Whispering gallery modes in quantum dot-embedded dielectric microspheres for tagless remote refractometric sensing
This thesis presents the development of a refractometric sensor based on quantum dot-embedded polystyrene microspheres. The technique uses optical resonances within a microsphere, known as Whispering-Gallery Modes (WGMs), which produce narrow spectral peaks. The basic theory of WGMs is reviewed and specifically discussed for biosensing application. The spectral shifts of WGM peaks are sensitive to changes in the local refractive index. In the experiments, two-photon excited luminescence from the quantum dots couples into several WGMs within the microresonator. By optimizing the detection area, the spectral visibility of the WGMs is improved. The spectral shifts are measured as the surrounding index of refraction changes. The experimental sensitivity is about five times greater than that predicted by Mie theory. The sensor element is based on commercially available dielectric microspheres with a diameter about 10 ?m. Thus, the technique is more economic and suitable for sensing applications, compared to microspheres of 100 ?m in size which can only be made in the laboratory.