International audienceWhispering-gallery modes (WGM) on a spherical surface were first described by Lord Rayleigh at the beginning of the last century, but only after the invention of laser did they start to have some scientific relevance and only during the last two decades there has been a substantial move towards real devices and practical applications. WGM resonators have peculiar properties, the most notable being the potential of having an ultrahigh quality factor Q, which makes them very appealing both as laser cavities and as extremely sensitive sensors. Among the different types of WGM resonators, the microspherical ones represent a very important category, due to their simplicity, easy fabrication, and very high quality. In this review we provide a description of their fundamental properties and we summarize recent works on their application as filters, sensors and lasers

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70SiO 2 – 30HfO 2 planar waveguides, doped with Er3+ concentrations ranging from 0.3 to 1 mol %, were prepared by sol-gel route, using dip-coating deposition on silica glass substrates. The waveguides show high densification degree, effective intermingling of the two components of the film, and uniform surface morphology. Propagation losses of about 1 dB/cm were measured at 632.8 nm. When pumped with 987 or 514.5 nm continuous-wave laser light, the waveguides show the I413/2→I415/2 emission band with a bandwidth of 48 nm. The spectral features are found independent both on erbium content and excitation wavelength. The I413/2 level decay curves presented a single-exponential profile, with a lifetime between 2.9 and 5.0 ms, depending on the erbium concentration.

Optical resonators play an ubiquitous role in modern optics. A particular class of optical resonators is constituted by spherical dielectric structures, where optical rays are total internal reflected. Due to minimal reflection losses and to potentially very low material absorption, these guided modes, known as whispering gallery modes, can confer the resonator an exceptionally high quality factor Q, leading to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. These attractive characteristics make these miniaturized optical resonators especially suited as laser cavities and resonant filters, but also as very sensitive sensors. First, a brief analysis is presented of the characteristics of microspherical resonators, of their fabrication methods, and of the light coupling techniques. Then, we attempt to overview some of the recent advances in the development of microspherical biosensors, underlining a number of important applications in the biomedical field.

Microbubble resonators combine the unique properties of whispering gallery mode resonators with the intrinsic capability of integrated microfluidics. Here an improved fabrication method of microbubble resonators is presented, based on the heating of a slightly pressurized capillary by a rotating arc discharge. Rotation of the electrodes, moved out of a fiber splicer, ensures a homogeneous distribution of the heat all over the capillary surface. The demonstrated microbubble resonators have Q factors up to 6×10(7) at 1550 nm. Microbubbles were filled with water and aqueous solutions of ethanol in order to test the refractive index sensing capabilities of such resonators, which also show a good temporal stability. The limit of detection of our microbubble resonator sensor is 10(-6) RIU.

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.

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Rare earth-activated 1-D photonic crystals were fabricated by RF-sputtering technique. The cavity is constituted by an Er3+-doped SiO2 active layer inserted between two Bragg reflectors consisting of ten pairs of SiO2/TiO2 layers. Scanning electron microscopy is employed to put in evidence the quality of the sample, the homogeneities of the layers thickness and the good adhesion among them. Near infrared transmittance and variable angle reflectance spectra confirm the presence of a stop band from 1500 nm to 2000 nm with a cavity resonance centered at 1749 nm at 0° and a quality factor of 890. The influence of the cavity on the ⁴I₁₃/₂ -->⁴I₁₅/₂ emission band of Er3+ ion is also demonstrated.