Hydrogel-Based Hybrid Microcavity for a Plasmonic-Enhanced Laser Sensor.

Hybrid microcavity systems combining optical resonators with responsive materials offer a promising route toward tunable, multifunctional photonic devices. Here, we demonstrate a hydrogel-based microfiber laser enhanced by plasmonic nanoparticles that enables single-mode operation with high sensitivity. The hydrogel network acts as a disordered scattering medium, inducing random lasing, while the microfiber geometry supports whispering-gallery modes with strong optical feedback. By tuning the microfiber diameter, we systematically investigate the interplay between microcavity modes and scattering. Incorporation of Au nanoparticles further enhances the optical confinement through localized surface plasmon resonances, providing single-mode lasing control. The resulting device exhibits strong humidity responsiveness and operational stability, with a sensitivity of 103 pm/% RH and a rapid response time of 3.2 s. The results reported here provide a versatile approach for integrating smart materials and microcavities, advancing the development of ultrasensitive hydrogel-based photonic sensors.