Size and morphology dependent evolution of resonant modes in ZnO microspheres grown by hydrothermal synthesis

We report the correlation between inner morphology, size and whispering gallery mode (WGM) behavior in ZnO microspheres (MSs) grown by hydrothermal method. WGMs in different ZnO microspheres with diameters in the range of 2 - 6 μm were analyzed by a modified refractive index (MRI) scheme. We found that the size dependence of WGMs in our system is more complicated than others because of the appearance of porosity inside each sphere. Such features might account for the refractive index change and peak shift. Despite that, our MRI scheme can detect such complex features and reproduce universal relations between all important quantities of a microsphere WGM resonator.