光學微共振腔技術近十年來已被廣泛的運用來實現矽晶調變器。利用高素質量之微共振腔可達到在頻率上極小線寬之共振環境。此設計可提供於光通訊系統在高頻環境時的反應速度及精準頻率選擇。在過去設計晶載調變器用於在光連結及通訊時，為了達到高靈敏度及高消光比，會將耦合環境設計在臨界耦合。也就是說在輸出波導位置因破壞性干涉條件不會有光波穿透。而所有輸入的能量則被儲存/消耗於共振腔內。微共振腔除了在線性系統下被廣泛運用，更被視為晶載非線性系統的重要元件-利用其腔內共振之極高能量，可用於做非線性光訊號之產生。然而此產生機制將嚴重改變共振腔的耦合環境，大幅降低能量耦合之效率。本計畫將討論於設計高效率之非線性光學元件之耦合特性，希望在非線性環境下增加其能量利用率。預期在利用光學梳之模型能建立轉換效率>40%之非線性元件。第一部分將討論氮化矽及氮化鎵二種不同材料微共振腔之線性耦合。利用製程技術之硬遮罩提高共振腔之平整度以降低傳遞損耗。第二部分討論在非線性光訊號(如二倍頻及光學梳)產生時所造成的入射能量衰減。設計在非線性模式下最佳耦合之操作點。最後提出可調變干涉共振腔之設計，達到整合非線性訊號調變器。 ;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.
