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In the process of reducing [http://www.selleckchem.com/products/Gefitinib.html Gefitinib nmr] the size from the classical Fabry�CPerot 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]. The principle of WGM resonators, in reality, is not new at all, but dates back to 1910�C1912 when John William Strutt (Lord Rayleigh) studied the characteristics of the so-called whispering gallery, namely the circular gallery running around the interior of the dome of St Paul��s Cathedral in London [4]. According to Rayleigh��s observations and to the later studies (see, for instance, [5]), it was proven that [http://www.selleckchem.com/products/Dasatinib.html Dasatinib cost] structures having circular symmetry may sustain the so-called whispering gallery modes that can be interpreted as acoustic or electromagnetic waves that circulate and are strongly confined within the structure. In terms of geometric optics, the confinement is described by the optical rays which are totally internally reflected and focused by the surface itself. Due to minimal reflection losses and to potentially very low material absorption (e.g., in dielectric materials), these resonators can reach exceptionally high quality factors Q = ��/���� (where �� is the wavelength at which a resonance occurs and ���� the linewidth of the resonant wavelength), up to 1011, compared to values around 105 for the best Fabry�CPerot resonators. To be more precise, a theoretical analysis indicated values of Q close to 105 for ideal lossless photonic-bandgap Fabry�CPerot resonators, reducing to Q > 2000 for real dielectric materials [6]. The ultra-high Q values of WGM resonators lead to very high energy density (of the order of GW/cm2), very narrow resonant-wavelength lines ([http://en.wikipedia.org/wiki/Oxygenase Oxygenase] and RF communications, quantum optics and electrodynamics and sensing [7,8,9]. Since the basic geometric requirement, i.e., circular symmetry, is quite easy to fulfill, several different WGM structures may be considered and have actually been successfully tested, in both two-dimensional (e.g., ring resonators) and three-dimensional shapes; the latter ones include cylindrical (optical fibers, microcapillaries, microdisks, microtoroids, etcetera) and spherical (microspheres and microbubbles) structures, as well as more complex ones (droplets, microbottles, etcetera) [7]. The above requirement, however, is not compulsory, and for instance, non-circularly symmetric WGM cavities, such as 2D racetrack resonators and 3D elliptical resonators, have been developed, which may have the advantage of an easier coupling of the light thanks to a longer interaction length and/or of a directional emission [10,11].