  We demonstrate a novel artificial optical material, a photonic hyper-crystal,
which combines the most interesting features of hyperbolic metamaterials and
photonic crystals. Similar to hyperbolic metamaterials, photonic hyper-crystals
exhibit broadband divergence in their photonic density of states due to the
lack of usual diffraction limit on the photon wave vector. On the other hand,
similar to photonic crystals, hyperbolic dispersion law of extraordinary
photons is modulated by forbidden gaps near the boundaries of photonic
Brillouin zones. Three dimensional self-assembly of photonic hyper-crystals has
been achieved by application of external magnetic field to a cobalt
nanoparticle-based ferrofluid. Unique spectral properties of photonic
hyper-crystals lead to extreme sensitivity of the material to monolayer
coatings of cobalt nanoparticles, which should find numerous applications in
biological and chemical sensing.
