  We propose a new paradigm for realizing bound states in the continuum (BICs)
by engineering the environment of a system to control the number of available
radiation channels. Using this method, we demonstrate that a photonic crystal
slab embedded in a photonic crystal environment can exhibit both isolated
points and lines of BICs in different regions of its Brillouin zone. Finally,
we demonstrate that the intersection between a line of BICs and line of leaky
resonance can yield exceptional points connected by a bulk Fermi arc. The
ability to design the environment of a system opens up a broad range of
experimental possibilities for realizing BICs in three-dimensional geometries,
such as in 3D-printed structures and the planar grain boundaries of
self-assembled systems.
