Trapping of Nanoparticles with Optical Waveguides

Abstract

Over the last few years, the notion that links optical trapping with strong intensity of light (high energy photon) not only forced the modification of optical tweezer, but it also open up the door for evanescent wave field trapping. While optical tweezer is merely suitable for trapping micro-sized particles, trapping by evanescent field of a channel waveguide enables both micro and nanosized particles to be trapped and propel as well. Indeed, nowadays, various structures of channel wave guides are designed to secure higher intensity of light for significantly better trapping purposes. The goal of this study is mainly to examine and better understand features related to trapping of particles on three different structures of a waveguides: straight, loop and ring resonators. We also propose new method to characterize the ring resonator waveguide. Though there are limitations to this method, it is possible to measure power in and out of the ring. Besides, the characteristics result shows too much power loss. From the straight waveguide experiment we confirm that gold particles of diameter 200nm and 500nm are trapped and propelled above the waveguide by the evanescent field. The speed obtained from the 200nm diameter analysis reaches up to 420µm/s for 700mW laser power, which considerably faster than the previously reported values. Given the advantages of the applications of loop waveguides, to stop particles by standing waves or counter propagating beams, we are able to clearly observe this phenomenon in our experiment for 1.02µm diameter polyester particles, in contrast to gold nanoparticles due to weak gradient forces. Maintaining similar analysis for ring resonator waveguides, however, the lacking of particle trapping or propulsion is observed for gallium arsenide nanowires, due to their asymmetric structure besides the low power. Weak gradient force and low power in the gold, and low power though strong gradient force in polystyrene ring waveguides are responsible for lack of trapping and propulsion in the nanoparticles. Even though the priority is of this thesis is the experimental essence, the theories of optical waveguides and optical trapping forces are briefly reviewed.