Phase Transition Monitoring of Liquid Metal Based on Temperature Sensors of Packaged Microbubble Resonators

The packaged microbubble resonators (PMBRs) provide a promising platform for investigating the interaction between fluid and optical microcavity. In this work, PMBRs have been applied to study the resonance wavelength shift with temperature in a hollow PMBR and a water-filled PMBR. The mode sensitivity is 0.0213 and −0.0573 nm/°C. After filling the PMBR with liquid metal (LM), the phase transition process is monitored by using the mode sensitivity change. Below the melting point (MP) (around 15.2 °C), the optical mode sensitivity is −0.0083 nm/°C, while above the MP, the sensitivity increases to −0.0841 nm/°C. The PMBR provides a new method to study the phase transition process of phase change materials. Moreover, LM greatly improves the thermal sensitivity of the optical mode of the PMBR. The thermal optical coefficient of LM is around $- 3.4\times 10^{-{2}}$ RIU/°C (liquid phase) according to the theoretical model estimation. The coefficient of second and third orders of magnitude is higher than the traditional dielectric material, which will open a new chapter in the application research of LM in thermo-optical response.