative agreement with our experimental data and indicate that tilting (applying magnetic field) can increase (decrease) the scatterings and the activation energy, resulting in enhancement (reduction) of resistivity. All those results suggest that the intrinsic inhomogeneity and the lattice structure play the significant roles in the electrical conductivity and anisotropic transport properties.

Peaks separation of the nonlinear refraction and nonlinear absorption induced by external electric field

To investigate the relation between the nonlinear susceptibility and the wavelength, Ag:BaTiO3 composite films were synthesized on MgO(100) substrates under different external electric fields using pulsed laser deposition. The nonlinear optical properties of the films were measured by z-scan in the wavelength of 355–650nm. It is found that with increasing the electric field, the peak of the nonlinear refraction index shifts to longer wavelength (redshift) and is separated from the peak of the nonlinear absorption. With the separation, the values of ∣Reχ(3)∕Imχ(3)∣ can be enhanced obviously.

Enhancement of optical nonlinearity in Ag:BaTiO3 composite films by applying an electric field during growth

We prepared the Ag:BaTiO3 composite films by pulsed-laser deposition technique. The films were grown on MgO (100) substrates under a nitrogen pressure of 10.0 Pa at 600°C, and a parallel electric field was applied on the substrate during growth. The chemical natures of Ag in the films were measured by x-ray photoelectron spectroscopy. The linear optical properties of the films were studied in the wavelength range of 330 to 700 nm. The third-order nonlinear optical susceptibilities of the films were determined by z-scan method at the wavelength of 532 nm with a laser duration of 10 ns. The figure of merit (FOM) was ∼1 order of magnitude larger than that of the films that were grown without an electric field and reached 1.62×10^−10 esu cm. Our experimental results have demonstrated that an external electrical field can induce the metal-particle orientation in order and therefore enhances the FOM of the metal-dielectric composite films.

Studies on the cluster sizes in the mixed-phase thin films

Based on the phase separation scenario, by simulating the resistivity of La0.33Pr0.34Ca0.33MnO3 film with the known coexisting-phase sizes, the authors have obtained the theoretical ferromagnetic (FM) cluster sizes of La2∕3Ca1∕3MnO3 film, showing around 0.6μm near Tc. Subsequently the insulator-metal transition occurs when the cluster grows up to 0.7μm, and the abrupt drop in resistivity occurs when the size is around 0.9μm. Furthermore, the FM cluster sizes grow with the magnetic field. The obtained results indicate that the model and method can be used to predict the critical size of the clusters at the phase transition for the mixed-phase materials.