Abstract
Whispering gallery mode (WGM) resonators have become increasingly diverse in terms of both architecture and applications, especially as refractometric sensors, allowing for unprecedented levels of sensitivity. However, like every refractometric sensor, a single WGM resonator cannot distinguish temperature variations from changes in the refractive index of the surrounding environment. Here, we investigate how breaking the symmetry of an otherwise perfect fluorescent microsphere, by covering half of the resonator with a high-refractive-index (RI) glue, might enable discrimination of changes in temperature from variations in the surrounding refractive index. This novel approach takes advantage of the difference of optical pathway experienced by WGMs circulating in different equatorial planes of a single microsphere resonator, which induces mode-splitting. We investigated the influence of the surrounding RI of the microsphere on mode-splitting through an evaluation of the sphere’s WGM spectrum and quality factor (Q-factor). Our results reveal that the magnitude of the mode-splitting increases as the refractive index contrast between the high-refractive-index (RI) glue and the surrounding environment increases, and that when they are equal no mode-splitting can be seen. Investigating the refractive index sensitivity of the individual sub modes resulting from the mode-splitting unveils a new methodology for RI sensing, and enables discrimination between surrounding refractive index changes and temperature changes, although it comes at the cost of an overall reduced refractive index sensitivity.
Highlights
Whispering gallery mode (WGM) resonators are small, axially symmetric dielectric devices, ranging from a few microns to hundreds of microns in diameter, with the unique ability to trap light by total internal reflection
We have investigated a novel WGM resonator platform, where part of the resonator is embedded into a high-refractive-index medium while the rest is left exposed to the surrounding environment
The intriguing behavior of the resulting WGM spectra reveals that the RI mismatch between the high RI medium and the surrounding environment can be evaluated from the effective
Summary
Whispering gallery mode (WGM) resonators are small, axially symmetric dielectric devices, ranging from a few microns to hundreds of microns in diameter, with the unique ability to trap light by total internal reflection. The optical wave circulating along the resonator’s inner surface generates optical resonances, whose spectral positions are strongly dependent on the refractive index contrast between the resonator and its surrounding environment as well as the resonator’s shape [1] This distinctive feature of WGMs, combined with their extremely high quality factor (Q-factor), defined as the ratio between the resonance wavelength and its full width at half maximum (FWHM), in addition to the small mode volume possible, have enabled WGM resonators to become prime candidates for highly sensitive, label-free bio-chemical sensors [1,2,3,4,5,6,7]. While single molecule detection is a tremendous achievement, with applications in fundamental research, it significantly restricts the dynamic range of the concentration where the sensor can be operated, thereby limiting its interest for the vast majority of medical diagnostic applications where detection of specific biomarkers, such as proteins, in the larger pg/mL to ng/mL range is required
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