Silicon quantum dots in microdisk resonators: Whispering-gallery modes, stress-induced Q-factor tuning and enhancement

We report on visible light emission from Si quantum dot (QD) based optically active microdisk resonators. Room temperature photoluminescence from single microdisks shows the characteristic modal structure of whispering-gallery modes (WGM). Highest quality factors of up to 7000 at visible wavelengths, where Si QDs absorb strongly, have been measured for the first time. Apart from conventional flat and circular resonators, we demonstrate for the first time a new class of active microdisk resonators with out-of-plane bending. In these devices, composed of silicon nitride, Si3N4 and Si QD-rich silicon oxide, SiOx, materials, the engineered stress at the interfaces results in bent-up (cup-like) and bent-down (umbrella-like) resonators, depending on which material is used as a top layer. Both type of bent devices support perfectly WGMs and, quite unexpectedly, offer a rich and interesting physics, in particular, the possibility of tuning and enhancing the Q-factor band of WGM modes in bent disks. Generally, the wavelength dispersion of two main different loss-channels, the material absorption and radiative losses, result in a limited bandwidth where the highest Q-factors can be observed (Q-band). We show here, that in a bent resonator, with respect to a flat one (same diameter, thickness and amount of Si3N4 and SiOx materials in both), the maximum of the Q-factor band blue-shifts by more than 70nm (from 832nm to 760nm). In addition, the absolute maximum of Q-band in a bent resonator is 3-4 times higher than that of the flat disk at the wavelength of 760nm. We explain this phenomenon by a smart interplay between the modified dispersions of material absorption and radiative loss related Q-factors (simultaneous increase of Q(mat) and decrease of Q(rad)). Importantly, this tuning scheme does not require larger device sizes, but rather utilizes self-adjustment properties of originally stressed resonator core. Remarkably, the bent resonators benefit from unmodified free-spectral range and cleaner WGM spectra due to the absence of higher order mode families.