Optical Trapping and Manipulation by Surface Plasmon Resonance Structures
Optical tweezers based on far-fields have proven highly successful for manipulating objects larger than the wavelength, but face difficulties at the nanoscale. The trapping potential varies with the cube of the object diameter (for a nanosphere), so small objects require high laser powers. Similarly, positional control with a precision commensurate with object size is difficult, as the potential well in which the object is trapped has a width of roughly half the wavelength. This has motivated interest in trapping particles with nanostructures supporting surface plasmons, as they enable intense fields confined to sub-wavelength dimensions.
In this thesis, we introduce three designs for plasmonic trapping in one-, two- and three-dimensions. In the first experiment, we propel gold nanoparticles using surface plasmon polaritons (SPPs) excited on a gold film. This work makes use of the fact that near-field coupling between the gold nanoparticle and gold film greatly enhances the attractive force between them. In the second experiment, we demonstrate a scannable plasmonic trap using SPPs excited on a gold stripe. When a single laser beam illuminates the stripe, SPPs propagating along the stripe are excited. Polystyrene particles in the water medium above the stripe are drawn onto it by optical forces and propelled along the length of the stripe by the SPPs. We also show that when two laser beams illuminate the stripe, counter-propagating SPPs can be excited. The particles are then trapped in three dimensions, as the forces on them along the stripe direction cancel. We show that variation of the relative intensities of these two laser beams enables the particle position to be scanned. In the last experiment, the trapping and rotation of nanoparticles using a template-stripped plasmonic nanopillar incorporating a heat sink is demonstrated. The work addresses three important issues: thermal management, the engineering of the near-field distribution to make it optimal for trapping, and ease of fabrication. A number of interesting possibilities exist for extending this work. The work presents a new method for achieving optical manipulation of nanoparticles in a lab-on-chip system with a large array of optical traps that operate at low laser power.