Enhancing Long-term Stability of Ultrahigh Q Silica Microcavities with Hydrophobic PFO Surface Treatment

Significance 

Silica microcavities are tiny, highly engineered structures made of silicon dioxide, known for their ultralow intrinsic optical loss, have been pivotal in achieving ultrahigh quality factor (Q) values necessary for various nonlinear optical processes in compact devices. These processes are critical for a wide range of applications, including but not limited to, optical signal processing, sensing, and quantum information technologies. The ability of these microcavities to trap light efficiently, thereby enabling high circulating intensity, is a cornerstone for the realization of these applications. However, the degradation of Q values due to surface absorption loss, primarily from water molecule absorption on the silica surface, has been a critical bottleneck. This degradation increases the threshold power required for nonlinear processes and adversely affects the optical mode stability and overall device performance. To this account, a new study published in ACS Applied Materials & Interfaces and conducted by Jiadu Xie, Yang Wang, Hui Kang, Jinsong Cheng, and led by Professor Xiaoqin Shen from the School of Physical Science and Technology at ShanghaiTech University, the authors addressed the degradation of Q in silica microcavities, particularly focusing on overcoming the challenge of Q value degradation over time due to surface absorption losses. Their experiments were centered on the fabrication, characterization, and performance evaluation of silica microcavities treated with a hydrophobic layer to prevent this degradation. The findings from these experiments demonstrated the effectiveness of their approach in maintaining ultrahigh Q values over time and highlighted the potential for significant advancements in nonlinear photonic applications. The innovative approach introduced by the research team involves the fabrication of a new type of silica microcavities with a hydrophobic surface layer composed of 1H,1H,2H,2H-perfluorooctyl (PFO) molecules. This layer effectively prevents both high and low affinity water molecule absorption on the silica surface, addressing the core issue leading to Q degradation over time. The study carefully outlines the fabrication process, leveraging a self-assembled monolayer approach to graft PFO molecules onto the silica surface. This process is elegantly demonstrated to retain the ultrahigh Q values of the devices, essentially unchanged over extended periods, even when stored in air at room temperature with varying humidity levels.

The researchers initially prepared silica microcavities using standard processes, likely involving laser machining or wet etching to achieve optimized geometries with high intrinsic Q values. The surfaces of these microcavities were then treated with a chemical vapor of PFO precursors, leading to the formation of a self-assembled monolayer of PFO molecules that covalently bonded to the silica surface. This process aimed to create a dense, uniform hydrophobic layer on the microcavities. The authors confirmed the successful grafting of the hydrophobic PFO layer on the silica microcavities and characterized the surface properties related to hydrophobicity. They used X-ray photoelectron spectroscopy to analyze the surface chemical composition, confirming the presence of PFO molecules and employed atomic force microscopy to assess the surface morphology and the thickness of the hydrophobic layer, ensuring uniform coverage and the expected monolayer thickness. Moreover, they evaluated quantitatively the hydrophobic nature of the treated silica surfaces, which is critical for inhibiting water molecule absorption. Surface contact angles of water droplets on treated vs. untreated silica surfaces were measured, providing a direct assessment of the surface hydrophobicity. To evaluate the long-term Q stability of both treated and untreated silica microcavities under varying environmental conditions. The researchers measured the Q values of microcavities immediately after fabrication using a tapered optical fiber coupling method and continuous wave laser and then over extended periods (up to nine months) in air with varying humidity levels. The ability of the microcavities to generate optical frequency combs with ultralow threshold performance was assessed over time, indicating the practical implications of sustained Q values on device performance. The authors confirmed the effective grafting of a hydrophobic PFO layer on silica microcavities, evidenced by XPS and AFM results. The treated microcavities demonstrated significantly higher water contact angles compared to untreated ones, indicating successful enhancement of surface hydrophobicity. Moreover, the treated microcavities showed remarkable stability in their Q values over time, even under varying humidity conditions, contrary to the untreated counterparts that experienced significant Q degradation. The ability of the hydrophobically treated microcavities to generate optical frequency combs with ultralow threshold performance was not only maintained but remained stable over long periods, showcasing the efficacy of the treatment in preserving the nonlinear optical performance of the devices.

In conclusion, the research led by Professor Xiaoqin Shen and colleagues represents a significant advancement in addressing a long-standing challenge in the utilization of silica-based microcavities for nonlinear photonic applications, particularly in the context of maintaining ultrahigh Q values during storage or use in atmospheric conditions. The introduction of hydrophobic PFO-coated silica microcavities with stable ultrahigh Q values over time enhances the practical utility of these devices and also paves the way for the development of advanced photonic systems.

Reference

Xie J, Wang Y, Kang H, Cheng J, Shen X. Hydrophobic Silica Microcavities with Sustainable Nonlinear Photonic Performance. ACS Appl Mater Interfaces. 2023;15(34):41067-41072. doi: 10.1021/acsami.3c06300.

Go to ACS Appl Mater Interfaces.

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