SENSORS: Optical Whispering-Gallery Modes for Diverse Sensing Applications

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This Sensor proposal focuses on transparent fused-silica microspheres, which supportwhispering-gallery modes (WGMs)- light trapped by total internal reflection and orbiting justbeneath the surface. These modes, excited by optical tunneling from an adjacent tapered fiber,have an evanescent component outside the surface and a very sharp frequency resonance (highQ). The mode amplitude and resonant frequency can be very sensitive to changes in themicroresonator's environment, and thus enable sensing of, for example: strain, acceleration,thermal effects, trace gases, chemicals, and modifications of the surface.This project will: further develop and advance previously-demonstrated methods ofWGM evanescent-wave sensing of trace gases in the atmosphere and chemicals in solution;extend earlier applications of WGM microsensors, in measuring thermal and absorptiveproperties, to the characterization of transparent thin films; pursue novel techniques that enablethe multiplication of sensitivity enhancements, by combining effects such as modal interferencewith WGM microsensing; and apply to WGM microresonators methods of surface preparationthat have proved useful in fiber sensors for gases, chemicals, and biological agents. Inaccomplishing these objectives, all aspects of the project will benefit from: improving controland coupling techniques, utilizing various redundant measurements to characterize WGMs,studying the effects of changing system configurations, and expanding the numerical modelingof the microsensors. Specific parts of the project will involve: implementing wavelength-modulationspectroscopy with the WGM locked to resonance with a frequency-scanning laser,and ruggedizing the sensor by attaching the coupler(s); measuring the thermal accommodationcoefficient and optical absorption coefficient of a thin film coating the microresonator; usingcavity ringdown of the scattered light, and applying controlled multimode-taper coupling to theWGMs of the microresonator, to amplify its absorption-detection sensitivity; and combiningsurface-enhanced Raman scattering with WGMs by coating the microresonator with a thin filmcontaining gold nanoparticles, as well as using the techniques of surface coating with xerogels orantibodies for biosensing.Intellectual Merit: Even a supposedly well-understood system can exhibit novelbehavior under extreme conditions, or when interacting with another system. The exquisitelyhigh Q of a WGM provides extreme conditions here, and interactions between WGMs andsurface or coupling effects give a multiplication of sensitivity-enhancement factors.Broader Impacts: The graduate students involved will be learning fundamental physicsas well as becoming grounded in real-world applications; they will also be directly involved indissemination of results through conference presentations and peer-reviewed journalpublications. This work will benefit the University's multidisciplinary Photonics degreeprograms and its interdepartmental Center for Sensors and Sensor Technologies. The sensingtechniques to be developed here have many potential benefits to society, as has already beenrecognized by the Oklahoma Center for the Advancement of Science and Technology, NASA'sMarshall Space Flight Center, and Nomadics, Inc., through their support of and collaboration onrelated work.