Sensitive optical biosensor based on whispering-gallery modes of dielectric microspheres

During the last decade, label-free optical biosensors have become valuable tools for clinical and military use as well as drug discovery. They are important devices for a range of applications going from detection of infectious agents, toxins, proteins and DNA to investigation of whole cell behaviour. Modern label-free optical biosensors, like the widely utilised surface plasmon resonance (SPR) method, are based on an evanescent light field travelling along a planar surface, probing the target material deposited on the surface. Recently, a novel type of biosensor based on evanescent field coupling has been proposed. This sensor exploits narrow-linewidth photonic resonances of dielectric microspheres, the so called whispering-gallery modes (WGMs), having a field confined close to the inner surface of the sphere by total internal reflection. Because the light can orbit many thousand times before escaping the resonator, the detection sensitivity of this method as compared to planar surface-based methods is expected to be greatly enhanced.