Optical Plasmonic Trapping

This work focuses on trapping subwavelength objects using resonant plasmonic structures. Trapping entities include latex spheres, living cells and metallic particles; the plasmonic structures involved include subwavelength disks, subwavelength apertures, and dipole antennas. Furthermore, the plasmonic structures have been incorporated into microfluidics and trapping in such a novel lab-on-a-chip is successfully demonstrated. In this thesis, the experiments implemented are supported with abundant numerical simulations. The optical field confinement is computed using the Green's tensor technique and the optical forces are computed using the dipole approximation. In particular, two numerical models are studied in detail: one is the plasmonic disk based near field optical trapping and the other is the nanoantenna based near field optical trapping. The study of the optical forces in such plasmonic systems indicates that the strength of the optical forces exerted on a subwavelength object is determined by the polarizability of the object and the gradient of the optical field. The sign of the optical forces can be accurately predicted by comparing the resonance wavelength of the subwavelength object λobject and the resonance wavelength of the plasmonic structure λstructure: if λobject < λstructure, the object is always attracted towards the plasmonic structure; if λobject > λstructure, the object is always repulsed away by the plasmonic structure. Furthermore, trapping of particles with dispersive optical properties may cause a significant spectral shift due to the near field interaction of the adjacent plasmonic entities. For example, the trapping of a 20 nm gold nanosphere in the gap of a dipole antenna with dimensions 90 × 40 × 40 nm3 results in a 40 nm shift of λstructure to the red. Two experiments are studied in great detail. The first one reports the first integration of plasmonic trapping with microfluidics for lab-on-a-chip applications. Subwavelength gold disks integrated into a three-layer microfluidic chip are used to demonstrate the trapping of latex spheres and living S.cerevisiae cells. The plasmonic substrate and the microfluidics, are fabricated with standard photolithography and soft lithography, respectively. This technique enables cell immobilization without the complex optics required for conventional optical tweezers. The second experiment demonstrates the use of individual plasmonic apertures to trap nanoparticles in the near field regime. Apertures are prepared using focused ion beam technique over a layer of gold film. Experiments are performed under a commercial scanning confocal microscopy. Trapping of 20 nm fluorescent polystyrene particles is tracked simultaneously by the fluorescent signal emitted by the trapped particles and the reflection pattern of the apertures. The correlation of those two signals offers further degree of freedom to qualitatively analyze the trapping properties: the number of trapped nanoparticles per aperture and the position where the trapped nanoparticles stay in the aperture. This trapping approach offers an alternative method to immobilize nanoparticles with great versatility: no additional optical components are required since it is directly operated with the commercial microscopy and integration with microfluidics should not be difficult. Comparing with conventional optical trappings, this plasmonic structure based near field trapping opens new perspectives to manipulating particles with a nanometer accuracy. The benefits of such plasmonic trapping in microfluidics are optical simplicity, low power consumption and compactness: addressing high quality single-cell data with good selectivity; massive and parallel manipulations for high-throughput analysis; time and reagent saving; reusable, cheap fabrication cost and versatility. It is an effective, versatile and compact manipulation tool, and should pave the way for advanced studies down to the single molecule level.