High Q/V Microdisk Resonators for Observation of Purcell Effect in Silicon Nanocrystals

Observation of Purcell enhancement, stimulated emission and nonlinear optical effects in microresonators depend critically on their quality factors (Q) and mode volumes (V). Ideally, a high value of Q/V ratio is desired. However, in most systems, there is a trade off between achieving a higher Q and reducing the size of the resonator (small V). For resonators containing Si nanocrystals (nc-Si), the Q-limiting optical loss mechanisms are: (a) the radiation loss resulting from low indices of refraction of the host material, (b) the sidewall scattering loss and (c) the material absorption. Based on these considerations, the aim of the present work is to experimentally characterize the quality factors and mode volumes of microdisk resonators containing luminescent silicon nanocrystals. This study helps us estimate the dimensions of optimal structures needed to observe the abovementioned effects (in particular, the Purcell effect) for nc-Si in the visible wavelength range. We report fabrication and characterization of micron sized, nc-Si coated, silicon nitride microdisk resonators with diameters ranging from 2 mum to 10 mum. Consistent with ray-optics and finite element simulations (FEM), small (diameter < 3 mum) microdisks appear to be limited by radiation loss whereas larger microdisks suffer from material absorption/sidewall scattering loss. These two competing mechanisms lead to a parameter window which achieves maximum Q/V for our system. We use these experimentally obtained values to predict the range of maximum Purcell enhancement achievable in our structures.