Silicon-based Nano-waveguides/Devices And Their Application For Optical Sensing

Because of the CMOS compatibility, silicon nanophotonics has become very attractive and consequently has been confidently regarded to be a promising platform for high-performance, low-cost, and ultra-dense optical interconnection and optical lab-on-a-chip. In this thesis, two types of silicon-based nano-photonic waveguides are considered, i.e., silicon-on-insulator (SOI) nanowires and silicon hybrid plasmonic waveguides. Some nano-photonic devices, e.g., novel optical microcavities have been developed for optical sensing as well as optical interconnection.Three kinds of optical microcavities based on SOI nanowire are investigated theoretically and experimentally in order to improve the performances of optical sensor. (1) A Mach-Zehnder interferometer (MZI)-coupled microring is demonstrated experimentally to obtain a high sensitivity as well as a large range for measuring the change of the refractive index. For the present MZI-coupled microring, there is a major resonance wavelength with a high extinction ratio (16-36 dB) over a very large wavelength span. Consequently, a very large quasi-free spectral range (>120 nm) is achieved, which helps to obtain a large measurement range. The MZI-coupled microring sensor is used for measuring the change of the ambient refractive index ranging from 1.0 to 1.538, and the sensitivity is as high as 111 nm/RIU. (2) A digital optical sensor based on two cascaded rings is demonstrated experimentally on the plaftform of SU-8/SiO2/Si rather than SOI. The present digital optical sensor shows an ultra-high theorectical sensitivity (-104 nm/RIU), which is over two orders higher than that of a regular single-ring sensor. By using the present digital optical sensor, it becomes convenient to use an array waveguide grating (AWG) as an integrated optical micro-spectrometer to monitor the peak shift of the spectral response. Therefore, it is promising to realize a low-cost and portable highly-sensitive optical sensor system on a single chip; (3) An F-P microcavity based on SOI nanowires is presented. Bragg gratings are used as the reflectors of the F-P microcavity in order to have a very high reflectivity. The fabricated devices exhibit a moderate Q-factor of 2600, a large FSR of 21 nm, and an extinction ratio of 13 dB. The experimental data agrees well with the simulation result. The demonstrated F-P microcavity is suitable to be used as an optical sensor. It is possible to achieve a higher Q-factor by improving the design of the Bragg grating reflectors by, e.g., widening the width of the Bragg gratings to reduce the scattering loss.Silicon hybrid plasmonic waveguides are also theoretically investigated in this thesis as a way to realize an ultra-desnse photonic integrated circuit. The silicon hybrid plasmonic waveguide has a thin SiO2 layer sandwiched between a silicon-on-insulator (SOI) rib and a metal cladding. The field enhancement in the nano-layer provides a nano-scale confinement of the optical field (e.g., 100nm×5nm@λ= 1550nm). The propagation length of this hybrid plasmonic waveguide is about 100μm, which provides a better compromise between the loss and the confinement than many plasmonic waveguides. Furthermore, silicon hybrid plasmonic waveguide is also potential to be compatible with the CMOS processing and enables the hybrid integration with SOI nanowires on the same chip. Some passive elements based on Si hybrid plasmonic waveguides, e.g., bending radius, directional coupler, multimode interference (MMI), and Y-branch structure, are designed by using a three-dimensional finite-difference time-domain (3D-FDTD) method and finite-element method (FEM). The minimal bending radius for low-loss is around 800 nm, which makes the device with an ultrasmall footprint. The designed 1 X 2 MMI has a nano-scale size of only 650 nm×530 nm. The designed Y-branch is also very small, i.e., about 900 nmX 600nm. The fabrication tolerance is also analyzed and it is shown that the tolerance of the waveguide width is as large as±50 nm. Furthermore, the MMI and Y branch have a very broad band of over 500 nm. Finally a 1×4 power splitter with a device footprint of 1.9μm×2.6μm is also presented using cascaded Y-branches. Therefore, the silicon hybrid plasmonic waveguide provides a very good platform to have ultracompact devices with relatively low loss, large fabrication tolerance as well as broad-band for data-communications and telecommunications.