Tuning a microsphere whispering-gallery-mode sensor for extreme thermal stability

Abstract

The reactive sensing application of optical microspheres can be plagued by local temperature fluctuations. Fluctuations due to laser heating or ambient changes in the lab environment cause resonance shifts that appear as noise or an underlying drift in the sensor data. Here, we show that thermal fluctuations can be exactly compensated in virtually any local medium (i.e., “analyte”) by the application of a high-index coating on the surface of the microsphere. The coating precisely controls the extent of the field penetration into the surroundings in such a way that the thermal shifts associated with the three layers (the glass sphere, the coating, and the exterior medium) can be exactly balanced. The conditions required for thermal stability were investigated theoretically; on the basis of these calculations a real sphere was then synthesized that showed excellent stability for aqueous solutions.