Silicon photonic devices for sensing and microparticle manipulation

Abstract

Lab-on-a-chip (LOC) devices are portable, easy-to-use and low-cost for refractive index sensing and bioparticle sorting in tiny volume samples. Optofluidics combining integrated photonics and microfluidics is a promising technology for LOC due to its features of miniaturization, high sensitivity and mass production. In this thesis, we propose and demonstrate silicon photonic device-based refractive index sensors and optical tweezers arrays as building blocks for optofluidics. On the refractive index sensor front, we propose coupled-resonator optical waveguide (CROW)-based sensors for refractive index sensing using spatial domain detection. The conventional optical sensors are typically based on resonance wavelength shift detection in spectral domain. The measurements rely on bulky and expensive equipment (wavelength-tunable laser or spectrometer) which are not suitable for LOC applications. The CROW sensors exhibit broadband transmissions with split modes corresponding to the eigen states. In the spatial domain, the internal field distributions along the CROW show distinguished patterns at different eigen states. Therefore, the refractive index change could be detected spatially by pattern recognitions at fixed probe wavelength. We fabricate racetrack microring cavity CROW sensors in silicon-on-insulator (SOI) substrates. The light scattering from each cavity is captured by an infrared camera and integrated as a pixel to represent the internal field spatial distribution of the CROW. By identifying the pixelized spatial patterns through the fourier transform algorithm, we demonstrate an 8-element CROW sensor with a detection limit of 0.0082 RIU corresponding to 5% mass concentration change of NaCl solutions. We also demonstrate proof-of-concept sensing experiment using gaplessly coupled microdisk CROW on silicon nitride-on-silica substrates. On the optical tweezers array front, we invent Silicon-on-insulator Multimode-interference (MMI) waveguide-based ARrayed optical Tweezers (SMART) technique for two-dimensional microparticle trapping and manipulation. The two-dimensional optical tweezers or optical lattices are significant tools for trapping multiple particles simultaneously and sorting particles by size or refractive index differences. The optical lattice generation techniques (acousto-optic deflectors and holographic optical tweezers) usually require electro-optical devices and sophisticated optics which cannot be integrated into a LOC system. We utilize the self-imaging phenomena in the MMI waveguide to generate optical lattices. We demonstrate arrayed trapping of 1μm- and 2.2μm-sized polystyrene particles in a microfluidic cell with static fluidic. We simulate the multiple physical processes involved in the SMART technique including optical force, thermal-induced flow and fluidic force using the finite-element method. We demonstrate moving, splitting and combining of clusters of microparticles by shaping the optical tweezers array. We also demonstrate 2.2μm polystyrene particle fractionation in optical lattices generated by the SMART technique assisted by a controllable microfluidic flow with a speed from ~2 μm/s to ~100 μm/s. We demonstrate particle guiding along the optical lattice direction with up to ~12° relative to the flow direction, particle blocking in a linear optical lattice and particle accelerations from ~20 μm/s (flow velocity) to ~60 μm/s in a non-uniform optical lattice.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology