Trapping and manipulation of micron and submicron particles

Micro/nano trapping and manipulation techniques have applications ranging from examining bacteria and exploring biochemistry at the cellular level to condensed matter physics. While conventional optical trapping techniques are widely used, their diffraction limited resolution and non-planar nature make them unsuitable for some applications. Near-field trapping techniques involving plasmonic structures can be used to address some of these issues. These structures can create a near-field spot size smaller than what is possible using diffraction limited optics. The small spot size makes it possible to manipulate subwavelength-sized objects. Their planar nature also makes parallelization easier for lab-on-a-chip applications. Another method having some of the same benefits is dielectrophoresis. Dielectrophoretic methods use electrical excitation of micro-electrodes at radio/microwave frequencies instead of optical excitation to create the trapping/manipulation force. Though lacking the resolution of near-field schemes, dielectrophoretic methods have longer range, material selectivity, and the ability to apply both attractive and repulsive forces. While near-field methods and dielectrophoretic methods both have been applied for trapping micro-/nano-sized objects, their application in controlled motion/manipulation of objects has not been widely investigated. My work involved designing devices which utilize these schemes and are capable of controlled manipulation of micron and submicron-sized objects. The developed technology is aimed towards biochemistry applications. In this thesis, I start with an overview of conventional particle manipulation schemes. Some theoretical discussions on gradient forces and the mathematics used for calculating those forces are presented. A Brownian dynamics model used for analyzing the behavior of colloidal particles in the force-field is also discussed. Next, I present my work on plasmonic C-shaped engravings/apertures for trapping and manipulating nanoparticles. The electromagnetic response of the plasmonic structure, profile of the generated force, and the behavior of a colloidal particle near the structure are discussed. Experimental results of particle manipulation are presented. Some of the limitations of this scheme are mentioned prior to introducing dielectrophoretic manipulation techniques. Dielectrophoresis solves many of the challenges associated with near-field methods at the cost of resolution. I present dielectrophoretic devices which are capable of moving a micro-particle over a distance of hundreds of microns. Controlled motion along a straight line and along a curved path is achieved by using micro-electrode arrays with appropriate electrical switching routines. Lastly, I discuss optically induced dielectrophoresis which incorporates aspects of both optical schemes and dielectrophoretic schemes. Rather than using physical micro-electrodes, the designs use optical illumination to excite photoconductors which behave like virtual electrodes to produce dielectrophoretic forces. The photoconductor can be selectively excited using focused lasers or plasmonic apertures. A hybrid approach utilizing the high resolution of plasmonic C-apertures and the long range of dielectrophoretic forces is developed. Particle manipulations using these techniques are discussed. Finally, concluding remarks are made and potential future works are listed

Keywords: Electrical Engineering.
Advisor: Hesselink Lambertus 
Brongersma Mark L. 
Fan Shanhui
Solgaard Olav