Whispering-gallery-mode (WGM) excitation, a phenomenon that occurs when light is trapped in a resonant cavity, has many applications in lasers, filters, and sensors. Previous research has demonstrated optical biosensors based on WGM in single microspheres, which are limited in sensitivity. Scientists at the Research and Development Division of Fujirebio (Tokyo, Japan) have reported a new detection scheme that uses WGM excitation in clusters of microspheres, the spectra of which distinguishes one cluster from another—even chemically identical ones.
Alexandre Francois and Michael Himmelhaus used clusters of polystyrene microspheres doped with a fluorescent laser dye (Coumarin 6G) to enable excitation of WGM from a continuous-wave helium-cadmium laser emitting at 442 nm. In this technique using microparticle clusters, the radius of the microspheres can be as small as 5 µm without losing the WGM effect. The “fingerprint” of these tiny but sensitive clusters can be traced even in an aqueous environment, meaning that they can be used in situ for medical applications. The new scheme is significant for its potential ability to integrate numerous detection sites on a single biosensor chip. Contact Michael Himmelhaus at [email protected].