Haixi Zhang

Plasmonic optical trap having very large active volume realized with nano-ring structure.

The feasibility of using gold nano-rings as plasmonic nano-optical tweezers is investigated. We found that at a resonant wavelength of λ=785 nm, the nano-ring produces a maximum trapping potential of ~32k(B)T on gold nanoparticles. The existence of multiple potential wells results in a very large active volume of ~10(6) nm(3) for trapping the target particles. The report nano-ring design provides an effective approach for manipulating nano-objects in very low concentration into the high-field region and is well suited for integration with microfluidics for lab-on-a-chip applications.

Thermal gradient induced tweezers for the manipulation of particles and cells

Optical tweezers are a well-established tool for manipulating small objects. However, their integration with microfluidic devices often requires an objective lens. More importantly, trapping of non-transparent or optically sensitive targets is particularly challenging for optical tweezers. Here, for the first time, we present a photon-free trapping technique based on electro-thermally induced forces. We demonstrate that thermal-gradient-induced thermophoresis and thermal convection can lead to trapping of polystyrene spheres and live cells. While the subject of thermophoresis, particularly in the micro- and nano-scale, still remains to be fully explored, our experimental results have provided a reasonable explanation for the trapping effect. The so-called thermal tweezers, which can be readily fabricated by femtosecond laser writing, operate with low input power density and are highly versatile in terms of device configuration, thus rendering high potential for integration with microfluidic devices as well as lab-on-a-chip systems.

Synthesis of size-controlled silver nanodecahedrons and their application for core–shell surface enhanced Raman scattering (SERS) tags

We report the synthesis of silver nanodecahedrons (Ag NDs) and their use as surface enhanced Raman scattering (SERS) nano-composites. The as-prepared Ag NDs possess strong localized surface plasmon resonance (LSPR) with widely tunable peaks between 420–660 nm, which was formerly not possible, thus greatly improving the prospect of using silver nanoparticles for SERS applications. The growth of large size Ag NDs (LSPR peak longer than 490 nm) results from a seed-mediated step involving the reduction of silver cations by photo-excitation (illumination wavelength at 500 nm). Ag ND-SERS composites formed from a layer-by-layer coating technique show strong Raman signal enhancement and good material stability because of passivation effects from the coating. The reported silica-coated Ag NDs may be used with other molecular species to take advantage of field enhancement for a variety of applications, including non-linear harmonics generation and fluorescence enhancement.

Metallic diffraction grating enhanced coupling in whispering gallery resonator

For the first time, metallic diffraction grating is investigated to enable efficient coupling in the whispering gallery resonator (WGR). Six-fold field enhancement in the resonator is achieved with respect to their dielectric counter-parts. This higher coupling efficiency is attributed to the surface plasmon excitation which drives the whispering gallery mode along the grating. Fano resonances have been observed in optical reflection. With the metallic grating, single-port end-fire WGR configuration becomes possible - a scheme that has not been demonstrated in any other WGR coupling devices. Hence, it serves as a prototype for portable whispering gallery devices potentially useful in sensing, switching and nonlinear applications.

Diffraction resonance with strong optical-field enhancement from gain-assisted hybrid plasmonic structure

We propose and analyze the diffraction coupling of localized plasmon resonances (LPRs) through gain-assisted propagation surface plasmons (PSPs). The coupling process involves localization of incident light by LPR and LPR-PSP interaction. We demonstrate a significantly strong enhancement of electromagnetic power for LPRs in the event of diffraction resonance through incorporation of experimentally feasible optical gain to the PSP. Based on such phenomenon, we propose a hybrid plasmonic structure, which would potentially give rise to device realization of the nano-lasers. In addition, it is also a promising platform for applications such as surface enhanced Raman scattering, nonlinear optics, plasmonic trapping, etc.

Experimental and Theoretical Investigation of Macro-Periodic and Micro-Random Nanostructures with Simultaneously Spatial Translational Symmetry and Long-Range Order Breaking

Photonic and plasmonic quasicrystals, comprising well-designed and regularly-arranged patterns but lacking spatial translational symmetry, show sharp diffraction patterns resulting from their long-range order in spatial domain. Here we demonstrate that plasmonic structure, which is macroscopically arranged with spatial periodicity and microscopically constructed by random metal nanostructures, can also exhibit the diffraction effect experimentally, despite both of the translational symmetry and long-range order are broken in spatial domain simultaneously. With strategically pre-formed metal nano-seeds, the tunable macroscopically periodic (macro-periodic) pattern composed from microscopically random (micro-random) nanoplate-based silver structures are fabricated chemically through photon driven growth using simple light source with low photon energy and low optical power density. The geometry of the micro-structure can be further modified through simple thermal annealing. While the random metal nanostructures suppress high-order Floquet spectra of the spatial distribution of refractive indices, the maintained low-order Floquet spectra after the ensemble averaging are responsible for the observed diffraction effect. A theoretical approach has also been established to describe and understand the macro-periodic and micro-random structures with different micro-geometries. The easy fabrication and comprehensive understanding of this metal structure will be beneficial for its application in plasmonics, photonics and optoelectronics.

Quasi-uniform excitation source for cascade enhancement of SERS via focusing of surface plasmons

A novel surface-enhanced Raman scattering (SERS) excitation source based on focusing of surface plasmons around the center hole of a metal disk for cascaded enhancement is put forward and studied theoretically. The device offers intense SERS excitation with quasi-uniformity and horizontal polarization over a comparatively large hole through the combination of electromagnetic field focusing and hole plasmon resonance. As revealed by finite-difference time-domain (FDTD) method, the intensity spectra and the characteristics of the near field for the wavelength range of 650-1000nm exhibit a number of enhancement modes. Electric field intensity of the optimal mode enhances the SERS signal inside the hole by over four orders. An analytical model was also developed to gain precise interpretation on FDTD results. Our model also reveals the possibility of achieving eight orders of enhancement by optimizing the scale of the disk. In addition to generation of highly optimized hot spots, the large active hole also offers potential applications in fluorescence enhancement and nonlinear spectroscopy.

Dressing plasmon resonance with particle-microcavity architecture for efficient nano-optical trapping and sensing.

We propose a particle-microcavity scheme for efficient optical trapping and sensing. When a resonant plasmonic nanoparticle (NP) is placed inside a microcavity with high Q-factor, sensitivity is enhanced in the far-field extinction while near-field around the NP is barely affected. Stable near-field and high sensitivity for optical trapping and ultrasensitive detection of nanosized targets are therefore realized simultaneously. Such a particle-microcavity system opens up a new hybrid nanophotonic device platform that combines the unique merits of conventional and plasmonic integrated photonics.

Theoretical investigation of optimal propagation performance in multilayer long-range surface plasmon waveguide

A theoretical study of a long-range surface plasmon waveguide is reported. The proposed waveguide has a similar configuration to the widely employed Insulator-Metal-Insulator (IMI) structure. Propagation performance with respect to propagation distance and mode confinement are investigated. As a result, the effect of an additional intermediate dielectric layer to the long-range plasmonic waveguide is discussed. Finally, optical gain is proposed as a practical solution to resolve the tradeoff between propagation loss and mode confinement.