Low Q-factor Silicon Photonic Cavities for Optical Filtering and Single-Photon Detection

  • Mack H Johnson

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Integrated silicon photonics offers great promise for both classical and quantum photonic applications. The high index-contrast of the silicon-on-insulator material system enables compact sub-micron size waveguides to be realised. Furthermore, the technology is compatible with on-chip single-photon detectors. This thesis targets the design and fabrication of components relevant for a dedicated detector chip that can be used in chip-to-chip quantum photonic experiments where high performance single-photon detectors and on-chip optical filters are required.
A design for optical filtering on-chip is presented based on ring resonators that is fully compatible with single-photon detector design and fabrication. An absorptive nanowire can be fabricated onto the ring resonator and the device can be operated in the critical coupling regime. Design considerations for the nanowire are given and up to 33 dB extinction is shown in a single ring, whilst up to 52 dB extinction is shown in a linear cascade of three rings. This low Q-factor filtering approach promises to suppress the build up of electric field intensity inside the critically coupled resonators, preventing spurious non-linear effects such as single-photon generation within the cavity. The advantages and disadvantages of this technology are discussed and contrasted with existing approaches.
A waveguide crossing for SOI strip waveguides based on multi-mode interference is designed and experimentally measured with 0.043 dB device loss and -50.2 dB crosstalk with a footprint of 14.3 x 14.3 µm2. Furthermore, the backscatter of the waveguide crossing was also evaluated to be between -35 and -55 dB by optical time-domain reflectometry. This technology can find applications in dense on-chip integration, where many low loss interconnect components are required such as large port optical switching networks and larger scale quantum photonic experiments.
Finally, considerations for experimental realisation of a previous on-chip detector proposal based on a racetrack resonator operating in the critical coupling regime is considered. The cavity design is modified by implementing the designed waveguide crossing, allowing the nanowire to be routed to electrical contacts without inducing significant losses. The structure is further modified according to practical considerations for the detector, which improves the loss of the crossing structure t o0.0125 dB. Studies of the absorption of the nanowire in the wave guide crossing region are performed. The detection efficiency and timing jitter are explored for the modified design, showing that detection efficiencies on-chip above 95% can be reached under certain conditions. Racetrack resonators with the waveguide crossing region are experimentally tested, showing similar performances to standard racetrack resonators. Furthermore, 24 dB of critical coupling is observed in these structures, which is necessary for a high performance detector.
Date of Award23 Jan 2020
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorMark G Thompson (Supervisor) & Dondu Sahin (Supervisor)

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