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

Area of Science:

• Optoelectronics
• Chemical Sensing
• Materials Science

Background:

• Whispering gallery mode (WGM) microtoroid resonators offer high sensitivity for single-molecule detection.
• Traditional fiber taper coupling limits WGM resonator applications to controlled lab settings due to fragility.

Purpose of the Study:

• To develop a robust and stable free-space optical coupling scheme for WGM microtoroid resonators.
• To enable ultra-sensitive gas-phase chemical sensing outside of laboratory environments.
• To demonstrate selective detection of volatile thioethers using novel polymer functionalization.

Main Methods:

• Integration of frequency-locked WGM microtoroids with a free-space optical coupling.
• Synthesis of polymer materials using reversible addition-fragmentation chain-transfer (RAFT) polymerization for sensor functionalization.
• Utilizing 2-chloroethyl ethyl sulfide (2-CEES) as a model target for volatile thioether detection.

Main Results:

• Achieved a room-temperature detection limit of 25 parts per trillion for 2-CEES, a significant improvement over previous methods.
• Demonstrated preservation of ultra-high Q factors (>10^7) after polymer coating, ensuring sensor performance.
• Maintained mechanical resilience and stability under ambient conditions in a compact, coin-sized chamber.

Conclusions:

• Established a scalable platform for ultra-sensitive and rugged gas-phase chemical sensors using free-space-coupled WGM microtoroids.
• The developed sensor technology has broad applications in defense, security, and environmental monitoring.
• Continuous frequency locking allows for unattended, long-duration monitoring capabilities.