Confining light in a 3D cavity superlattice in a 3D photonic band gap

The confinement of light in three dimensions (3D) is an ongoing pursuit in Nanophotonics, since it allows for ultimate control over photons [1]. A powerful tool to this end is a 3D photonic band gap crystal with a tailored defect that acts as a cavity or even a waveguide. When a one-dimensional array of cavities is coupled, a well-known waveguide, known as a CROW (coupled resonator optical waveguide) [2]. However, 3D superlattices of coupled cavities that are shielded inside a 3D band gap have hardly been studied to date. Recently, theory predicted the occurrence of intricate “Cartesian light”, wherein light propagates by hopping only in high symmetry directions in space [3].
To experimentally study light propagation in a 3D superlattice of cavities in a 3D band gap, we have fabricated 3D nanostructures from silicon by reactive ion etching. The photonic crystal has the inverse woodpile structure that is composed of two perpendicular sets of pores (in the X and Z directions) with a cubic diamond-like symmetry. By intentionally making two perpendicular pores smaller light is confined at the intersection: a cavity. By fabricating an array of defect pores we realize a 3D cavity superlattice [4]. The challenge we address here is to experimentally identify modes in this complex 3D system.
We measure reflectivity of the crystals with a large aperture (NA = 0.85). With our setup we also record light scattered in the lateral direction. In presence of the intentional defects, we observe characteristic troughs in the reflectivity peak that contains the band gap. The corresponding lateral scattering spectrum shows matching peaks that are potential superlattice modes. To confirm superlattice modes, we record spectra at different locations on the crystal. Laterally scattered peaks that reproduce at different positions are thus not random speckle, but superlattice modes. Thus, this results represents a first demonstration of the 3D guiding of light through a 3D cavity superlattice.