  Hexagonal boron nitride (hBN) is a wide bandgap van der Waals material that
is emerging as a powerful platform for quantum optics and nanophotonics. In
this work, we demonstrate whispering gallery mode silica microresonators
hybridized with thin layers of epitaxially grown hBN that exhibit high optical
quality factor $> 7 \times 10^5$. Measurements of the effect of hBN thickness
on optical $Q$ and comparison with a theoretical model allows the linear
optical absorption coefficient of the hBN films to be estimated. These high-$Q$
devices will be useful for applications in quantum and nonlinear optics, and
their hybridized geometry provides a sensitive platform for evaluating losses
in hBN and other 2D materials.
