  Bound states in the continuum (BICs) are radiationless localized states
embedded in the part of the parameter space that otherwise corresponds to
radiative modes. Many decades after their original prediction and early
observations in acoustic systems, such states have been demonstrated recently
in photonic structures with engineered geometries. Here, we put forward a
mechanism, based on waveguiding structures that contain anisotropic
birefringent materials, that affords the existence of BICs with fundamentally
new properties. In particular, anisotropy-induced BICs may exist in symmetric
as well as in asymmetric geometries; they form in tunable angular propagation
directions; their polarization may be pure transverse electric, pure transverse
magnetic or full vector with tunable polarization hybridity; and they may be
the only possible bound states of properly designed structures, and thus appear
as a discrete, isolated bound state embedded in a whole sea of radiative
states.
