Optical sensing by polystyrene microspheres

Trapped light inside the microsphere can make resonance, if the original and returned light happens to be in phase after a few round trips. If the Refractive Index (RI) of the surrounding medium increases, the resonance peaks, known as Whispering Gallery Modes (WGMs), are red-shifted. In this work, the minimum detection limit (DL) of polystyrene microspheres, 10 μm & 25 μm in diameter, is measured while gain medium is embedded Quantum Dots (QDs) on the microspheres surface. For the smaller microsphere, the sensitivity of TE and TM modes are measured individually. Comparing results with available literatures shows a slight improvement of the microspheres´ DL when coating with QDs rather than doping with dyes. Furthermore, the smaller microsphere shows to generate higher sensitivity and lower DL.