Abstract
Whispering gallery mode (WGM) microresonators, thanks to their unique properties, have allowed researchers to achieve important results in both fundamental research and engineering applications. Among the various geometries, microspheres are the simplest 3D WGM resonators; the total optical loss in such resonators can be extremely low, and the resulting extraordinarily high Q values of 108–109 lead to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. They can also be coated in order to better control their properties or to increase their functionality. Their very high sensitivity to changes in the surrounding medium has been exploited for several sensing applications: protein adsorption, trace gas detection, impurity detection in liquids, structural health monitoring of composite materials, detection of electric fields, pressure sensing, and so on. In the present paper, after a general introduction to WGM resonators, attention is focused on spherical microresonators, either in bulk or in bubble format, to their fabrication, characterization and functionalization. The state of the art in the area of biosensing is presented, and the perspectives of further developments are discussed.
Highlights
From the 1960s on, the term optical cavity was immediately been associated with a laser device.In the last few decades, the quest for miniaturization and integration has led to the development of optical microcavities, namely structures that enable the confinement of light in microscale volumes, which are becoming more and more important in a number of applications other than “simple” lasers
The application area where WGMRs have better proven their unique properties is that of sensing; in very high Q resonators, the trapped photons are able to circulate on their orbits several thousand times before exiting the WGMR by the loss mechanism, and this very long interaction path makes possible the detection of very small external perturbations, such as those induced by the presence of a nanoparticle [129]
A theoretical analysis of the sensing capability of microbottle and microbubble WGMRs under different conditions is presented in [149]; it has been estimated that about a 10-fold sensitivity enhancement over a microsphere biosensor could be achieved and that nanoparticles less than 20 nm in radius could be detected. Another analysis of the optical properties of microbubble WGMRs, using numerical simulation results based on FEM, has evidenced that, when the wall thickness diminishes to a certain scale, the whispering gallery mode (WGM) are dominated by the presence of the liquid core, and the microbubble modes operate in the so-called quasi-droplet regime; this provides an ultra-sensitive way to detect liquid optical properties [150]
Summary
From the 1960s on, the term optical cavity was immediately been associated with a laser device. In the process of reducing the size from the classical Fabry–Perot resonator, other geometries and principles have emerged, resulting in novel classes of microcavities, the most important ones being the whispering gallery mode (WGM) resonators and the photonic crystals [1,2,3]. A class of asymmetric resonant cavities (ARC) has been proposed, in which substantial deformation from cylindrical or spherical symmetry leads to partially chaotic ray dynamics [12]. Even free space coupling is possible in such asymmetric structures, where mode matching of a focused Gaussian beam allows an efficient excitation of high-Q WGMs through a process called “chaos-assisted dynamical tunneling” [12,13,14,15,16]. Specific field, namely biosensing, achieved by exploiting the properties of a specific structure, namely bulk and hollow microspheres
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