Wang Zhen-Lin

Ordered Gold Nanobowl Arrays as Substrates for Surface-Enhanced Raman Spectroscopy

We demonstrate that an interlinked gold half-shell array fabricated by metal deposition on a sacrificial two-dimensional colloidal crystal template can show a large enhancement in surface-enhanced Raman spectroscopy at its main transmission resonance. It is further observed that Raman signal enhancement shows a noticeable difference when reversing the orientations of the Au nanobowls in relation to the underlying flat dielectric substrate. As the pump laser wavelength is tuned in the vicinity of the resonant plasmonic mode of the structure, the enhancement on an upward Au nanobowl array can be five-fold compared to that on a downward one. Numerical simulation confirms that for the upward nanobowls, a strong localized mode inside the Au nanobowls is formed at the resonant excitation wavelength, which helps to explain this observed extra enhancement in Raman scattering.

Preparation of Silver-Coated Polystyrene Composite Particles

We report a feasible approach to the preparation of monodispersed metal-shell composite microspheres based on a combination of surface reaction and surface seeding techniques. The method was implemented for coating polystyrene (PS) spheres with silver shell having a variable thickness by controlling the amount of reagents in the reaction procedure. These composite spherical particles in dimensions of the submicrometer range may become attractive building blocks for the creation of metallo-dielectric photonic band gap materials when they are organized into crystals.

Pure Electric and Pure Magnetic Resonances in Near-Infrared Metal Double-Triangle Metamaterial Arrays

We experimentally and numerically investigate the optical properties of metamaterial arrays composed of double partially-overlapped metallic nanotriangles fabricated by an angle-resolved nanosphere lithography. We demonstrate that each double-triangle can be viewed as an artificial magnetic element analogous to the conventional metal split-ring-resonator. It is shown that under normal-incidence conditions, individual double-triangle can exhibit a strong local magnetic resonance, but the collective response of the metamaterial arrays is purely electric because magnetic resonances of the two double-triangles in a unit cell having opposite openings are out of phase. For oblique incidences the metamaterial arrays are shown to support a pure magnetic response at the same frequency band. Therefore, switchable electric and magnetic resonances are achieved in double-triangle arrays. Moreover, both the electric and magnetic resonances are shown to allow for a tunability over a large spectral range down to near-infrared.

Confined Mie Plasmons in Monolayer Hexagonal-Close-Packed Metallic Nanoshells

Using a double templating method by electroless deposition within a templating organic porous mold, we fabricate a monolayer of hexagonal-close-packed metallic nanoshells, each with a small opening. Light transmission spectra of the metallic nanoshell arrays are measured, which show transmission resonances at specific wavelengths whose positions are observed to be independent of the incident angle as well as light polarizations. More interestingly, the resonance wavelengths of Mie plasmon modes are also independent of the surrounding medium. Further numerical simulations confirm these transmission resonances and reveal that they are attributed to the excitations of highly localized dipolar, quadrupolar and hexapolar Mie plasmon modes, which are highly confined within metallic nanoshells.

An Array of One-Dimensional Porous Silicon Photonic Crystal Reflector Islands for a Far-Infrared Image Detector

With the aid of photolithography, an array of one-dimensional porous silicon photonic crystal reflector islands for a far infrared image detector ranging from 10 μm to 14 μm is successfully fabricated. Silicon nitride formed by low pressure chemical vapor deposition (LPCVD) was used as the masking layer for the island array formation. After etching, the microstructures were examined by a scanning electron microscope and the optical properties were studied by Fourier transform infrared spectroscopy, the result indicates that the multilayer structure could be obtained in the perpendicular direction via periodically alternative etching current in each pre-pattern. At the same time, the island array has a well-proportioned lateral etching effect, which is very useful for the thermal isolation in lateral orientation of the application in devices. It is concluded that regardless of the absorption of the deposition layer on the substrate, the localized photonic crystalline islands have higher reflectivity. The designed islands structure not only prevents the cracking of the porous silicon layers but is also useful for the application in the cold part for the sensor devices and the interconnection of each pixel.

A Stable Porous Silicon Dielectric Reflector with a Photonic Band Gap Centred at 10μm

By pulsed anodic etching at low temperature, we prepared a porous silicon reflector with a photonic band gap centred in the long-wavelength infrared spectral region (centred at about 12 μm). After proper oxidation process, the stable reflector structure, which can reflect electromagnetic wave from 8μm to 12μm (centred at 10μm) within wide incidence angles (about 50°), is obtained. The wavelength shift of absorption peak of Si-H and Si-O shows the influence of oxidation process and indicates the stability of oxidized porous silicon dielectric reflector, which offers possible applications for the room temperature infrared sensor.

Fabrication of Two-Dimensional Arrays of Micron-Sized Gold Rings Based on Preferential Nucleation at Reentrant Sites

A templating method for fabricating two-dimensional (2D) arrays of micron-sized gold rings is reported. The microstructures are formed by electroless plating in a through-porous polymer membrane on a silicon substrate obtained from a closed-packed silica colloidal crystal. Our results show that the sizes of gold rings can be altered by varying electroless plating conditions for the porous polystyrene membranes. Moreover, we explain the growth mechanism of gold rings using the classical crystal growth theory that is preferential nucleation at reentrant sites.

A Facile Synthesis of Silver-Coated Composite Particles by Swelling Surface Method

We report a facile and rapid method for fabrication of composite particles consisting of a polystyrene (PS) core and a uniform silver shell. The process involves the PS colloid surface swelling, the anchoring of silver ions and nanoparticles onto the surfaces, and the subsequent growth of metal seeds in a short period. The present approach has the advantages of simplicity and high efficiency. The TEM images show the morphology of the obtained PS core-silver shell particles, and their chemical composition and crystallinity are analysed by x-ray diffraction. To our knowledge, this is the first study based on swelling PS surface for synthesis of silver-coated PS particles and may be implemented for preparing other metal-coated PS particles.

Observation of Optical Bistability in Two-Dimensional Volume Refractive Index Grating Thermally Fixed in LiNbO3:Fe Crystal

Experimental observation of optical bistability in a nonlinear two-dimensional volume refractive index grating is reported. The index grating in LiNbO3: 0.1 wt.% Fe photorefractive single crystal was fabricated by using hologram recording and subsequent thermal fixing techniques. Due to the thermal fixing, the stored 2D volume index grating shows no optical erasure during readout. An explanation of the nonlinear phenomena is also given in terms of the intensity-enhanced bulk photovoltaic effects in LiNbO3:Fe material.

Zero-backward scattering by Metallo-Dielectric core-shell nanostructures

Based on the Mie theory, we propose and investigate a “metal (M)-low-permittivity (LP)-high-permittivity (HP)” core-shell nanoparticle that possesses the high directional zero-backward scattering characteristics. We analyze the effect of the LP layer on the wavelength and strength of the electric or magnetic dipolar resonance, and reveal that the zero-backward scattering and dramatically enhanced forward scattering are easy to be obtained by modulating the thickness and refractive index of the middle LP layer, which can engineer the dipolar electric and magnetic modes in the M-LP-HP core-shell nanoparticles to coincide spectrally with the same strength, thus satisfying the first Kerker condition of zero backward scattering. We also demonstrate that the zero-backward scattering can be tuned over a wide range from the near-infrared to visible by varying the metal core size and the outer radius of the HP shell or by changing the refractive index of the HP shell. Our results can provide a scientific direction for designing the nanostructures and devices based on the zero-backward scattering.