  Optical microresonators with low quality factor ($Q$) can be efficiently
excited by and scatter freely propagating optical waves, but those with high
$Q$ typically cannot. Here, we present a universal model for resonators
interacting with freely propagating waves and show that the stored energy of a
resonator excited by a plane wave is proportional to the product of its $Q$ and
directivity. Guided by this result, we devise a microdisk with periodic
protrusions in its circumference that couples efficiently to normally incident
plane waves. We experimentally demonstrate several microdisk designs, including
one with a radius of $0.75{\lambda_0}$ and $Q$ of 15,000. Our observation of
thermally-induced bistability in this resonator at input powers as low as 0.7
mW confirms strong excitation. Their small footprints and mode volumes and the
simplicity of their excitation and fabrication make wavelength-scale,
free-space-coupled microdisks attractive for sensing, enhancing emission and
nonlinearity, and as micro-laser cavities.
