Yuanjun Yan

Selective electroless silver plating of three dimensional SU-8 microstructures on silicon for metamaterials applications

We report a method for selective silver coating of SU-8 structures on Si substrates by treating the sample with radio frequency plasma prior to electroless plating. Silver films with high conductivity of 9 × 10−8Ω.m and low surface roughness of 9 nm have been obtained. When combined with two-photon lithography, this process can be utilized for three-dimensional metamaterials applications. Unlike previous work on selective coating, our process can coat directly on SU-8 photoresist that is widely used for two-photon lithography and does not require any resin modification.

Continuous-flow C. elegans fluorescence expression analysis with real-time image processing through microfluidics

The nematode Caenorhabditis elegans has become an essential model organism in neuroscience research because of its stereotyped anatomy, relevance to human biology, and capacity for genetic manipulation. To solve the intrinsic challenges associated with performing manual operations on C. elegans, many automated chip designs based on immobilization-imaging-release approaches have been proposed. These designs are prone to limitations such as the exertion of physical stress on the worms and limited throughput. In this work, a continuous-flow, high-throughput, automated C. elegans analyzer based on droplet encapsulation and real-time image processing was developed to analyze fluorescence expression in worms. To demonstrate its capabilities, two strains of C. elegans nematodes with different levels of expression of green fluorescent protein (GFP) were first mixed in a buffer solution. The worms were encapsulated in water-in-oil droplets to restrict random locomotion. The droplets were closely packed in a two-layer polydimethylsiloxane (PDMS) platform and were flowed through a narrow straight channel, in which a region of interest (ROI) was defined and continuously recorded by a frame acquisition device. Based on the number of pixels counted in the selected color range, our custom software analyzed GFP expression to differentiate between two strains with nearly 100% accuracy and a throughput of 0.5 seconds/worm.

Flow effects in the laser-induced thermal loading of optical traps and optofluidic devices

Flow effects on the thermal loading in different optofluidic systems (optical trap and various microfluidic channels) have been systematically explored by using dye-based ratiometric luminescence thermometry. Thermal images obtained by fluorescence microscopy demonstrate that the flow rate plays a key role in determining both the magnitude of the laser-induced temperature increment and its spatial distribution. Numerical simulations were performed in the case of the optical trap. A good agreement between the experimental results and those predicted by mathematical modelling was observed. It has also been found that the dynamics of thermal loading is strongly influenced by the presence of fluid flow.

A microfluidic device with integrated optics for microparticle switching

We report a high efficiency and noninvasive microfluidic particle switching device with integrated optical microstructures. Microfluidic channels are combined with a cylindrical microlens and an optical fiber to achieve on-chip optical switching of colloidal particles without the need for an optical microscope. A laser beam is coupled into an optical fiber and redirected by the microlens. The angle of incidence of the optical force can be changed by varying the position of the optical fiber relative to the microlens. Under certain circumstances, a switching efficiency approaching 100% was achieved with a relatively fast response time for a solution containing 10 μm polystyrene spheres.

Radio frequency plasma pre-treatment for selective electroless Ag coating of three-dimensional SU-8 microstructures

Three dimensional metamaterials are fabricated using direct laser writing in SU-8 polymer followed by an electroless coating process. A method has been developed to allow for selective electroless plating of SU-8 microstructures with a smooth conformal coating of Ag. The process utilizes radio frequency plasma pretreatment to modify the SU-8 surface so that Ag ions can nucleate on the surface, leaving the substrate uncoated. An array of split ring resonators and other 3D microstructures are used to demonstrate how the technique can be applied to metamaterials applications.

Tailoring fabrication parameters of silver helical metamaterials for enhanced polarization properties in terahertz regime

Metallic helices have been extensively researched and demonstrated for their application as broadband circular polarizers in different frequency regimes. For making such 3D helices, two photon lithography (TPL) has been employed in conjunction with electroplating of metals. Recently, our group has demonstrated selective silver electroless plating of two photon fabricated polymer (SU-8) structures on silicon substrate. This procedure allows us to make metal-coated polymer helices. In this work, we examine how these fabrication process parameters could be tailored to obtain higher extinction ratios for circular polarizers in THz regime (or MIR regime). We further analyze the role of aspect-ratio of helices in their polarizing action. We will present both simulation and experimental results to show the improved performance of the polarizers.

A Continuous-Flow C. elegans Sorting System with Integrated Optical Fiber Detection and Laminar Flow Switching

Sorting of C.elegans genotypes is a routine task in most C.elegans research labs. Conventionally, the worms are sorted manually which is both labor intensive and slow. In recent years microfluidics has become a useful and important tool for biologists to study C.elegans, including sorting. Although they have shown successful sorting results, the immobilization of worms which was adopted in most of these works causes aversive stimulation to the worms and their behavior will potentially be altered when carrying out post-sorting characterizations.In this work, both worm detection and switching are achieved without any intervention of the continuous worm flow. The genotypes of the worms are detected by integrated optical fibers based on their fluorescence, without the need for immobilization. Switching is based on the steering of laminar flow boundaries. A novel design that integrates two control inlets dynamically switches the fluidic flow to desired outlets by changing the relative pressure in the control inlets, which cause the two laminar flow boundaries to steer.Compared to previously reported microfluidic C. elegans sorting devices, sorting in this system is conducted in a continuous flow environment without any immobilization technique or need for multilayer mechanical valves to open and close the outlets. The continuous flow sorter not only increases the throughput but also avoids any kind of invasive or possibly damaging mechanical or chemical stimulus. We have characterized both the detection and the switching accuracy of the sorting device at different flow rates, and efficiencies approaching 100% can be achieved with a high throughput of about 1 worm/s. To confirm that there was no significant damage to C. elegans following sorting, we recovered the sorted worms, finding no differences in behavior and propagation compared to control.

Selective electro-less plating of SU-8 microstructures fabricated using two-photon polymerization

In this paper, we report a method for fabricating an array of 3D silver-coated polymer microstructures. The process involves three dimensional nanolithography using two-photon polymerization of SU-8 photoresist that has been spin coated onto a silicon substrate. The resulting polymer structures are selectively coated with silver using an electroless silver nitrate plating solution. We utilize a radio frequency plasma pretreatment process to modify the surface of the polymeric microstructures prior to electroless silver plating. Optimization of this pretreatment process allows us to selectively silver plate the polymer structures whilst leaving the silicon substrate uncoated. This technique opens up the possibility of fabricating true three dimensional metallic nanostructures for metamaterials and plasmonics applications.