  A high-sensitivity thermal sensing is demonstrated by coating a layer of
polydimethylsiloxane (PDMS) on the surface of a silica toroidal microresonator
on a silicon wafer. Possessing high-Q whispering gallery modes (WGMs), the
PDMS-coated microresonator is highly sensitive to the temperature change of the
surroundings. We find that, when the PDMS layer becomes thicker, the 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 thermal sensor is also analyzed to
reach 10-4 K.
