Study on stable optical trapping and manipulation in free-space on a chip using an open trenched waveguide

Abstract

Using an optical mode formed in a photonic device, optical trapping force is generated due to a change in the momentum as light passes through a particle, and can selectively control the position of the micro-particles in space. In particular, optical waveguides that can be fabricated by semiconductor processing have been actively studied since those structures can be integrated with microfluidic systems. However, the optical trapping using conventional optical waveguides employs the near-field generated from the surface of the structure. Because there is a strong force toward the surface, physical contact between the particle and the surface occurs during the trapping. The Contact can have physical and chemical effects on the particles. Therefore, it is essential to implement a system that can control the light spatially away from the surface.

In this dissertation, we propose inverted rib-type waveguides with an open trench to overcome limitations in the planar structure, and demonstrate stable, free-space optical trapping and manipulation in an integrated microfluidic system. The open trench designed to be deeper and wider than the optical mode enables full utilization of the optical power away from all surfaces. First, numerical simulations are performed to generate the optical mode in the free space in the microfluidic channel and to analyze the optical trapping force for the three - dimensional space. After integration with polydimethylsiloxane (PDMS) microfluidic channel for particle delivery, 0.65 $\mu$m and 1 $\mu$m diameter polystyrene beads were trapped in free space of the trench. The position of the trapped particle was controlled experimentally by controlling the relative optical power at the end of the waveguides. And the optical trapping force in each direction was analyzed by comparing with numerical simulation.