Free‐Space‐Coupled Frequency‐Locked Microtoroid Resonators With Reactive Polymer Functionalization for Part‐Per‐Trillion Gas Detection

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 ultrasensitive 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 ultrahigh Q factors (>10 7 ) 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 ultrasensitive, rugged detectors with broad applications in defense, security, and environmental monitoring.