Microdisk, Microring, Microcylinder, and various curvilinear-shape
optical resonators with sizes from submicron to hundreds of microns
have become a widely used technology. We refer to them collectively as
optical microresonators. These optical microresonators can be used as
highly compact tunable optical filters integrated on chip. They can also
be used to form wavelength-scale optical cavities for realizing microcavity
lasers and various microcavity devices. There are few systematic
studies of the limitations of these microresonators. Knowing their limitations
is important for various practical applications. We first review
the various progresses in these optical microresonators, followed by a
discussion of the main factors affecting the cavity Q factor of these
microresonators. To understand radiation loss, we show a numerically
accurate method to compute the radiation loss using conformal transformation.
We discuss how to simulate lasing properties of these optical
microresonators by using a multi-level multi-electron Finite-Difference
Time-Domain (MLME FDTD) quantum model for the semiconductor
medium. We then discuss how to compute radiation loss and scattering
loss using a FDTD method based on an active-lasing approach
and to compare the results to the conformal transformation results.
Lastly, we address the important question of how to optimize output
coupling of the lasing light in these optical resonators utilizing either
a conventional tangential waveguide coupling method or a novel radial
waveguide coupling method.