Spherical microresonators coated with a high refractive index coating for different applications

We will discuss several applications of the coating of silica microspheres. Silica microspheres fall in the group of so called whispering gallery mode (WGM) resonators. They are known to have a very high Q-factor and a low modal volume. Therefore they are suitable for a number of di®erent applications such as lasing, frequency comb generation or sensing. In this paper we will discuss three di®erent applications. Firstly we will discuss the sensing properties of the coated microsphere. We have found that a high refractive index coating can greatly improve the sensing properties of a silica microresonator, improving the theoretical sensing sensitivity from 5 nm/RIU for a blank silica sphere to a maximum of 145 nm/RIU for an amorphous silicon coating. We will also discuss some experimental results dealing with the coating of silica microspheres with a LPCVD a : Si coating. Second application which we will discuss is lasing and optical ampli¯cation. By coating the microsphere using an optically active material, microsized lasers can be produced. We have coated a silica microsphere using an erbium doped glass (70SiO2- 30HfO2) and have shown that such a system exhibits lasing. Finally we will discuss the possibility to tailor the modal dispersion of the resonator by applying a high refractive index coating on top of the microsphere. This can be a crucial step for improving the applicability of the microsphere in non-linear optics applications, such as for four-wave mixing. We used erbium doped silica hafnia glass as a probe to study the in°uence of the coating on the modal dispersion inside the microresonator and have found that the coating can be used to tailor the modal dispersion in a very wide range in both the normal and anomalous regime.