Abstract
Asymmetric microsphere resonant cavities (ARCs) allow for free-space coupling to high quality (Q) whispering gallery modes (WGMs) while exhibiting highly directional light emission, enabling WGM resonance measurements in the far-field. These remarkable characteristics make “stand-off” biodetection in which no coupling device is required in near-field contact with the resonator possible. Here we show asymmetric microsphere resonators fabricated from optical fibers which support dynamical tunneling to excite high-Q WGMs, and demonstrate free-space coupling to modes in an aqueous environment. We characterize the directional emission by fluorescence imaging, demonstrate coupled mode effects due to free space coupling by dynamical tunneling, and detect adsorption kinetics of a protein in aqueous solution. Based on our approach, new, more robust WGM biodetection schemes involving microfluidics and in-vivo measurements can be designed.
Highlights
Optical microresonators have been demonstrated as powerful biodetection tools, recently even reaching single nanoparticle and single virus detection as well as monitoring specific interaction kineticsSensors 2015, 15 of single DNA oligonucleotides [1,2,3,4,5,6]
While free space coupling can be achieved through the incorporation of engraved gratings, or though the placement of nano-scatterers along the resonator boundary, here we focus on the free space coupling mechanisms inherent in Asymmetric microsphere resonant cavities (ARCs) due to their ease of fabrication and simplified integration into any possible analyte solution for the purposes of biosensing [27,28]
Consistent with the trends for coupling efficiency predicted by our Poincare Surface of Section (PSOS) plots, we only observe excitation of higher Q resonances (WGMs) with SF-6 ARCs in water, as we will show in the following, and report no observation of microcavity resonances of significant Q factors for free space coupling to SMF-28 ARCs after their immersion in an aqueous environment
Summary
Optical microresonators have been demonstrated as powerful biodetection tools, recently even reaching single nanoparticle and single virus detection as well as monitoring specific interaction kinetics. Prism coupling has mitigated many of these challenges [3], it is necessary to explore entirely new coupling methods that do not rely on evanescent couplers, to optimize the design and effectiveness of future biodetection platforms [17,18] One such method, free space coupling, relies on mode matching of a focused Gaussian beam for efficient excitation of high-Q WGMs in asymmetric cavities through a process called “chaos assisted dynamical tunneling” [19,20,21,22]. Directional emission from ARCs makes “stand-off” biodetection a possibility, where the necessary optics can be placed far away from the microresonator and measurements can be made in the far field This scheme allows for a greater simplified incorporation into micro-fluidics and may enable novel in-vivo measurements. Placing a photodetector in the far field emission pattern, we demonstrate stand-off WGM biodetection by monitoring adsorption kinetics of bovine serum albumin (BSA) onto a microsphere ARC resonator
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