Waveguide-Resonator Coupling Design for High Efficient Nonlinear Photonics(1/2)

Optical microresonators have been well studied as on-chip modulator in silicon photonics. Due to the high quality factor and narrow linewidth, it provides superior performance for high speed optical communication and interconnection. In a linear system, critical coupling shows no light is passing through the waveguide after the resonator via destructive interference at the output waveguide. The propagating wave is coupled into the waveguide resonator and the energy is stored / lost inside the resonator. It gives the most efficient power transfer and high sensitivity / extinction ratio for an optical modulator. This behavior is fully realized in modern silicon photonics design. Recently, microresonators have been proven to be useful in nonlinear photonics, especially in Kerr frequency comb generation. However, unlike the coupling in a cold cavity, the nonlinear dynamics strongly changes the coupling behavior in the presence of energy transition between nonlinear signal and the input pump; this effectively alters the coupling condition and therefore the coupling design at the cold cavity is not suitable for efficient power transform in the resonator. To realize this effect, we aim to utilize two promising platforms in semiconductor industry – silicon nitride and gallium nitride resonator, and design resonators with optimized power coupling in the presence of nonlinear process. By considering the pump power transfer (loss) to the parametric signal, the effective propagation loss of pump line inside the resonator is expected to be increased and therefore pushes the resonator coupling toward under-coupled regime. For this proposal, we plan to design a strongly over-coupled resonator at the cold cavity. To achieve this, two criteria need to be satisfied – First, the propagation loss (or intrinsic loss) should be minimize which mean a resonator with high quality factor. Hardmask patterning / waveguide geometry design / strain relaxed trench will be utilized to meet this requirement. Second, strong coupling is necessary between bus and resonator waveguide. Shrinking gap size down to 300 nm will be investigated. In addition, damascene patterning will be considered to achieve waveguide with critical coupling. It helps to build an efficient / high power platform in the application of on-chip nonlinear photonics. Last, we propose integrated interferometry-based microresonators for electrically controllable comb generation with different repetition rate. This integrated device also works as a compact modulator for specific resonances.