Sensors based on evanescent field perturbation of microresonators

We consider a whispering gallery mode (WGM) sensor where a dielectric perturber extends into the evanescent tail of the microresonator. The position of the perturber rather than the morphology of the resonator is modified by a change in the measured property, inducing a shift in the WGM. This approach can be used for both three-dimensional (spheres, toroids) and planar (disks, rings) resonator geometries. We demonstrate, through analysis and experiments, the feasibility of this sensor approach for both geometries by using a sphere resonator and an on-chip ring resonator. Experiments show that the motion of the perturber across the evanescent tail on the resonator's outer surface leads to a measurable shift in the resonator WGM. In the latter experiment, a ferromagnetic structure is attached to the perturber stem so that the system acts as a magnetic field detector. The results show a consistent relationship between the mode shift and the magnetic field strength.