Free-Space-Coupled Frequency-Locked Microtoroid Resonators with Reactive Polymer Functionalization for...

Whispering gallery mode (WGM) microtoroid resonators combined with frequency locking offer single-molecule detection sensitivity, but their dependence on fragile fiber tapers has limited their use to controlled laboratory environments. Here, we overcome this barrier by integrating frequency-locked WGM microtoroids with a robust free-space optical coupling scheme, enabling ultra-sensitive and stable gas-phase chemical sensing. Using polymer materials synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization, we demonstrate selective and durable interaction with volatile thioethers, with 2-chloroethyl ethyl sulfide (2-CEES, a common mustard gas simulant) serving as a model target. Importantly, the polymer functionalization preserves ultra-high Q factors (>10⁷) after coating, ensuring performance is not compromised. Housed in a compact, coin-sized chamber, the sensor achieves a room-temperature detection limit of 25 parts per trillion-three orders of magnitude more sensitive than prior reports-while maintaining mechanical resilience under ambient conditions. Continuous frequency locking enables unattended, long-duration monitoring. This first demonstration of free-space-coupled WGM microtoroids for chemical sensing with novel polymer materials establishes a scalable platform for ultra-sensitive, rugged detectors with broad applications in defense, security, and environmental monitoring.