  Owing to their ability to confine electromagnetic energy in ultrasmall
dielectric volumes, micro-disk, ring and toroid resonators hold interest for
both specific applications and fundamental investigations. Generally,
contributions from various loss channels within these devices lead to limited
spectral windows (Q-bands) where highest mode Q-factors manifest. Here we
describe a strategy for tuning Q-bands using a new class of micro-resonators,
named micro-kylix resonators, in which engineered stress within an initially
flat disk results in either concave or convex devices. To shift the Q-band by
60nm towards short wavelengths in flat micro-disks a 50% diameter reduction is
required, which causes severe radiative losses suppressing Q's. With a
micro-kylix, 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. Micro-kylix devices can
provide new functionalities and novel technological solutions for photonics and
micro-resonator physics.

Optics