Using the lasing threshold in whispering gallery mode resonators for refractive index sensing

Lasing whispering gallery mode resonators, such as dye doped microspheres and microcapillaries, have shown tremendous potential for refractive index sensing and biosensing applications owning to the narrower resonances achieved upon lasing. This has enabled higher resolution on the determination of the resonance wavelength shift induced by a variation of the surrounding refractive index and as a consequence to reach lower detection limit compared with their fluorescent counterparts. The sensing procedure in both cases relies on tracking the wavelength shift of individual modes, therefore requiring high resolution spectral analysis. This stringent requirement not only prevents any viable commercial prospects due to high equipment cost but more importantly imposes a technological limit, related to the equipment spectral resolution, on the achievable detection limit. In this paper, we show for the first time that the lasing threshold and eventually the resonances intensity can be used for inferring changes of refractive index around a 15 μm dye doped polystyrene instead of the mode tracking procedure. The sensing mechanism relies on the spoiling of the resonator Q factor upon change of refractive index which eventually increases the lasing threshold. In addition to allow free space excitation and collection, alleviating the need for phase matched prism or fiber taper, this novel approach promises to reach lower detection limit by suppressing the need of high resolution spectral analysis of the whispering gallery mode spectra but instead relying on cost effective and highly sensitive intensity measurements.