Annular photonic crystals.

A new type of two-dimensional photonic-crystal (PC) structure called annular PC composed of a dielectric-rod and a circular-air-hole array in a square or triangular lattice such that a dielectric rod is centered within each air hole is studied. The dielectric rods within the air holes greatly modify the dispersion diagram of the photonic crystal despite the fact that the percentage of volume occupied by the dielectric rods may be small (<12%). Increasing the radius of the inner-dielectric rod, starting from zero to a critical value, reduces the band gap and closes it completely as expected, because of the addition of more dielectric material inside the unit cell. Continuing to increase the radius of the rod above the critical value surprisingly creates another photonic band gap. Comparison of the dispersion diagrams of the new structure and the original lattice (circular air hole square/triangular array in dielectric background) reveals that the photonic band gap is considerably enhanced in size for both square and triangular lattice with the new structure. This approach preserves the symmetry of the structure and provides a complete photonic band gap away from the close-packed condition and at low normalized frequencies.