High-quality factor microsphere as sensors

Whispering gallery mode (WGM) microspheres have gained prominence as sensors in various applications due to their ability to significantly enhance light-matter interaction. In fact, the sensitivity of these sensors is directly correlated with their quality factor (Q), and melted glass microspheres typically exhibit a Q-factor of approximately $10^{5}$ when used as sensors (functionalized and/or in the measurement environment). Usually, the predominant sensing mechanism involves monitoring mode shifts, where the resonant wavelength of the WGM is employed to measure the desired signal. However, at higher Q-factors, characterized by narrower bandwidths, sophisticated equipment is required for accurate measurements. In this study, we utilize an active microsphere as a sensor with an exceptional Q-factor exceeding $10^{8}$ in the measurement environment, thanks to its gain. We opt for an alternative sensing mechanism that leverages the microsphere’s gain to measure the signal of interest. Lastly, we compare this innovative sensing approach with other techniques commonly employed in optical sensors based on WGM micro resonators.