A Versatile Approach to Stabilize Whispering Gallery Microresonators Toward Reliable Photonic Labeling.

Whispering gallery mode (WGM) microresonators provide ultrasensitive optical fingerprints that are ideal for photonic labels capable of detecting minute variations in size and refractive index. WGM microresonators can be obtained through convenient self-assembly protocols, for instance, by loading polystyrene microparticles with colloidal quantum dots. However, despite their ultrasharp spectral signatures, these resonators exhibit significant thermal and temporal drifts that limit their applications. We combine experiments and modeling to show that these drifts originate from the slow, days-long release of residual solvent trapped within the microparticles, which can be monitored via WGM spectroscopy. We establish a simple protocol to suppress this instability, locking the resonances into stable, sharply defined modes. This approach provides a straightforward route for long-term stabilization of WGM microresonators, advancing their reliable use as robust optical platforms, noninvasive sensors, and precise self-trackers of physicochemical changes at the microscale.