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

Fruitful analogies exist between waves like light or sound that propagate in mesoscopic photonic or phononic metamaterials and the elementary excitations in atomic crystals like phonons, electron and spin waves. A peculiar class of wave transport is discretized transport with hopping in all three dimensions on superlattices, as demonstrated in phonons, electrons and spins, but not yet for light. Here, a superlattice is a periodic arrangement of a supercell that consists itself of multiple unit cells of an underlying crystal structure. In this work, we experimentally observe light waves propagating by hopping between neighbouring cavities along high-symmetry Cartesian directions in space. The hopping transport leads to the appearance of defect bands in the 3D photonic band gap, as theoretically identified by scaling and machine learning methods. Cartesian light is a completely new mode of light propagation (e.g., different from CROWs) that opens the door to a plethora of applications.