Jiajie Chen

Plasmonic graded nano-disks as nano-optical conveyor belt

We propose a plasmonic system consisting of nano-disks (NDs) with graded diameters for the realization of nano-optical conveyor belt. The system contains a couple of NDs with individual elements coded with different resonant wavelengths. By sequentially switching the wavelength and polarization of the excitation source, optically trapped target nano-particle can be transferred from one ND to another. The feasibility of such function is verified based on the three-dimensional finite-difference time-domain technique and the Maxwell stress tensor method. Our design may provide an alternative way to construct nano-optical conveyor belt with which target molecules can be delivered between trapping sites, thus enabling many on-chip optofluidic applications.

Trapping and assembling of particles and live cells on large-scale random gold nano-island substrates

We experimentally demonstrated the use of random plasmonic nano-islands for optical trapping and assembling of particles and live cells into highly organized pattern with low power density. The observed trapping effect is attributed to the net contribution due to near-field optical trapping force and long-range thermophoretic force, which overcomes the axial convective drag force, while the lateral convection pushes the target objects into the trapping zone. Our work provides a simple platform for on-chip optical manipulation of nano- and micro-sized objects, and may find applications in physical and life sciences.

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.

Plasmonic random nanostructures on fiber tip for trapping live cells and colloidal particles

We demonstrate optical trapping on a gold-coated single-mode fiber tip as excited by 980-nm laser radiation. The trapping force here is not due to common plasmonic localization, but dominated by the combined effect of thermophoresis and thermal convection. The reported scheme only requires simple thin-film deposition. More importantly, efficient broadband plasmonic absorption of the gold random nanostructures, aided by purely Gaussian excitation profile from the fiber core, has led to very low trapping-power threshold typically in hundreds of microwatts. This highly versatile fiber-based trapping scheme clearly offers many potential application possibilities in life sciences as well as engineering disciplines.

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.

Plasmonic absorption activated trapping and assembling of colloidal crystals with non-resonant continuous gold films

Here we report the realization of trapping and assembly of colloidal crystals on continuous gold thin films based on the combined effect of thermophoresis and thermal convection associated with plasmonic optical heating. In the system, the stabilized trapping phenomenon is driven by thermophoretic forces caused by a temperature gradient which pushes the target particles from cold to hot regions and always in an opposite direction to the axial convective drag forces. Furthermore, the lateral convective flow of an aqueous medium accelerates the formation of the trap considerably by dragging target particles into the hot region from a long distance. The influence of salt concentration on the trapping behavior has also been investigated. Typically the threshold optical power density is in the order of microwatts per square micrometer (∼μW μm−2). We anticipate that the reported optical trapping approach may find many potential applications in biophysics, life sciences, and lab-on-a-chip devices.

Plasmonic Absorption Induced Optical Tweezers with in situ Surface Enhanced Raman Scattering Capability

We report a new trapping technique based on localized temperature gradient induced by plasmonic absorption. Versatile optical tweezers for trapping of live cells and the capability of conducting SERS have been demonstrated.

Optofluidic guiding based on plasmonic absorption

We demonstrate a novel microfluidic guiding technique by using plasmonic energy absorption in gold nano-islands produced by thermal annealing. Live cells and nanoparticles in aqueous medium can be readily guided with good speed control.