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.

Magnetorheological polydimethylsiloxane micro-optical resonator.

We investigate the possibility of using magnetorheological polydimethylsiloxane (MR-PDMS) spheres as micro-optical resonators. In particular, the effect of a magnetic field on the whispering gallery modes (WGM) of these resonators is studied. The applied field induces mechanical deformation, causing shifts in the WGM. The microspheres are made of PDMS with embedded magnetically polarizable particles. An analysis is carried out to estimate the WGM shifts induced by an external magnetic field. An experiment is also carried out to demonstrate the magnetic field-induced WGM shifts in an MR-PDMS microsphere. The results indicate that MR-PDMS microspheres can be used as high-Q-factor tunable optical cavities with potential applications in sensing.

Tuning of whispering gallery modes of spherical resonators using an external electric field.

In this paper we investigate the electrostriction effect on the whispering gallery modes (WGM) of polymeric microspheres and the feasibility of a WGM-based microsensor for electric field measurement. The electrostriction is the elastic deformation (strain) of a dielectric material under the force exerted by an electrostatic field. The deformation is accompanied by mechanical stress which perturbs the refractive index distribution in the sphere. Both strain and stress induce a shift in the WGM of the microsphere. In the present, we develop analytical expressions for the WGM shift due to electrostriction for solid and thin-walled hollow microspheres. Our analysis indicates that detection of electric fields as small as ~500V/m may be possible using water filled, hollow solid polydimethylsiloxane (PDMS) microspheres. The electric field sensitivities for solid spheres, on the other hand, are significantly smaller. Results of experiments carried out using solid PDMS spheres agree well with the analytical prediction.

Micro-optical force sensor concept based on whispering gallery mode resonators.

A micro-optical force sensor concept based on the morphology-dependent shifts of optical modes of dielectric microspheres is investigated. The optical resonances, commonly referred to as the whispering gallery modes (WGM), were excited by evanescently coupling light from a tunable diode laser using a tapered single-mode fiber. A compressive force applied to the sphere induces a change in both the shape and the index of refraction of the sphere leading to a shift in WGM. By tracking the shifts, the force magnitude is determined using solid silica as well as solid and hollow Polymethyl-methacrylate (PMMA) microsphere resonators. A measurement sensitivity as high as dlambda/dF=7.664 nm/N was demonstrated with a 960 mum hollow PMMA sphere.