  Whispering gallery mode (WGM) cavity is important for exploring physics of
strong light-matter interaction. Yet it suffers from the notorious radiation
loss universally due to the light tunneling effect through the curved boundary.
In this work, we propose and demonstrate an optical black hole (OBH) cavity
based on transformation optics. The radiation loss of all WGMs in OBH cavity is
completely inhibited by an infinite wide potential barrier. Besides, the WGM
field outside the cavity is revealed to follow $1/r^\alpha$ decay rule based on
conformal mapping, which is fundamentally different from the conventional
Hankel-function distributions in a homogeneous cavity. Experimentally, a
truncated OBH cavity is achieved based on the effective medium theory, and both
the Q-factor enhancement and tightly confined WGM field are measured in the
microwave spectra which agree well with the theoretical results. The circular
OBH cavity is further applied to the arbitrary-shaped cavities including
single-core and multi-core structures with high-Q factor via the conformal
mapping. The OBH cavity design strategy can be generalized to resonant modes of
various wave systems, such as acoustic and elastic waves, and finds
applications in energy harvesting and optoelectronics.
