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Liquid Droplet Microresonators

1
Consiglio Nazionale delle Ricerche, Istituto Nazionale di Ottica (INO), via Campi Flegrei 34—Comprensorio A. Olivetti, 80078 Pozzuoli (Na), Italy
2
Consiglio Nazionale delle Ricerche, Istituto Nazionale di Ottica (INO), Largo E. Fermi 6—50125 Firenze, Italy
*
Author to whom correspondence should be addressed.
Sensors 2019, 19(3), 473; https://doi.org/10.3390/s19030473
Received: 21 December 2018 / Revised: 17 January 2019 / Accepted: 18 January 2019 / Published: 24 January 2019
(This article belongs to the Special Issue Resonator Sensors 2018)
We provide here an overview of passive optical micro-cavities made of droplets in the liquid phase. We focus on resonators that are naturally created and suspended under gravity thanks to interfacial forces, illustrating simple ways to excite whispering-gallery modes in various slow-evaporation liquids using free-space optics. Similar to solid resonators, frequency locking of near-infrared and visible lasers to resonant modes is performed exploiting either phase-sensitive detection of the leakage cavity field or multiple interference between whispering-gallery modes in the scattered light. As opposed to conventional micro-cavity sensors, each droplet acts simultaneously as the sensor and the sample, whereby the internal light can detect dissolved compounds and particles. Optical quality factors up to 107–108 are observed in liquid-polymer droplets through photon lifetime measurements. First attempts in using single water droplets are also reported. These achievements point out their huge potential for direct spectroscopy and bio-chemical sensing in liquid environments. Finally, the first experiments of cavity optomechanics with surface acoustic waves in nanolitre droplets are presented. The possibility to perform studies of viscous-elastic properties points to a new paradigm: a droplet device as an opto-fluid-mechanics laboratory on table-top scale under controlled environmental conditions. View Full-Text
Keywords: whispering gallery modes; droplet micro-cavity; free-space laser excitation; optical Q-factor; cavity ring-down spectroscopy; cavity optomechanics whispering gallery modes; droplet micro-cavity; free-space laser excitation; optical Q-factor; cavity ring-down spectroscopy; cavity optomechanics
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MDPI and ACS Style

Giorgini, A.; Avino, S.; Malara, P.; De Natale, P.; Gagliardi, G. Liquid Droplet Microresonators. Sensors 2019, 19, 473.

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