Three-dimensional photonic band gap cavity with finite support: Enhanced energy density and optical absorption

We study numerically the confinement of light in a three-dimensional (3D) photonic crystal cavity in a diamondlike inverse-woodpile structure. We present a versatile field-field cross-correlation method to identify resonances in the finite-support crystal with defect states in the 3D band gap of the infinite crystal. We argue that the five eigenstates of our 3D photonic band gap cavity have quadrupolar symmetry, in analogy to d -like orbitals of transition metals. It is remarkable that quality factors up to Q = 1000 appear in such thin structures of only three unit cells, which is attributed to the relatively small Bragg length of the perfect crystal. We find that the optical energy density is remarkably enhanced at the cavity resonances by up to 2400 the incident energy density in free space or up to 1200 × the energy density of the equivalent effective medium. We find that an inverse woodpile photonic band gap cavity with a suitably adapted lattice parameter reveals substantial absorption in the visible range. Below the 3D band gap, Fano resonances arise due to interference between the discrete fundamental cavity mode and the continuum light scattered by the photonic crystal. Hence, our study concludes that inverse woodpile cavities offer interesting perspectives for applications in optical sensing and photovoltaics.