Label-Free Optical Ring Resonator Bio/Chemical Sensors

Abstract

Optical micro-ring resonator sensors are an emerging category of label-free optical sensors for bio/chemical sensing that have recently been under intensive investigation. Researchers of this technology have been motivated by a tremendous breadth of different applications, including medical diagnosis, environmental monitoring, homeland security, and food quality control, which require sensitive analytical tools. Ring resonator sensors use total internal reflection to support circulating optical resonances called whispering gallery modes (WGMs). The WGMs have an evanescent field of several hundred nanometers into the surrounding medium, and can therefore detect the refractive index change induced when the analyte binds to the resonator surface. Despite the small physical size of a resonator, the circulating nature of the WGM creates extremely long effective lengths, greatly increasing light–matter interaction and improving its sensing performance. Moreover, only small sample volume is needed for detection because the sensors can be fabricated in sizes well below 100 μm. The small footprint allows integration of those ring resonator sensors onto lab-on-a-chip types of devices for multiplexed detection. This chapter gives an introduction to the ring resonator sensing principles. Different ring resonator configurations are illustrated as well, including microspheres, microfabricated planar ring resonators, and capillary-based opto-fluidic ring resonators. Their sensing performances are evaluated and compared quantitatively. Finally, the future development for ring resonator sensors is discussed.