A high-sensitivity temperature sensor is demonstrated by coating a layer of polydimethylsiloxane (PDMS) on the surface of a silica toroidal microresonator on a silicon chip. Combining both the advantages of the WGM microcavity (with ultrahigh Q factor) and PDMS (with large thermal effect), the PDMS-coated microresonator is highly sensitive to the temperature change of the surroundings. We find that, when the PDMS layer becomes thicker, the fundamental WGM experiences a transition from red- to blue-shift with temperature increasing due to the negative thermal-optic coefficient of PDMS. The measured sensitivity (0.151 nm/K) is one order of magnitude higher than pure silica microcavity sensors. The ultra-high resolution of the temperature sensor is also analyzed to reach 10-4 K. With both high sensitivity and resolution, the thermal sensor can be employed to monitor a slight temperature variation which cannot be realized with conventional temperature sensor. Its on-chip feature can also fulfill the demand for integration and miniaturization in optics.
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