As the demand for continuous, in-situ surveillance and monitoring of systems and environments rapidly increases, users seek miniaturized sensory devices with fast response times. Optical dielectric resonators supporting whispering gallery modes are a class of cavity-based devices with exceptional properties such as very high-power density, very narrow spectral linewidth, and extremely small mode volume. The footprint of these sensors is extremely small compared to that of current state-of-the-art sensor technology, and the advantages of taking measurements of microvolume samples are significant: Microvolume samples enable the performance of large numbers of parallel analyses using microarrays with a dramatic reduction in analysis time. In this feasibility study, we investigated the use of microsphere resonators for microscale optical gas sensors, the operation of which relies on the properties of highly confined whispering gallery modes. Infrared light is coupled in and out of the microspheres from either tapered optical fibers or microprisms placed in proximity of the microspheres via evanescent coupling. Cavity-enhanced absorption spectroscopy for gas detection occurs when the microsphere is immersed in the targeted gasses, providing high sensitivity and specificity in a multi-gas mixture. This approach may enable a means of developing embedded sensors that can be directly integrated into spatially confined systems that handle hazardous gasses that are ordinarily difficult to access.
Due to minimal reflection losses and potentially very low material (glass) absorption, optical dielectric resonators can reach exceptionally high quality factors (Armani and Vahala 2006, Heebner et al. 2007). Therefore, cavity-enhanced absorption spectroscopy (CEAS) can be performed when the sensor’s microsphere is immersed in an environment with the targeted analytes. The analytes will induce losses that weaken the coupling efficiency and deteriorate the resonance amplitude. Additionally, when the background is altered by the presence of host species, spectral shift and broadening in the cavity resonances are induced. These effects are also exploited as very effective detection mechanisms since the resonances are very narrow (down to 100KHz). Furthermore, by adopting surface adsorption (i.e., coating the resonator surface with films targeting the type of gas of interest) the sensor’s detection abilities not only become more specific but also become stronger due to the forced proximity of gasses to the sensor’s electrical field.
Whispering gallery modes (WGMs) are a well known phenomenon. Lord Rayleigh discovered that the refocusing effect of a curved surface such as that of the inside of a cathedral’s dome on the sound waves traveling along the cathedral’s gallery would also apply to electromagnetic waves (1910). Total internal reflection guides the light beam along the curved surface in wavelengths that satisfy the phase-matching condition (see figure). Very limited examples of WGM resonators (WGMRs) for trace-gas detection exist currently. Our goal during this study was to advance this area by combining WGMRs with tunable laser-diode spectroscopy, an approach we currently employ for gas detection in headspace surveillance and older gas-spectroscopy studies (Bora et al. 2012). We were able to demonstrate that WMGRs naturally enable CEAS and provide a sensing platform that is significantly scaled-down in size, nonintrusive, sensitive, and fast. The method is based on recording changes in the resonant dip depth (peak height) observed in the reflection (transmission) throughput caused by the gas absorption when the resonator is locked at the wavelength matching the absorption line.
During this feasibility study, we investigated and developed microsphere resonators. Specifically, our objectives were to demonstrate that we could (1) build WGMRs housed in various types of glasses (e.g., SiO2, ZrF4, As2S3, and As2Se3) for near-infrared (NIR) to short-wavelength infrared (SWIR) absorption; (2) design WGMRs using analytical models to guide fabrication and create one-dimensional models to study the coupling of microspheres to either prisms or tapered fibers, thus providing guidance in the characterization; (3) arrange optical systems for WGMRs’ characterization setups and CEAS; (4) conduct preliminary experiments using CEAS; and (5) fabricate tapered fibers since they are important for investigating best coupling practices and are not commercially available. Our research into fabricating tapered fibers led to a new in-house capability at Lawrence Livermore National Laboratory for chemically and mechanically tapering fibers with diameters of less than 15 μm.
Our research into the development of a miniature, deployable platform for real-time gas-phase detection may significantly improve the performance of current spectroscopic sensors developed for nuclear stockpile stewardship tasks by decreasing volume and power requirements while maintaining or even potentially improving their current sensitivity. Additionally, this study enhances the Laboratory’s core competencies in nuclear, chemical, and isotopic science and technology. WGM-CEAS microsensors can also play a key role in monitoring air quality in evapotranspiration cycles. Other fields, from bioremediation to food safety and system safeguards, would benefit from the use of this sensor technology.
During this study, we laid the foundations for future development of CEAS using WGMRs, demonstrating the feasibility of the approach and providing the infrastructures to proceed toward testing the resonators in the environments of interest. We developed models to guide the design and fabrication of the resonators, as well as sustain the coupling and detection setups; we developed the knowledge base and multiple in-house methodologies to produce both microspheres and tapered fibers that will benefit other projects in the future; and we screened various coupling mechanisms leading to various available configurations for different microresonators. Finally, we now have a better understanding of the equipment requirements and of the environmental factors to be taken into account for higher confidence in the real-time gas measurements.
Armani, A. M. and K. J. Vahala. 2006. "Heavy Water Detection Using Ultra-High-Q Microcavities." Optics Letters 31(12):1895–1898.
Bora, M., et al. 2012. "Multiplexed Gas Spectroscopy Using Tunable VCSELs." Proceedings of SPIE Vol. 8366 836607-1. doi: 10.1117/12.920803.
Heebner J. E., et al. 2007 Optical Microresonators (Springer).
Rayleigh, Lord. 1910. "The Problem of the Whispering Gallery." The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 20(120): 1001–1004. doi: 10.1080/14786441008636993.
Bond, T. 2018. "MicroResonators for Compact Optical Sensors (mRCOS)." 32nd Compatibility and Aging Study Workshop, Livermore, October 2018. LLNL-PRES-759729.