Hybrid Plasmonic Devices for Optical Communication and Sensing
Tid: Må 2017-05-22 kl 10.00 - 12.00
Föreläsare: Xu Sun
Plats: Sal C, Electrum, Kistagången 16, Kista
Title: Hybrid Plasmonic Devices for Optical Communication and Sensing
Candidate: Xu Sun
Time: Monday May 22, 2017, at 10.00
Location: Sal C, Electrum, Kistagången 16, Kista
Opponent: Dr. Laurent Vivien, CNRS director, Centre for Nanoscience and Nanotechnology, University of Paris Sud, Orsay, France
Supervisor: Univ. Lekt. Lech Wosinski
Abstract: Silicon (Si) and Si-on-insulator (SOI) platforms based technology is well- developed and is regarded as a most promising technology for the realization of photonics integrated circuits for optical communication, interconnect, bio-sensing, etc. However, due to silicon’s indirect bandgap, absence of detection properties at 1.3-1.6μm and the difficulty in shrinking device size, hybrid Si devices are widely investigated in recent years, addressing compactness and versatility. Integration of Silicon and plasmonic materials (gold, silver and cooper are commonly used) can break the so called diffraction limit and permit much reduced mode size in relation to Si photonic, allowing photonics on-chip components with ultra-compact size. However, the large plasmonic temporal losses due to the damping of free electrons’ oscillations, in general increase with reduced mode size and hence limit the propagation length and thus applicability. Hybrid plasmonic (HP) waveguides, a multi-layer waveguide structure supporting a hybrid mode of surface plasmonics and Si photonics, is a compromise way to integrate plasmonic materials into Si or SOI platforms, which can guide optical waves of sub-wavelength size, and with relative low propagation loss. In this thesis, several HP waveguides and devices are developed for the purposes of optical communications and sensing.
The developed HP waveguides are divided into lateral and vertical structures, where different materials are either placed side by side (lateral) or arranged as layers in vertical direction. With lateral structures, an air gap between plasmonic material and the silicon core can be formed, which can be used for optical sensing and modulation applications with very high sensitivity to the refractive index change of the materials inside the gap. The single-slot HP ring resonator sensor with 2.6μm radius can give a quality factor (Q factor) of 1300 at the communication wavelength of 1.55μm with a device sensitivity of 102nm/RIU (RIU = refractive index unit). For the double-slot HP waveguide, the sensitivity is even higher. The Mach-Zehnder interferometer (MZI) with a 40μm double-slot HP waveguide has a device sensitivity around 474nm/RIU. The partly opened silicon side-coupled double-slot HP ring resonator has a device sensitivity of 687.5nm/RIU, however, at the expense of low Q factor (300) due to the different modes propagated inside. Further optimizations (simulation results) show that the Q factor can be improved to over 1000. For vertical HP waveguide structures, the fabrication is simpler than for the lateral ones, with easier the overlayer alignment. Further, an all-optical switching HP donut resonator with a photothermal plasmonic absorber is developed, utilizing the thermal expansion effect of silicon to shift the resonant peak of the HP resonator. The active area has a radius of 10μm to match the core size of a single-mode fiber. By applying 10mW power of the driving laser to the absorber, the resonator transmitted power can be changed by 15dB, with an average response time of 16μs. Using the same fabrication flow, and removing the oxide materials using hydrogen fluoride wet etching, a hollow HP waveguide is fabricated for liquid sensing applications. The experimentally demonstrated waveguide sensitivity is about 0.68, which is more than twice that of pure Si waveguide device. Microelectromechanical systems (MEMS) can also be integrated into vertical HP waveguides. By tuning the thickness of the air gap, over 20dB transmitted power change was experimentally demonstrated. This can be used for optical switching applications by either changing the absorption or phase of the HP devices.
All components presented here are novel HP structures showing useful performance in optical communications and bio-sensing applications, and can be used to enhance the performance and broaden the functionality of silicon or SOI based photonic integrated circuits.