Near-field mapping of optical eigenstates in coupled disk microresonators

We report on the real-space experimental observation of whispering-gallery modes in coupled disk microresonators excited by a tapered optical fiber. Using a high-resolution scattering scanning near-field optical microscope (SNOM) technique, the resonator modes can be perturbed, leading to altered transmission and reflection through the tapered fiber. The correlation of the perturbed signals with the position of the perturbing SNOM tip results in intensity maps of the optical resonator modes under investigation. A combination of a coupled-mode theory of the excited microresonator including the SNOM tip and a two-dimensional modal expansion technique can be used to understand different regimes of the perturbation, leading to qualitatively different transmission and reflection maps for the same optical resonator mode. Detailed measurements of the intensity distributions of eigenmodes of two and three coupled disks in different arrangements are presented and compared to theoretical calculations.