Whispering gallery mode (WGM) resonances were observed in the emission spectrum of Ru(bpy) 3 2+ (bpy = 2,2'-bipyridine) in a single laser-trapped water droplet levitated in air. The emission decay profiles of Ru(bpy) 3 2+ in the water droplets comprised fast and slow decay components. The emission lifetime of the slow decay component was independent of the diameter of the droplet, and corresponded to the value in a bulk aqueous solution. On the other hand, the emission lifetime of the fast

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... component decreased with decreasing the droplet diameter, which could be ascribed to the cavity-enhanced spontaneous emission. The decrease in the emission lifetime of the fast decay component as a function of the droplet diameter was explained on the basis of cavity quantum electrodynamic (QED) effects. It was shown that the mode characteristic of WGM resonances and the enhancement factor of the radiative rate of Ru(bpy) 3 2+ were controlled by the size of the water droplet.