Novel structures and applications of leaky thin-ridge silicon waveguides
Creators: name: Yego, Kiplimo
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400909
subject: Photonic and electro-optical devices, sensors and systems (excl. communications)
classificationCode: 510204
subject: Photonics, optoelectronics and optical communications

description: The ability to utilize signals at optical frequencies, as opposed to say microwave frequencies, provides much more bandwidth and signal transmission speed to meet the increasing telecommunication demands in today's world. The ability to integrate optical circuits in the same manner as in electronic integrated circuits means that optical devices can be miniaturized and can even complement today's complex electronic circuits and devices. Silicon nanophotonics is a highly attractive platform for emerging integrated optical solutions in areas including optical signal transmission, signal processing, optical sensing and optical computing. This is primarily because the silicon platform is compatible with CMOS fabrication processes, which through significant investment have developed and matured over many years to serve the electronics industry. Transitioning into an optical platform that can exploit this vast electronics manufacturing industry is viable particularly for enabling low cost mass manufacturing of integrated photonic circuits. High refractive index contrast silicon waveguide platforms such as silicon-on-insulator (SOI) enable strong confinement of light in sub-micron waveguides as well as the sharp bending of waveguides with minimal loss. The SOI platform has therefore attracted research interest into the development of compact integrated silicon photonic circuits.<br><br>Thin-ridge SOI waveguides are particularly promising because they minimize signal transmission loss by significantly reducing the waveguide etch-depth and therefore reducing scattering losses due to sidewall roughness. However, a consequence of the reduced etch-depth is the possibility for TM guided modes to couple to highly coherent TE radiation in the adjacent slab. This TM-TE coupling phenomenon, named lateral leakage radiation, is the subject of this thesis. The main aim of this thesis is to investigate the possible exploitation of this inherent TE-TM coupling relationship. The novel structures presented herein could have potential applications which include optical biosensing, polarization rotation and resonant optical filtering.<br>The main contributions of this research work include first and foremost the discovery of a resonant TE-TM coupling effect in thin-ridge waveguides. This resonance effect has a canonical Lorentzian response and the quality-factor can be controlled by adjusting the waveguide dimensions. It is also shown that several such resonator waveguides can be cascaded in a coupled resonator topology to realize higher order Chebyshev filter responses. Another contribution in this thesis is that a holographic-based grating structure exploiting the TM-TE coupling in thin-ridge waveguides can be used to efficiently convert a Gaussian TE slab beam into a collimated TM slab beam. It is shown that an apodized grating is the most suitable design for achieving this goal. Lastly, it is also shown through simulation that the lateral leakage effect can be utilized as a biosensor to measure refractive index changes at the surface of a thin-ridge waveguide caused by the deposition of biomolecules. A tapered thin-ridge waveguide in tandem with a planar lens structure is proposed as a potential sensor topology for evanescent field biosensing.<br><br>In summary, it has been shown that lateral leakage in thin ridge waveguides can be enhanced using unique waveguide structures and exploited for integrated optical applications.