Alexandre Fran ç ois *, S. Afshar , K.J. Rowland, M. Henderson , Tanya Monro

1. Alexandre François*, S. Afshar, K.J. Rowland, M. Henderson, Tanya Monro The Institute for Photonics and Advanced Sensing (IPAS) School of Chemistry and Physics, The University of Adelaide,, Australia *e-mail: alexandre.francois@adelaide.edu.au Motivation Experimental setup Our aim is to create optical biosensing technologies which can be use for in-vivo applications. For this specific application, we envisioned combining a dye doped Whispering Gallery Modes (WGM) resonator with a suspended core microstructured optical fiber (MOF). Positioning the resonator (polystyrene microsphere) onto a hole of a MOF enables the utilisation of such sensor remotely, providing the adequate pump wavelength and collecting the WGM modulated fluorescence signal through the fiber. Here, we show that this simple architecture offers other advantages depending on how the resonator sits onto the MOF hole. We demonstrate that it is possible to enhance the radiation efficiency of the WGM by a factor 10 by breaking the symmetry of the resonator when its diameter matches the holes diameter of the MOF. Enhancement of the radiation mode as function of the diameter mismatch between the resonator and the fiber hole Larger resonator diameter compared to the fiber hole: Øsphere=20mm Øhole=15mm Complete mismatch between the fiber hole and resonator diameter: Øsphere=25mm Øhole=15mm Almost perfect match between the resonator and the fiber hole: Øsphere=16mm Øhole=15mm Enhancing the radiation efficiency of dye doped microresonators using coupling effects with a suspended core microstructured optical fiber In each case, the microresonator’s WGM spectra has been recorded in water, before (free floating) and after the attachment of the microsphere to the MOF’s tip (attached). The excitation and collection efficiency of the optical setup doesn’t vary between the measurements of each sphere as indicated by the good match of the fluoresscence background level. • Enhancement of specific modes depending on their polarization are observed when the dye doped resonator is sitting into a hole of a suspended core MOF. • The enhancement is highly depending on the diameter mismatch between the resonator and the MOF hole. Influence of the enhancement of the radiation on the lasing behaviour • The radiative enhancement of the WGM observed as function of the diameter mismatch between the resonator and the MOF hole was characterized in both below and above the lasing threshold of the WGM. • A factor 10 increase of the WGM intensity below the lasing threshold was fund for an almost perfect diameter match. • Above the lasing threshold, the lasing efficiency of resonator sitting onto a MOF hole with the same diameter is also increase by a factor 10. Conclusion We experimentally demonstrated that positioning a dye doped microsphere into the hole of a suspended core MOF has a significant effect on the emission efficiency of the WGMs. We investigated the dependence of this enhancement as a function of the diameter mismatch between the MOFs hole and the sphere and found out that the maximum enhancement is reach when half the sphere protrudes into the MOF hole, which is equivalent to introducing a higher refractive index ring on one equatorial plane of the sphere. We believe iitmay be possible to induce the same effect to smaller microsphere by tailoring the diameter of the MOF hole, resulting in highly sensitive WGMs sensor with increased resolution. More generally, this work introduces the concept of using external microstructures as a means of controlling and enhancing the performance of resonators.