R. F. Xiao

Large third-order optical nonlinearity in Au:SiO2 composite films near the percolation threshold

Very large third-order optical nonlinearity, χ(3)∼2.5×10−6 esu, measured by a degenerate four wave mixing method using a short pulse (70 picosecond) laser, has been found in the rapid-thermal annealed Au:SiO2 composite films at concentrations below the Au percolation threshold. The dependence of the χ(3) on Au concentration, p, follows a cubic power law. The maximum figure of merit, χ(3)/α (with α being the absorption coefficient) is about 10−11 esu cm. We explain this result as due to local field enhancement arising from the Mie resonance of the Au nanoclusters, with strong interaction between the nanoclusters further promoting the effect.

Large third-order optical nonlinearity in Au:TiO2 composite films measured on a femtosecond time scale

The wavelength dependence of the third-order nonlinear optical susceptibilities, χ(3), of the Au:TiO2 composite films with Au concentration varying from 15% to 60% (volume fraction), was measured by a degenerate four-wave mixing (DFWM) technique using a probe laser with a pulse width of 200 fs. It was found that, with the wavelength of the probe laser close to the surface plasmon resonance (∼680 nm), both the χ(3) and the figure of merit, χ(3)/α (α is optical absorption coefficient) were significantly enhanced. The maximum value of the χ(3) was 6×10−7 esu and occurred at an Au concentration of about 38%. Femtosecond time-resolved DFWM measurements revealed that the response time of the optical nonlinearity in the Au:TiO2 films is extremely fast. The time-resolved DFWM results suggest that the main physical mechanism involved in the optical nonlinearity in Au:TiO2 films on the femtoseconds time scale is the interband electric–dipole transition, and the hot electron excitation only partially contributes to ...

Origin of third-order optical nonlinearity in Au:SiO 2 composite films on femtosecond and picosecond time scales

Three sorts of probe laser, which have pulse durations of 200 fs, 35 ps, and 70 ps, were employed in the measurement of the third-order nonlinear optical susceptibility x((3)) in Au:SiO(2) composite films in a degenerate four-wave mixing scheme. We found that the composite films at their absorption peak (~550 nm) had a maximum x((3)) , which depends strongly on the pulse width of the probe laser. The value of x((3)) measured with a 70-ps laser was ~30 times larger than that measured with a 200-fs laser. The time-resolved measurements revealed that the optical nonlinearity on the femtosecond time scale is attributable mainly to contributions from the interband electric-dipole transition (especially at low concentrations) and partly to those from hot electrons rather than being dominated by hot-electron excitation in the picosecond regime.

Third-order optical nonlinearity enhancement through composite microstructures

Formulas for the evaluation of third-order optical susceptibility of nonlinear composites are derived in the mean-field approximation for four different microstructures corresponding to those described by the Maxwell–Garnett theory, the Bruggeman self-consistent theory, the Sheng theory, and the differential effective medium theory. A commonly encountered error in the literature is pointed out, with the correct formulation given. Anomalous dispersion, i.e., surface plasmon resonance of coated spheres, is identified as the source of large optical nonlinear susceptibility enhancement. Examples are given that demonstrate this microstructural enhancement effect. Comparison with experimental data on AuSiO2 granular films shows that large enhancement in the third-order Kerr-type nonlinear susceptibility can indeed be realized at compositions below the percolation threshold, with prediction of even larger enhancement possible.

Second harmonic generation in periodically domain-inverted Sr0.6Ba0.4Nb2O6 crystal plate

Quasiphase-matched second harmonic generation (SHG) has been realized in periodically domain-inverted strontium barium niobate (Sr0.6Ba0.4Nb2O6, so-called SBN) crystal plate induced by an external electric field. Both the coercive field and Curie temperature have been determined through in situ observation of the second harmonic signals. The measured SHG conversion efficiency is about 10% at a fundamental wavelength of 977.8 nm.

Preparation of crystalline beta barium borate (β‐BaB2O4) thin films by pulsed laser deposition

Crystalline beta barium borate (β‐BaB2O4) thin films have been prepared on silicon (001) and sapphire (001) substrates by the pulsed laser deposition technique. The crystallinity of these films was found to depend sensitively on the deposition temperature. At a deposition temperature around 800 °C and in the presence of 75 mTorr of O2 gas, films grown on Si(100) substrates are typically polycrystalline with x‐ray diffraction peaks along (104) and (001) orientations while the film grown on a sapphire (001) substrate is highly textured with (001) orientation (c axis) normal to the film surface. The film grown on sapphire (001) substrate exhibits a maximum effective second harmonic generation coefficient deff of 2.2 pm/V, which is comparable to that of bulk β‐BaB2O4 single crystals.

Third harmonic generation through coupled second-order nonlinear optical parametric processes in quasiperiodically domain-inverted Sr0.6Ba0.4Nb2O6 optical superlattices

A one-dimensional quasiperiodically domain-inverted optical superlattice has been designed and fabricated in a strontium barium niobate (Sr0.6Ba0.4Nb2O6) single-crystal plate. A third harmonic output generated through coupled second-order nonlinear parametric processes with a conversion efficiency of about 1.6% was obtained when both the second harmonic and the third harmonic were simultaneously quasiphase matched in the crystal.

Pulsed laser deposition of optical waveguiding strontium barium niobate films

Strontium barium niobate waveguiding thin films were prepared by pulsed laser deposition. The relationship between the film structure and the deposition temperature were analyzed by x-ray diffraction and atomic force microscopy. Waveguiding properties such as refractive index and mode dispersion were examined and a dispersion relation was obtained for the (001)-oriented films.

Amorphous KNbO3 thin films with ferroelectriclike properties

Amorphous KNbO3 films are prepared by a low-cost sol–gel method. These films show a smooth surface morphology and have good waveguiding properties. Ferroelecticlike hysteresis loops are observed in these amorphous KNbO3 films with a remanant polarization of about 6μC/cm2. After corona poling, a stable effective second harmonic generation coefficient of 0.523 pm/V is achieved.

Preparation of silicon oxynitride (SiOxNy) thin films by pulsed laser deposition

The pulsed laser deposition technique was used to grow silicon oxynitride (SiOxNy) thin films at low temperatures (25–300 °C). The thin films were found to be smooth in surface morphology, extremely inert in HFH2O solution, and highly transparent in the optical range from 0.3 μm to 5 μm. The refractive index is tunable between 1.4 and 2.1 by the addition of oxygen in substitution for nitrogen in the thin films.