Resonant Detection of Nano to Microscopic Objects Using Whispering Gallery Modes

A micron sized glass sphere is able to confine light to its interior volume. The trapped light describes an orbital trajectory circumnavigating just below the microsphere surface. Whenever the light ray tries to escape it is sent back on its circular path by total internal reflection. The light orbit closes in on itself several thousand times and thus creates an optical resonance. The unprecedented narrow linewidth of such a microsphere resonance (Q factors of up to 3x10⁹) allows precise measurement of its frequency. Dielectric microspheres of very high Q are thus the ideal choice for a resonant molecular sensor. Although the resonance is stealth, an evanescent field extends from the microsphere surface the distance of a wavelength into the surrounding medium. This thesis demonstrates how label-free molecules binding to the microsphere surface perturb the optical resonance by interaction with this evanescent field. The effect is demonstrated by surface adsorption of a protein (serum albumin). The general use as a biosensor is shown by specific detection of streptavidin binding to biotin. A first order perturbation theory describing the linear response of the sensor is presented. Molecular perturbation leads to a wavelength shift that can be measured with such high precision that single molecule detection seems theoretically possible. The experimental approach is extended to the multiplexed measurement of DNA hybridization using two microsphere resonators. This differential measurement allows the detection of a single nucleotide mismatch with a high signal to noise ratio. The effect of larger Mie particles such as bacteria and polystytrene nanospheres perturbing the cavity resonance is examined experimentally and theoretically. For such larger objects it is necessary to include the decay length of the evanescent field in the theoretical analysis. The Q spoiling which occurs for such large Mie particles is described by an analytic formula. Furthermore, a pairing effect is observed for polystyrene nanospheres with diameters of ~ a quarter wavelength polarized in the evanescent field of the microsphere resonance. A novel mechanism might be involved since the coupling cannot be explained by simple dipole-dipole interactions.