Silicon quantum dots in microdisk resonators: Stress-engineering of disk core for Q-factor tuning and enhancement

We report on visible light emission from Si quantum dot (QD)-based optically active microdisk resonators with highest quality factors of up to 7000 at visible wavelengths, where Si QDs absorb strongly. Generally, contributions from various loss channels within optically active micro-resonator devices lead to limited spectral windows, referred to as Q-bands, where highest mode Q-factors manifest. Here we describe a strategy for tuning Q-bands using a new class of micro-resonators, in which engineered stress within an initially flat disk results in either concave or convex devices after post formation. To shift the Q-band by 60nm towards short wavelengths in conventional flat micro-disks, for example, a 50% diameter reduction is required, which causes severe radiative losses, suppressing Q's. With our bent devices we achieve similar tuning and even higher Q's by two orders of magnitude smaller diameter modification (0.4%). The phenomenon relies on geometry-induced smart interplay between modified dispersions of material absorption and radiative loss-related Q-factors. 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.