Enhancing sensitivity of a whispering gallery mode biosensor by subwavelength confinement

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Abstract
The authors demonstrate enhanced sensitivity of a spherical whispering gallery modebiosensor (WGMB) by confining orbiting light near the surface using a subwavelength (sub- λ ) high refractive index layer on a fluorine dopedsilica microsphere (radius R ∼ 200 μ m ). Their experiments at a free space wavelength λ ∼ 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.