Waveguide-coupled spiral-shaped microdisk resonators in silicon nitride

In this thesis, we study waveguide-coupled spiral-shaped microdisk resonators in silicon nitride. We report our experiments and numerical simulations of the transmission spectra and mode distributions of a passive waveguide-coupled spiral-shaped microdisk resonator-based channel filters on a silicon nitride chip. Recently, unidirectional lasing emission in novel rotational asymmetric spiral-shaped microdisk resonators has been proposed and demonstrated. The spiral-shaped microdisk has a smoothly deformed cavity boundary with a characteristic notch structure due to radius mismatch, which exhibits chirality that enables preferential out-coupling of traveling-wave modes in only one sense of circulation. Although the detailed lasing mechanism for such active spiral-shaped microdisks is yet clearly understood, it is generally believed that the clockwise (CW) and counterclockwise (CCW) modes are asymmetric and encounter different losses at the notch, thereby different quality factors and lasing thresholds. However, to the best of our knowledge, the CW/CCW traveling-wave modes Q’s and lasing thresholds in spiral-shaped micropillar/disk lasers have yet been directly measured. Here, we report our systematic experiments and numerical simulations of passive spiral-shaped microdisk resonators in silicon nitride, with waveguides in/out-coupling non-evanescently at the spiral notch and evanescently at the cavity sidewall. We employ a linear-scanning tapered fiber tip in the vertical dimension to probe the cavity internal-field intensity distributions by measuring the out-of-plane scattering from the cavity. Surprisingly, both the transmission and out-of-plane scattering measurements reveal only reciprocal transmission spectra with input-port-dependent asymmetric modal distributions, yet there is no convincing evidence for the CW and CCW traveling-wave modes having different Q’s. Our numerical modelling using two-dimensional (2-D) finite-difference time-domain (FDTD) method also shows results consistent with experiments. Spiral microdisk is a promising microresonator which can be non-evanescently coupled. The implication is that spiral-shaped microdisk Raman silicon laser can be non-evanescently pumped through the spiral notch from a seamlessly butt-coupled waveguide