Tight-Binding Coupling of the microsphere resonators

We discuss the optical whispering gallery modes in single and coupled spheres in the small size range. We study the emission from a dye doped polystyrene spheres with diameters ranging from 2 to 5 mu m under quasi-steady state optical excitation. By studying the spectral characteristics of the emission of the individual 5 mu m sphere, we observe the transition from spontaneous emission to lasing. Although the dye molecule linewidth is broader than the free spectral range of whispering-gallery modes for such a sphere, more than 30% of the spontaneous emission is emitted into the lasing mode. By monitoring the frequencies of fluorescence peaks of individual spheres, we sort out two spheres with appropriate size matching and make them in contact. We observe coherent resonant coupling of optical whispering-gallery modes in fluorescence from two-sphere system (bisphere). By taking into account harmonic coupling of the whispering-gallery modes, the obtained features of the normal mode splitting are well explained by the tight binding photon picture.