title: Optical manipulation of metallic particles
Creators: name: Shen, Z
Publisher: name: University of Liverpool Repository
Year: 2017

description: Plasmonic tweezers and optical tweezers are two techniques for trapping and manipulating particles. Plasmonic tweezers utilizes localized surface plasmon field, whilst optical tweezers utilizes focused laser beam. In this thesis, these two techniques were applied for the manipulation of metallic particles in three basic forms: single particle, particle dimer and particle trimer. Firstly, the trapping of metallic particles was investigated through focused plasmonic tweezers when surface plasmons are excited by focused Radially Polarized Beam (RPB). The force exerted on the metallic particle is responsible for the trapping, which is found to be due to the sum of both gradient and scattering forces acting in the same direction established by the coupling between the metallic particle and focused plasmonic field. This contrasts the repulsion of metallic particles in optical tweezers. Focused plasmonic trapping of metallic particle enables actively moving metallic particle in a controlled way, which could be used for intracellular Surface Enhanced Raman Scattering (SERS) imaging. Secondly, the trapping of horizontally-oriented metallic particle dimers was theoretically studied through focused plasmonic tweezers when surface plasmons are excited by focused linearly-polarized beam. It was found that a Surface Plasmon Virtual Probe (SP-VP) pair was generated on a metal film. A formula is derived to represent the electric field of SP-VP pair, revealing that the spacing of the two virtual probes is wavelength-dependent. Each SP-VP is able to trap a metallic particle, thus the gap between the trapped particles of the dimer can be controlled by changing the excitation wavelength. This theory was further tested by successfully trapping nanosphere and nanorod metallic dimers with 10 nm gaps. The trapped dimer showed a typical electric field enhancement of more than 103 times, which is enough for single molecule SERS detection. Thirdly, a vertically-oriented dimer structure was proposed based on trapping of metallic nanoparticle by focused plasmonic tweezers. The vertically-oriented dimer can effectively make use of the dominant longitudinal component of the SP-VP thus providing much stronger electric field in the gap. Furthermore, for practical application the top nanoparticle of the dimer can be replaced with the tip of an atomic force microscope which enables the precise control of the gap distance of the dimer. Therefore the proposed vertically-oriented dimer structure provides both the scanning capability and the extremely-high electric field enhancement necessary for the high sensitivity Raman imaging. Lastly, the stable trapping and steady rotation of a metallic particle trimer were experimentally achieved by optical tweezers with optical vortex. The trimer particles are found to be confined inside the maximum intensity ring of a focused circularly polarized optical vortex. Theoretical analysis suggests that a large proportion of the radial scattering force pushes the particles together, whilst the remaining portion provides the centripetal force necessary for the rotation. The achieved steady rotation of the metallic particle trimer may lead to the development of microfluidics devices such as micro-rotor.