Selective Passive Tuning of Cavity Resonance by Mode Index Engineering of the Partial Length of a Cavity

Abstract: Cavities in large-scale photonic integrated circuits often suffer from a wider distribution of resonance frequencies due to fabrication errors. It is crucial to adjust the resonances of cavities using post-processing methods to minimize the frequency distribution. We have developed a concept of passive tuning by manipulating the mode index of a portion of a microring cavity. Through analytical studies and numerical experiments, we have found that depositing a thin film of dielectric material on top of the cavity or etching the material enables us to fine-tune the resonances and minimize the frequency distribution. This versatile method allows for the selective tuning of each cavity's resonance in a large set of cavities in a single fabrication step, providing robust passive tuning in large-scale photonic integrated circuits. We show that proposed method achieves tuning resolution below 1/Q and range upto 10^3/Q for visible to near-infrared wavelengths. Furthermore, this method can be applied and explored in various optical cavities and material configurations.