Nano-optical conveyor belt using plasmonic tweezers [electronic resource]

Manipulating nanoscopic objects with near-field optical devices is becoming increasingly attractive for optofluidic and lab-on-a-chip applications. While conventional optical tweezers have proven to be powerful tools in biology, physics and nanoscience, they suffer from the diffraction limit (~ half of the light wavelength) and hence low trapping efficiency for nanoscale objects. Among all the near-field optical approaches, resonant plasmonic nanostructures are particularly promising because of the significantly stronger and tighter trapping enabled by their extraordinary field enhancement combined with deeply subwavelength spot size. Furthermore a plasmonic trapping system is much more scalable and cost-effective, promising greater potential for on-chip integration. However one major problem of this approach is the difficulty of producing controlled movement over a reasonable range. While trapping of nanoscale objects with plasmonic tweezers has been successfully demonstrated, transport and manipulation over long distance has remained a considerable challenge. In this thesis I describe the use of plasmonic nanostructures to construct a nano-optical conveyor belt (NOCB) for long-range transport and manipulation. The first such nanostructure evaluated as the building block of the conveyor belt was the C-shaped engraving (CSE). Analysis indicated that this geometry exhibits extraordinary intensity enhancement (~ 500) due to plasmonic resonance and hence produces a much stronger (~ 20x) near-field trapping force than that of conventional optical tweezers under the same power density of illumination. Besides, these engravings can be densely packed together with negligible interference owing to the small skin depth of gold (20-30nm) in the NIR wavelength range. Closely packing a repeating chain of three or more plasmonic structures with separately addressable resonance then forms a "nano-optical conveyor belt", on which nanoparticles are transported through handoff between adjacent traps. This mechanism is clearly revealed by the numerical calculations of the optical force and potential profile along the conveyor belt. The device was fabricated using electron beam lithograpy and a dual-layer template-stripping process, which produces much smoother gold surface than direct deposition methods to facilitate particle transport. Using the as-made device we successfully demonstrated handoff between two orthogonally oriented CSEs for polystyrene spheres 200 nm, 390 nm and 500 nm in diameter, as well as continuous transport on a 4.5 μm long conveyor belt driven merely by polarization rotation. Finally the transport dynamics of the conveyor belt is studied with the help of Brownian motion simulations. The results predict the existence of a cutoff driving frequency which depends on the illumination power and a maximum transport speed. This nonlinear behavior promises more interesting applications such as sorting, routing and etc., all of which may be realized simply by desigining the proper illumination protocol.

Keywords: Electrical Engineering.
Advisor: Hesselink Lambertus primary advisor. Thesis advisor
Brongersma Mark L. advisor. Thesis advisor
Howe Roger Thomas advisor. Thesis advisor
Pease R. (R. Fabian W.) advisor. Thesis advisor