Observation of Cartesian light propagation through a three-dimensional cavity superlattice in a silicon photonic band gap crystal

We experimentally investigate peculiar light propagation inside a three-dimensional (3D) superlattice of resonant cavities that are confined within a 3D photonic band gap. To this end, we fabricated 3D diamondlike photonic crystals from silicon with a broad 3D band gap in the near-infrared and doped them with a periodic array of point defects. In position-resolved reflectivity and scattering microscopy, we observe narrow spectral features that match well with superlattice bands in band structures computed with the plane-wave expansion. The cavities are coupled in all three dimensions when they are closely spaced (aSL 3a), and uncoupled when they are further apart. The superlattice bands correspond to light that hops in high-symmetry directions in 3D ("Cartesian light") that opens applications in 3D photonic networks, 3D Anderson localization of light, and future 3D quantum photonic networks.