High-Q WGM Resonators Encapsulated in PDMS for Highly Sensitive Displacement Detection

Optical whispering gallery mode (WGM) microresonators are a class of optical devices with a superior capability to confine light within a small volume. These devices offer the advantages of high Q-factor, small mode volume, and strong light-matter interaction enabling high sensitivity. Fiber tapers have been used as an efficient tool to couple light in and out of WGM resonators, but their instability has limited their practical applications. Conventional packaging methods for WGM resonators rely on UV curable polymers with low refractive indices to improve robustness; however, they are costly and potentially toxic, and the interference from ambient moisture poses a critical issue. In this study, we demonstrate the packaging of WGM devices using non-toxic polydimethylsiloxane (PDMS). The Q-factor of 10⁷ is achieved at the 780 nm band. The PDMS packaging technique not only endows the WGM device with better robustness and compactness but also enhances the humidity resistance significantly. By making use of the unique flexibility of PDMS, we demonstrate displacement detection with a high sensitivity of ∼0.1 pm/µm and a detection limit as low as 600 nm.