Interactions of electromagnetic waves with micro/nano particles: Manipulation and characterization.

摘要

Interaction of electromagnetic waves with small particles has been extensively investigated for detection, characterization and manipulation of objects in the micro/nanoscale. Several advanced techniques based on light-particle interaction have been developed and widely used in biomedical research, e.g., trapping of micron-size particles by light and plasmonic photothermal therapy with gold nanoparticles.;In this thesis we have developed two new methods of optical trapping by use of planar metal nano-optic structures. One is based on a metallic nanoslit array structure that is designed to allow refractive transmission of light with proper phase retardation at each slit, shaping an incident light into a sharp focus. The metallic nanoslit array lenses were integrated with a fluidic channel cell, and optical trapping of polystyrene microspheres was demonstrated. Another optical trapping method developed in this thesis utilizes the diffraction phenomenon occurring at a thin-film metal edge. Interference of boundary diffraction and free-space transmission waves is found to generate highly localized distribution of light around the metal edge. The electromagnetic field distribution around the edge was calculated by finite-difference-time-domain method, and the 2D trapping forces were estimated by applying a ray optics model. Trapping of a 2-im-diameter polystyrene bead was demonstrated, and the trapping force (escape force) is measured to be about 2.2 pN at incident power of 32 mW. Selective trapping and sorting of microspheres by this metal edge trapping is also demonstrated.;Use of metal nanoparticle colloidal solutions in conjunction with radiofrequency (RF) waves has recently drawn attention as a possible means to deposit heat in a local confined space for cancer therapy. We investigated the heating effect of gold nanoparticle (Au-NP) colloids in the presence of RF electromagnetic wave, and explored the possible role of Au-NPs in RF energy absorption. Contrary to the previously-taken assumption in this field, we found that Au nanoparticles do not contribute to RF energy absorption. Au-NPs were physically separated from the colloidal solutions via centrifugation, and RF heating and electrical conductivity measurements were performed. The results show that the dominant mechanism of RF-radiation-to-thermal conversion is due to the Joule heating via ionic conduction in the electrolyte solutions.