We review the works performed on whispering gallery mode (WGM) micro-optical resonators made in rare-earth (RE) doped glasses for implementing low threshold and narrow line coherent sources. These types of micro-lasers, because of their small size and ease of fabrication can be a useful tool for the characterization of laser glasses and have several applications, especially as sensors.

Porous silicon has been intensely studied for the past several decades and its applications were found in photovoltaics, biomedicine, and sensors. An important aspect for sensing devices is their long–term stability. One of the more prominent changes that occur with porous silicon as it is exposed to atmosphere is oxidation. In this work we study the influence of oxidation on the sensing properties of porous silicon. Porous silicon layers were prepared by electrochemical etching and oxidized in a tube furnace. We observed that electrical resistance of oxidized samples rises in response to the increasing ambient concentration of organic vapours and ammonia gas. Furthermore, we note the sensitivity is dependent on the oxygen treatment of the porous layer. This indicates that porous silicon has a potential use in sensing of organic vapours and ammonia gas when covered with an oxide layer.

We report the effects of thermal oxidation on the sensing properties of porous silicon. Porous silicon substrates were prepared by electrochemical etching and thermally oxidized at different temperatures. A comparative EDS analysis shows that porous surfaces oxidized at higher temperatures have higher oxygen-to-silicon ratios. Our results indicate that the chemoresistive response due to the presence of isopropanol vapors at room temperature also increases with an increasing oxidation temperature. The presence of oxygen atoms in the PS layer could both protect the sensor from further atmospheric oxidation and increase its sensitivity.

Full exploitation of the unique properties of high quality factor micro-optical Whispering Gallery Mode (WGM) resonators requires a controllable and robust coupling of the light to the cavity, either for fundamental investigations or even more for practical applications. Fiber tapers are ideal phase-and-mode-matched couplers and are typically used for lab demonstrations in silica based micro-resonators or in low-index crystalline disks. Prism-based coupling basically adapts to any material and offers improved robustness and reliability for the implementation of devices based on larger resonators. We present the results of our studies on alternative methods based on integrated waveguides with specific reference to the coupling to lithium niobate disk resonators. We also demonstrate efficient coupling from fiber tapers to higher order azimuthal modes in coated microspheres and for third harmonic generation in silica microspheres. We finally propose a new method based on fiber gratings for improved robustness in biosensing applications.