Surface Plasmon Resonance Enhanced Photophoresis in Nano-metallic Colloids

Surface plasmon resonance (SPR) is the collective electrons excitations, which occurred at the metal-dielectric interfaces and can be induced by an oscillating electric field. Because of the large field enhancement, SPR has a wide range of applications in near field optics and nano-optics, such as biosensors, lab-on-a-chip devices, plasmonic waveguides, and cloaking devices.;In this thesis, we study the interparticle forces between metallic nano-sized objects. It is an important topic in nanotechnology for at least two reasons. Firstly, the study of the interparticle forces may provide methods to control the motion and position of nano-size objects, which can be used to fabricate artificial nano-structure by bottom up approach. Secondly, the force can change the arrangement of the particles in the nanodevices and hence affecting the property of the devices.;The interparticle forces of nano-sized dielectric particles are negligible, since the force is proportional to the volume of the objects. However, the interparticle forces of metallic particles will be greatly enhanced when SPR occurs, which is able to compensate the volume effect. This phenomenon is called surface plasmon resonance enhanced photophoresis (SPREP), which is one of the consequences of the rapid increase in the dipole moment in the particles.;This thesis is consisted of three main parts. In the first part, we study the SPREP between a graded metallic nanosphere and a point dipole which is undergo oscillation and precession. A first principle approach is applied to handle this problem. Our analytic solutions are able to capture the multipole effect, which cannot be neglected in highly non-uniform fields. We have analyzed three important physical quantities: the induced force, the induced torque, and the field distribution. Furthermore, we find that there is a binding between the nanoparticle and the dipole source, when the gradation of the graded particles is large enough. This study has a potential application in developing a novel kind optical tweezers.;In the second part, we study the SPREP between two metallic nanoparticles. The force spectra are calculated by two different methods: Bergman-Milton spectral representation and multiple image method. The binding between two nanoparticles is observed, which indicates a possible stable structure among the metallic clusters. The binding is caused by the excitation of collective plasmon modes, and the consequence that the resonance poles s ℓ are the functions of separation distances. This study may provide a better understanding in the structure formation of colloidal clusters in nano-scales.;Finally, we consider a many-particle system by the discrete dipole approximation (DDA) and effective medium theory. Although, the DDA is not an exact formalism, it is a suitable approximation for considering finite number of particles, if the distances among particles are not too close. When the number of particles in the host solution is large, we can use the effective medium theory to handle the problem. Instead of considering all discrete particles individually, we will consider the interaction between a single particle and a new effective host solution, where the dielectric function of the effective host solution is determined by the concentration of nanoparticles in the host solution.