Allowed light in forbidden gaps

Full control over light propagation and emission in 3D is an outstanding goal in nanophotonics and relevant to applications of 3D waveguiding, quantum information processing and Anderson localization of light. For this purpose, 3D photonic band gap crystals offer promising possibilities. Due to the vanishing density of states, there are no modes in the band gap and light propagation is prohibited in all directions. To realize fully controlled unconventional light propagation in the forbidden gap, 3D nanocavities are formed by tailoring defects in the photonic crystal. Theory predicts that coupled cavity resonances give rise to new, allowed bands. In such a system, photons hop predominantly in x,y,z directions, which is known as “Cartesian light”. We fabricated 3D cavity superlattices in 3D photonic band gap crystals made from silicon that operate at telecom wavelengths. We have developed a near-infrared optical microscope to collect scattering and reflection from the sample from two orthogonal directions. We collect broadband reflectivity to identify the gap and simultaneously collect spectra of the scattered light from the lateral side of the crystal. We find that intensity peaks in the lateral scattering spectra reproduce at different locations on the crystal that distinguishes them from speckle, and identify them as superlattice modes. The observations of Cartesian light open the avenue to 3D cavity superlattice physics, including 3D Anderson localization.