Enhancing sensitivity of a whispering gallery mode biosensor by subwavelength confinement

The authors demonstrate enhanced sensitivity of a spherical whispering gallery mode biosensor (WGMB) by confining orbiting light near the surface using a subwavelength (sub-lambda) high refractive index layer on a fluorine doped silica microsphere (radius R similar to 200 mu m). Their experiments at a free space wavelength lambda similar to 1310 nm show that the frequency shift sensitivity by changing the external refractive index is increased by more than 700% by adding a 340 nm thick polystyrene layer. This advance is expected to move the WGMB well into the lead as the most sensitive method for unlabeled biosensing.