SENSORS: Optical Whispering-Gallery Modes for Diverse Sensing Applications

Oklahoma State University, Stillwater OK

Investigators

Abstract

This Sensor proposal focuses on transparent fused-silica microspheres, which support whispering-gallery modes (WGMs)- light trapped by total internal reflection and orbiting just beneath 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 (high Q). The mode amplitude and resonant frequency can be very sensitive to changes in the microresonator'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 of WGM evanescent-wave sensing of trace gases in the atmosphere and chemicals in solution; extend earlier applications of WGM microsensors, in measuring thermal and absorptive properties, to the characterization of transparent thin films; pursue novel techniques that enable the multiplication of sensitivity enhancements, by combining effects such as modal interference with WGM microsensing; and apply to WGM microresonators methods of surface preparation that have proved useful in fiber sensors for gases, chemicals, and biological agents. In accomplishing these objectives, all aspects of the project will benefit from: improving control and coupling techniques, utilizing various redundant measurements to characterize WGMs, studying the effects of changing system configurations, and expanding the numerical modeling of the microsensors. Specific parts of the project will involve: implementing wavelength-modulation spectroscopy with the WGM locked to resonance with a frequency-scanning laser, and ruggedizing the sensor by attaching the coupler(s); measuring the thermal accommodation coefficient and optical absorption coefficient of a thin film coating the microresonator; using cavity ringdown of the scattered light, and applying controlled multimode-taper coupling to the WGMs of the microresonator, to amplify its absorption-detection sensitivity; and combining surface-enhanced Raman scattering with WGMs by coating the microresonator with a thin film containing gold nanoparticles, as well as using the techniques of surface coating with xerogels or antibodies for biosensing. Intellectual Merit: Even a supposedly well-understood system can exhibit novel behavior under extreme conditions, or when interacting with another system. The exquisitely high Q of a WGM provides extreme conditions here, and interactions between WGMs and surface or coupling effects give a multiplication of sensitivity-enhancement factors. Broader Impacts: The graduate students involved will be learning fundamental physics as well as becoming grounded in real-world applications; they will also be directly involved in dissemination of results through conference presentations and peer-reviewed journal publications. This work will benefit the University's multidisciplinary Photonics degree programs and its interdepartmental Center for Sensors and Sensor Technologies. The sensing techniques to be developed here have many potential benefits to society, as has already been recognized by the Oklahoma Center for the Advancement of Science and Technology, NASA's Marshall Space Flight Center, and Nomadics, Inc., through their support of and collaboration on related work.