Breaking the Loss Limitation of On-chip High-confinement Resonators (1609.08699v1)

Abstract: On-chip optical resonators have the promise of revolutionizing numerous fields including metrology and sensing; however, their optical losses have always lagged behind their larger discrete resonator counterparts based on crystalline materials and flowable glass. Silicon nitride (Si3N4) ring resonators open up capabilities for optical routing, frequency comb generation, optical clocks and high precision sensing on an integrated platform. However, simultaneously achieving high quality factor and high confinement in Si3N4 (critical for nonlinear processes for example) remains a challenge. Here, we show that addressing surface roughness enables us to overcome the loss limitations and achieve high-confinement, on-chip ring resonators with a quality factor (Q) of 37 million for a ring with 2.5 {\mu}m width and 67 million for a ring with 10 {\mu}m width. We show a clear systematic path for achieving these high quality factors. Furthermore, we extract the loss limited by the material absorption in our films to be 0.13 dB/m, which corresponds to an absorption limited Q of at least 170 million by comparing two resonators with different degrees of confinement. Our work provides a chip-scale platform for applications such as ultra-low power frequency comb generation, high precision sensing, laser stabilization and sideband resolved optomechanics.

Breaking the Loss Limitation of On-chip High-confinement Resonators

This paper by Xingchen Ji et al. addresses one of the significant challenges in advancing on-chip optical resonators: overcoming optical losses while maintaining high confinement and a high-quality factor (Q). On-chip resonators, especially those utilizing silicon nitride (Si₃N₄), have potent applications in metrology, sensing, frequency comb generation, and optical clocks. However, these resonators historically suffer from higher losses compared to their bulkier crystalline counterparts. Through a combination of surface smoothing techniques and optimized lithography processes, the researchers report achieving unprecedentedly high Q factors, significantly reducing optical losses for on-chip high-confinement resonators.

Key Advancements and Results

The paper elucidates a methodology to significantly curtail the scattering-induced losses in high-confinement Si₃N₄ ring resonators. Notably, it reports achieving Q factors of 37 million for a 2.5 µm wide ring and approximately 67 million for a 10 µm wide ring, underlining a systematic path to achieve these metrics by addressing surface roughness. Measurement revealed that the loss limited by material absorption was as low as 0.13 dB/m, corresponding to an absorption-limited Q of at least 170 million. This is achieved by fabricating resonators with optimized technique combinations—specified plasma etching adjustments to eliminate sidewall polymer residues and chemical-mechanical polishing (CMP) to reduce top surface roughness.

The researchers also highlight a novel application where reduced optical losses lead to lowered thresholds for optical parametric oscillation; this is demonstrated with sub-milliwatt threshold powers, which are notably the lowest reported for planar nonlinear platforms to date. The paper further presents extensive comparative analysis with other platforms and materials, as depicted in Table 1.

Implications and Future Directions

The findings hold significant implications for integrated photonics. By outstripping previous loss limitations related to both surface scattering and bulk material absorption, the work provides a scalable approach to achieving ultra-low-loss on-chip devices. Such advancement is a pivotal step toward realizing compact, high-performance photonic circuits with minimal power requirements—a crucial facet for the development of photonic systems in telecommunications and sensors.

From a theoretical perspective, this work contributes to the broader understanding of loss mechanisms in high-confinement waveguides, providing empirical quantification to isolate scatter-induced from absorption-induced losses. The meticulous process optimization demonstrated here could be adapted across various material platforms, hinting at the possibility of achieving ultra-high-Q resonators beyond Si₃N₄.

Future research can build upon this framework by exploring additional material systems or further refining the fabrication processes to push the boundaries of miniaturization and integration in photonic devices. Moreover, addressing the remaining constraints posed by bottom surface roughness, particularly the cladding/core interface, represents a potential pathway for further reduction of scattering losses, potentially unlocking even higher Q factors and lower power operational thresholds.

Overall, this paper illustrates a significant stride in photonic technology, offering a foundation for future innovations in integrated photonics that demand both high confinement and low losses.