  We present an absorbing boundary condition for electromagnetic frequency
domain simulations of photonic crystals and metamaterials. This boundary
condition can simultaneously absorb multiple Bloch-Floquet eigenmodes of a
periodic crystal, including both propagating and evanescent modes. The photonic
crystal or metamaterial in question can include lossy, active, anisotropic and
even bi-anisotropic inclusions. The absorbing boundary condition is dependent
on an orthogonality condition for Bloch-Floquet eigenmodes, a generalized
version of which is presented here. We test this absorbing boundary condition
numerically and present the results.
