Xiao-Yang Zhang

Self-Assembly of Large-Scale and Ultrathin Silver Nanoplate Films with Tunable Plasmon Resonance Properties

We describe a rapid, simple, room-temperature technique for the production of large-scale metallic thin films with tunable plasmonic properties assembled from size-selected silver nanoplates (SNPs). We outline the properties of a series of ultrathin monolayer metallic films (8-20 nm) self-assembled on glass substrates in which the localized surface plasmon resonance can be tuned over a range from 500 to 800 nm. It is found that the resonance peaks of the films are strongly dependent on the size of the nanoplates and the refractive index of the surrounding dielectric. It is also shown that the bandwidth and the resonance peak of the plasmon resonance spectrum of the metallic films can be engineered by simply controlling aggregation of the SNP. A three-dimensional finite element method was used to investigate the plasmon resonance properties for individual SNPs in different dielectrics and plasmon coupling in SNP aggregates. A 5-17 times enhancement of scattering from these SNP films has been observed experimentally. Our experimental results, together with numerical simulations, indicate that this self-assembly method shows great promise in the production of nanoscale metallic films with enormous electric-field enhancements at visible and near-infrared wavelengths. These may be utilized in biochemical sensing, solar photovoltaic, and optical processing applications.

Numerical analysis of deep sub-wavelength integrated plasmonic devices based on Semiconductor-Insulator-Metal strip waveguides

We report the first study of nanoscale integrated photonic devices constructed with semiconductor-insulator-metal strip (SIMS) waveguides for use at telecom wavelengths. These waveguides support hybrid plasmonic modes transmitting through a 5-nm thick insulating region with a normalized intensity of 200-300 μm(-2). Their fundamental mode, unique transmission and dispersion properties are consistent with photonic devices for guiding and routing of signals in communication applications. It has been demonstrated using Finite Element Methods (FEM) that the high performance SIMS waveguide can be used to fabricate deep sub-wavelength integrated plasmonic devices such as directional couplers with the ultra short coupling lengths, sharply bent waveguides, and ring resonators having a functional size of ≈1 µm and with low insertion losses and nearly zero radiation losses.

Synthesis of Silver Nanostructures by Multistep Methods

The shape of plasmonic nanostructures such as silver and gold is vital to their physical and chemical properties and potential applications. Recently, preparation of complex nanostructures with rich function by chemical multistep methods is the hotspot of research. In this review we introduce three typical multistep methods to prepare silver nanostructures with well-controlled shapes, including the double reductant method, etching technique and construction of core-shell nanostructures. The growth mechanism of double the reductant method is that different favorable facets of silver nanocrystals are produced in different reductants, which can be used to prepare complex nanostructures such as nanoflags with ultranarrow resonant band bandwidth or some silver nanostructures which are difficult to prepare using other methods. The etching technique can selectively remove nanoparticles to achieve the aim of shape control and is widely used for the synthesis of nanoflowers and hollow nanostructures. Construction of core-shell nanostructures is another tool to control shape and size. The three methods can not only prepare various silver nanostructures with well-controlled shapes, which exhibit unique optical properties, such as strong surface-enhanced Raman scattering (SERS) signal and localized surface plasmon resonance (LSPR) effect, but also have potential application in many areas.

Investigation on the role of the molecular weight of polyvinyl pyrrolidone in the shape control of high-yield silver nanospheres and nanowires

Serving as shape control agent, polyvinyl pyrrolidone (PVP) has been widely used in chemical synthesis of metal nanoparticles. However, the role of molecular weight (MW) of PVP has been rarely concerned. In this study, we show a facile method to control the shapes of silver nanocrystals using PVP with different MWs. PVPMW=8,000, PVPMW=29,000, PVPMW=40,000, and PVPMW=1,300,000 are compared in the present study. Surprisingly, high-yield silver rodlike nanostructures, nanospheres, and nanowires can be obtained under the same growth environment and reactant concentrations by simply changing the MW of PVP. The mechanism studies of the role of PVP with different MWs in the growth process were carried out systemically using the morphology and spectroscopic measurement, FT-IR spectrum analysis, and seed crystallization monitoring. The results indicate that the MW of PVP plays a determinant role in the morphology and optical property control of the silver nanocrystals. Meantime, the concentration of PVP was found to be an assistant factor to further improve the shape and the yield of the synthesized nanocrystals.

Subwavelength plasmonic waveguides based on ZnO nanowires and nanotubes: A theoretical study of thermo-optical properties

We show theoretically that plasmonic waveguide structures in ZnO nanowires and nanotubes working at optical frequencies can achieve photonic waveguiding in a subdiffraction limit. The output intensity distribution, propagation length, and thermo-optical properties with different waveguide configurations are investigated. Our results show that these waveguides have the potential to develop either high performance thermally controlled nanoscale plasmonic devices or thermally insensitive waveguides by optimizing waveguide configurations.

Integrated optical gyroscope using active Long-range surface plasmon-polariton waveguide resonator

Optical gyroscopes with high sensitivity are important rotation sensors for inertial navigation systems. Here, we present the concept of integrated resonant optical gyroscope constructed by active long-range surface plasmon-polariton (LRSPP) waveguide resonator. In this gyroscope, LRSPP waveguide doped gain medium is pumped to compensate the propagation loss, which has lower pump noise than that of conventional optical waveguide. Peculiar properties of single-polarization of LRSPP waveguide have been found to significantly reduce the polarization error. The metal layer of LRSPP waveguide is electro-optical multiplexed for suppression of reciprocal noises. It shows a limited sensitivity of ~10−4 deg/h, and a maximum zero drift which is 4 orders of magnitude lower than that constructed by conventional single-mode waveguide.

Controllable plasmonic antennas with ultra narrow bandwidth based on silver nano-flags

We show theoretically plasmonic antennas based on two-step chemical synthesized silver nano-flags constructed by a silver nanowire and a nanoplate. The silver nano-flag antennas exhibit high polarization sensitivity and diversity of spectral signatures dependent on the structural parameters arising from the observed mode competition. With specific configuration engineering, the antennas show controllable electric-field enhancement and ultra narrow bandwidths down to about 2 nm. These nano-antennas promise exciting applications in lasing spaser, optical modulation, and enhanced nonlinear processes.

Tunable Optical Ring Resonator Integrated With Asymmetric Mach–Zehnder Interferometer

We demonstrate theoretically a tunable optical ring resonator incorporating an asymmetric Mach-Zehnder interferometer (MZI) and two phase shifters. The optimal resonance state of the ring resonator with different geometries can be achieved by tuning the two embedded phase shifters. Distinct intensity and phase responses and transmission spectra characteristics are newly observed by setting different structural parameters such as the asymmetrical path lengths of the waveguides and the coupling ratio of the directional couplers in the MZI. The performance characteristics related to the radius of the ring cavity and the propagation losses in the waveguides are also discussed. At optimal resonance, it is shown that sharp intensity response and tunable narrow-bandwidth spectra can be achieved especially for resonators with a highly asymmetrical configuration. Such device has a potential in sensing, switching and filtering applications.

High quantum-yield luminescent MoS2 quantum dots with variable light emission created via direct ultrasonic exfoliation of MoS2 nanosheets

We report a sonication combined with ion intercalation method in an alkaline environment to fabricate MoS2 quantum dots (QDs), with the quantum-yield of the QDs increasing from 0.99% to 4.84% with the additive sodium hydroxide. The QDs displaying variable photoluminescence emission were systematically studied and applied to the bio-imaging field.

Plasmonic Properties of Welded Metal Nanoparticles

Metal nanostructures show great applications in chemical sensing, biomedical detection, optical-thermal therapy, and optical communications because of their electromagnetic field enhancement properties at the visible and the near-field infrared wavelengths. Such strong optical field enhancement induced by the localized surface plasmon resonance is dependent on the configurations and the sizes of the metal nanoparticles. We presented a numerical investigation of the plasmonic properties of the individually welded silver nanoparticles fabricated by nanojoining technique. It shows that the field enhancement factor in welded silver nanostructures is much larger than in separated silver nanoparticles. The size dependent localized surface plasmon resonance spectra and the polarization sensitivity property of such configurations are also discussed.