Yuxiao Wang

Dynamics of reverse saturable absorption and all-optical switching in C 60

The dynamic processes of reverse saturable absorption (RSA) in a solution of C60 are studied in detail by use of rate equations and the light-propagation equation for a five-energy-level model. The RSA experimental results for C60 in toluene solution with a nanosecond–picosecond Nd:YAG laser at 532 nm are consistent with the theoretical simulations. Effects of sample concentration and thickness on RSA and contributions of the singlet and triplet excited states to RSA for various laser-pulse widths and wavelengths are also analyzed. Finally, an all-optical switch based on excited-state nonlinear absorption and the pump-probe technique is implemented.

A theoretical and experimental study on optical limiting in platinum nanoparticles

Optical limiting performance of PVP-stabilized platinum nanoparticles in methanol is investigated with 8 ns pulses at 532 nm. The experimental results show that the platinum nanoparticles possess strong optical limiting effect. We propose a theoretical model based on Mie extinction theory to describe nonlinear optical limiting (OL) behavior of metal nanoparticles. The theoretical analysis and experimental results show that the main mechanism for OL of platinum nanoparticles can be attributed to interband transition of platinum during the excitation of nanosecond pulses.

Optical nonlinearities and optical limiting properties in gold nanoparticles protected by ligands

Abstract Optical nonlinearities of gold nanoparticles protected by ligands have been investigated by Z-scan technique. Absorptive and refractive optical limiting (OL) effects have been measured with 8 ns duration laser pulses at 532 nm. Significant differences have been found experimentally for these two nanoparticles that differ only by their ligands. Such differences indicate that nonlinear optical responses in two nanocomposites are strongly influenced by the surface plasmon resonance of gold nanoparticles and also the nature of ligands. Both of these two nanocomposites show stronger refractive optical limiting effects than C 60 in toluene.

A study on graphitization of diamond in copper-diamond composite materials

Abstract The graphitization of diamond particles in the copper–diamond composite materials used for low voltage electro-contacts was investigated by SEM, TEM and X-ray diffraction (XRD) analyses. The results show that although diamond is a metastable allotropic modification of carbon, the diamond particles are not graphitized to an apparent extent after sintering at 1150–1220 K in the copper–diamond composite. The reason for this phenomenon was discussed. The diamond powders prepared by explosion synthetic method are supposed to possess higher resistance to graphitization and thus benefit the performance of the copper–diamond composite used for electro-contacts.

Nonlinear excited state absorption in cadmium texaphyrin solution

Nonlinear absorption at 532 nm in a cadmium texaphyrin solution has been studied using 8 ns and 23 ps laser pulses. The experiments show that reverse saturable absorption occurs in the nanosecond case. For picosecond pulses, reverse saturable absorption occurs only at low fluences, and the transmission increases with increasing incident fluence at high fluences. A six‐level model is presented to explain these nonlinear absorption effects. Several photophysical parameters for cadmium texaphyrin, such as absorption cross sections at 532 nm and lifetimes, have been evaluated by theoretical simulations of the experimental results.

Excited-state absorption and optical-limiting properties of organometallic fullerene–C60 derivatives

Singlet excited-state absorption properties and optical limiting behaviors of organochromium and organomolybdenum C60 derivatives (C89H24CrO3P2 and C89H24MoO3P2) were studied by using a double frequency ns/ps Nd:YAG pulse laser system at wavelength of 532 nm and pulse width 21 ps. The experimental results demonstrated that the optical limiting behavior of organochromium C60 derivative is better than both organomolybdenum C60 derivative and fullerene–C60 itself. The experimental results agree well with the singlet excited-state absorption theory.

Fabrication of graphene nanogap with crystallographically matching edges and its electron emission properties

We demonstrate the fabrication of graphene nanogap with crystallographically matching edges on SiO2/Si substrates by divulsion. The current-voltage measurement is then performed in a high-vacuum chamber for a graphene nanogap with few hundred nanometers separation. The parallel edges help to build uniform electrical field and allow us to perform electron emission study on individual graphene. It was found that current-voltage (I-V) characteristics are governed by the space-charge-limited flow of current at low biases while the Fowler–Nordheim model fits the I-V curves in high voltage regime. We also examined electrostatic gating effect of the vacuum electronic device. Graphene nanogap with atomically parallel edges may open up opportunities for both fundamental and applied research of vacuum nanoelectronics.

Nonlinear optical properties in three novel nanocomposites with gold nanoparticles

Nonlinear optical properties of three novel nanocomposites with zerovalent noble metal gold nanoparticles were investigated by using Z-scan technique. Optical limiting effects has been measured with 8 ns pulses at 532 nm. The cross-sections of nonlinear absorption were obtained by the simulation with a simplified model in which the effective excited-state absorptions of three ligands in nanocomposites were considered. The nonlinear refractive indices were calculated from the data of Z-scan measurement. The experimental results were found to be significantly different in these nanocomposites. Optical nonlinearities in these nanocomposites can be attributed to the strong excited-state absorptions of the ligands and the surface plasmon resonance of gold nanoparticles.

Effect of Li+ ions on enhancement of near-infrared upconversion emission in Y2O3:Tm3+/Yb3+ nanocrystals

Near-infrared (NIR) to NIR upconversion emission is investigated in Tm3+/Yb3+/Li+ triply doped Y2O3 nanocrystals. Li+ ions doped in Y2O3:Tm3+/Yb3+ nanocrystals can greatly enhance the NIR upconversion emission intensity of Tm3+ ions. The abnormal shift of the (222) diffraction peak position determined from x-ray diffraction measurements is discussed, by introducing Li+ ions in the Y2O3:Tm3+/Yb3+ nanocrystals. The cause of the enhancement is the modification of the local symmetry induced by the Li+ ions, which increases the intra-4f transitions of Tm3+ ion. Li+ ions doped in Y2O3:Tm3+/Yb3+ nanocrystals also can reduce the OH groups, dissociate the Yb3+ and Tm3+ ion clusters, and create the oxygen vacancies, which are the other reasons for enhancing the upconversion emission intensities. This material may be promising for in vitro and in vivo bioimaging probes.

Strong optical limiting property of a novel silver nanoparticle containing C60 derivative

Abstract Optical limiting performance of novel silver nanoparticle containing C60 derivative is investigated under 532 nm laser pulse with 8 ns pulse width. Experimental results demonstrate that the nanoparticle performs with better optical limiting behavior than C60. An explanation based on nonlinear absorption and scattering is provided.