Silicon photonic devices utilizing lateral leakage behaviour
Creators: name: Dalvand, Naser
Publisher: name: RMIT University
Year: 2024

description: Transverse magnetic (TM)-like modes of thin-ridge Silicon-on-Insulator (SOI) waveguide can leak into radiating transverse electric (TE) slab modes in the lateral direction unless the waveguide width is maintained at a resonant, so called `magic' width. The highly evanescent TM mode has a relatively low effective index and a strong longitudinal field component. The higher effective index TE slab mode, when rotated about the axis of TM propagation, can achieve both phase matching and non-zero overlap resulting in strong radiation without any perturbation. This TE radiation occurs in equal amounts on both sides of the waveguide. At the `magic' width, there is destructive interference of radiation wave fronts resulting in low loss for the TM mode. Rather than viewing this leakage as a limitation, this thesis explores the ability to manipulate waveguides from low-loss guidance to efficient radiation and how this ability may present opportunities for new silicon photonic devices. In this thesis, first a Silicon-on-Insulator waveguide structure is proposed which, when excited with TM guided light, emits controlled TE polarized radiation from one side of the structure only. The validity of the proposed structure is analysed using eigenmode expansion and supermode techniques. Steps toward practical demonstration of the proposed structure are identified. Transitions from low-loss `magic' width to strongly radiating `anti-magic' width thin-ridge SOI waveguides operating in the TM mode, is analysed using a vector eigenmode expansion method for the first time to the best of my knowledge. It is shown that the transition produces a beam of TE radiation with a pattern which is strongly dependent on the geometry of the transition. It is shown that a controlled, highly coherent, and low-Transverse magnetic (TM)-like modes of thin-ridge Silicon-on-Insulator (SOI) waveguide can leak into radiating transverse electric (TE) slab modes in the lateral direction unless the waveguide width is maintained at a resonant, so called `magic' width. The highly evanescent TM mode has a relatively low effective index and a strong longitudinal field component. The higher effective index TE slab mode, when rotated about the axis of TM propagation, can achieve both phase matching and non-zero overlap resulting in strong radiation without any perturbation. This TE radiation occurs in equal amounts on both sides of the waveguide. At the `magic' width, there is destructive interference of radiation wave fronts resulting in low loss for the TM mode. Rather than viewing this leakage as a limitation, this thesis explores the ability to manipulate waveguides from low-loss guidance to efficient radiation and how this ability may present opportunities for new silicon photonic devices. In this thesis, first a Silicon-on-Insulator waveguide structure is proposed which, when excited with TM guided light, emits controlled TE polarized radiation from one side of the structure only. The validity of the proposed structure is analysed using eigenmode expansion and supermode techniques. Steps toward practical demonstration of the proposed structure are identified. Transitions from low-loss `magic' width to strongly radiating `anti-magic' width thin-ridge SOI waveguides operating in the TM mode, is analysed using a vector eigenmode expansion method for the first time to the best of my knowledge. It is shown that the transition produces a beam of TE radiation with a pattern which is strongly dependent on the geometry of the transition. It is shown that a controlled, highly coherent, and low-divergence TE beam can be emitted from a relatively compact linear taper. Methods for side lobe suppression are also analysed and avenues for more sophisticated beam shaping are identified drawing inspiration from leaky wave antennas. Novel concepts for polarization splitter-rotator and wavelength selective devices are presented utilizing the lateral leakage in taper transition from `magic' to `anti-magic' width thin ridge SOI waveguides. It is shown that a very compact TM-TE polarization rotator-splitter with a large bandwidth (with a 0.24dB bandwidth of 100nm) can be realized. A 4-channel wavelength de-multiplexer utilizing dispersive characteristics of the lateral leakage is proposed. It is numerically predicted that the proposed device would have relatively low propagation loss and cross-talk. Avenues to practical demonstration of the proposed structures are identified. In summary, it is shown that the lateral leakage phenomenon can be used to design interesting new devices. It is shown that rigorous numerical tools introduced in this thesis can be effectively used to design such devices. This could enable designing and developing other topologies which might utilize lateral leakage such as sensors and hybrid integrated devices.<br>