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A theoretical model for describing the emission spectra of microspherecavities is presented, and its prediction of detailed lineshapes of emissionspectra associated with whispering gallery modes (WGMs) of various orders inZnO microspheres (MSs) are verified experimentally by photoluminescence (PL)spectroscopy. The interplay of Purcell effect, quality factor, and leaky modesin spontaneous and stimulated emission spectra related to WGMs of all orders isrevealed. The key success of the theory is based on the expansion of the fullGreen function of the MS in terms of all possible resonance modes in complexfrequency space, which allows incorporation of contributions from leaky modes,stimulated emission processes, and Purcell effect. We show that the spontaneousemission spectrum calculated according to Mie theory (without Purcell effect)is dominated by the contribution of leaky modes, while the spontaneous andstimulated emission enhanced by Purcell effect are responsible for the main WGMresonance peaks observed experimentally. It is found that the stimulatedemission peaks are doubly enhanced by their respective mode quality factor Q:one factor from the Purcell effect and the other factor from the photon numberderived from the rate equation. After combining all these effects the theorycan provide a quantitative description of fine features of both TE and TM modes(including higher-order modes) observed in the PL spectra of ZnO MSs.Surprisingly, it is found that for ZnO MS with diameter larger than 5 $\mu m$,the PL emission spectrum is dominated by higher-order modes. The quantitativeunderstanding of the interplay of these emission mechanisms should prove usefulfor optimizing the performance of light-emitting devices based on microresonators.

Interplay of Purcell effect, stimulated emission, and leaky modes in the photoluminescence spectra of microsphere cavities