Study Of Micro-manipulation Using Silicon-based Near-field Optical Tweezers

The modern medical and biological research has improved to the manipulation of single biological particle, which brings lots of challenge to conventional manipulation method such as microfluidics. Luckily, Nanofluidics provides the solution. As a kind of non-contact, non-invasive method, optical tweezers have become one of the most important driving forces. However, this kind of conventional tweezers is limited by the diffraction limit and as a result it is not capable of manipulation of particles with the size smaller than 100nm. Fortunately, the development of nanofabrication enables us to fabricate fine structures to beat this limit. Then near field based optical tweezers are proposed. Meanwhile, the silicon platform owns the advantages of maturity and easy integration, becoming the most promising platform for the realization of near field based optical tweezers. Near field based optical tweezers include evanescent wave based optical tweezers and surface plasmonics based optical tweezers.Using different structures of silicon waveguides, we are able to realize various manipulations, including trapping, transportation, sorting, storage and sensing. In this paper, we propose several novel principles and structures to achieve various optical manipulations, including optical conveyor belt for trapping and transportation, switch system for logic control and sorting system to separate particles with different sizes into groups, which pave the way to the integration of light and fluidics. This paper includes three parts:1. Proposed an optical conveyor belt based on waveguide excitation and realized the precise transportation of particles. Because the evanescent wave is as large as several hundred meters, it is rather difficult for conventional optical tweezers to trap and manipulate particles with size smaller than 100nm. Plasmonics based optical tweezers provide a solution to this problem. For example a pair of metallic rods can excite the local enhanced optical field efficiently and can provide much higher gradient force, which contributes to particles trapping. This kind of structure also owns wavelength selectivity, related with rod dimensions, which enables us to excite certain local optical field. This conveyor belt consists of a silicon waveguide and metallic nanorods with graded dimensions, which are arranged alternatively. Different dimensions lead to different resonance wavelength. With the excitation of a certain wavelength, a certain set of hotspots will be delighted and particles can be trapped in these areas. When the wavelengths are changed alternatively, particles will be delivered into the next hotspot. Finally, an optical conveyor belt is proposed and particles can be moved forward and backward along a waveguide in a highly controlled way.2. Proposed a polarization-based particle guiding device which could guide particles into different ports with the switch of polarization of incident light. This structure serves as one of the fundamental parts in optical micro-manipulation area and is also the key part of the large-scale complex manipulation system. However, most of the guiding devices are based on multi-wavelength. In this paper, we propose a guiding device based on polarization. With the help of polarization sensitivity of slot waveguides, we can obtain the polarization dependent coupling between a conventional nanowire and a slot waveguide by optimizing the dimensional parameters of the structure. Thus, a polarization beam splitter is achieved, in which light of different polarization will be guided into different ports. Integrated with near-field optical tweezers, this splitter is capable of guiding particles into different ports by changing the polarization of excitation.3. Proposed a sorting system using the different response of particles with different dimensions to a same optical field. Sorting system is also a key technique on lab-on-a-chip system, which can sort particles with different sizes automatically. The system consists of two nanowires with different widths. The one with larger width owns a larger optical envelop, while the smaller one has a smaller envelop. When particles are first trapped in the small envelop, because the larger ones can sense more optical force in space, they may be dragged by the large optical envelop to the other port. However, the smaller ones cannot sense the large envelop and thus it will stay. Finally, particles with different sizes can be sorted in this device.