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22 April 2016 Whispering-gallery mode resonator sensors based on liquid droplets
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We present a simple and effective set-up to exploit the enhancement of passive optical cavities that are directly made from a liquid droplet. The optical resonances, corresponding to the so-called whispering-gallery modes (WGMs), are excited by a focused free-space beam edge-coupling scheme. Very narrow resonances are observed, both in the visible and near-infrared spectral regions, with quality (Q) factors ranging from 105 to 107 and beyond. Different methods for interrogation via frequency locking of a laser source to the WGM are shown. Locking of a diode laser to the equatorial modes of a liquid droplet resonator is demonstrated at 1560 nm and 663 nm. This approach makes high-performance optical sensing directly feasible in liquid samples with a number of advantages in view of their application for detection and quantification of bio-molecules.
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R. Zullo, A. Giorgini, S. Avino, P. Malara, P. De Natale, and G. Gagliardi "Whispering-gallery mode resonator sensors based on liquid droplets", Proc. SPIE 9727, Laser Resonators, Microresonators, and Beam Control XVIII, 972713 (22 April 2016);

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