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Rigorous characterisation of photonic devices incorporating novel phase change materials

Song, J. (2020). Rigorous characterisation of photonic devices incorporating novel phase change materials. (Unpublished Doctoral thesis, City, University of London)


The primary aim of this research work is to develop compact Si Photonic devices incorporating with novel phase change material at the operating wavelength of 1.55 μm. Miniaturisation of active photonic devices, such as optical modulators and switches is crucial to improve the integration density of photonic integrated circuits (PICs). The photonic device based on COMS compatible Si-on-insulator platform have shown great promising due to the footprint reduction and lower power consumption. However, Si modulation is not only limited in the electro-optic but also the thermo-optic effect, and the plasma dispersion effect is also not efficient. The phase change material (PCM) exhibiting a larger refractive index change during its phase transition can be integrated with Si waveguide to mitigate the present Si modulation limitations. The reversible phase transition can be triggered thermally, electrically, and optically in nanosecond. PCM such like GST also possess the self-holding features, so no sustaining power is required to maintain a given state.

Design, optimisations, and performance evolution of those Si-PCM composite waveguides are carried out by a full-vectorial H-field finite element method. Initially, modal solutions of a GST-clad Si waveguide and a GST-clad Si nanowire are investigated for comparison for a variety of waveguide parameters. The electro-absorption design is preferred due to a very high loss incurred in electro-refraction design. The coupling loss at input/output junctions for the composite waveguides are also analysed by the least squares boundary residual method. A compact 2-5 μm long electro-absorption modulator incurs only 0.36 dB insertion loss and more than 20 dB extinction ratio is shown to be possible. The new low-loss material GSST is also considered and shown that a 4.75 μm long modulator have a 0.135 dB insertion loss, only 38% of the GST based design and more than 20 dB extinction ratio. The proposed device design is also verified by a finite difference time domain method (FDTD). Additionally, an electrically driven optical switch based co-directional coupling between Si nanowire and ITO-GST-ITO waveguide is also investigated. The modal evolution of the complex supermodes is studied and employed to optimize the coupling length and ITO spacing. The phase-matching condition of the GST thickness is also achieved for efficient power transfer between the waveguides. A 1.7 μm long ON-OFF switch can be obtained with 0.56 dB insertion loss and 22 dB extinction ratio. Finally, a 1 x 2 optical switch based on a directional coupler comprises of a Si nanowire and GSST-loaded Si waveguide is reported. Seven combination parameter sets are reported for the desire coupling length ratio R = 2. The FDTD is used to verify the proposed design and study the power transmission characteristics. A compact and fabrication tolerant 12-16 μm long switch can be achieved based on an optimised directional coupler design with 15-20 nm GSST layer on the Si nanowires shows 0.33-0.35 dB insertion loss.

Publication Type: Thesis (Doctoral)
Subjects: Q Science > QA Mathematics
Q Science > QA Mathematics > QA75 Electronic computers. Computer science
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering
Date available in CRO: 24 Aug 2021 08:43
Date deposited: 24 August 2021
Text - Accepted Version
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