Interest in negative refraction has been motivated by the possibility of creating a “superlens” as proposed by Pendry (Phys. Rev. Lett. 85, 3966 (2000)). This theoretical work showed that a material capable of negative refraction amplifies evanescent waves and allows this material to act as a lens with a resolution not limited by working wavelength. Although theory and some experiments have shown that certain metamaterials and photonic crystals (PhCs) can act as superlenses, realistic demonstration of negative refraction in the optical and infrared range remains a challenge. This is because most metamaterials employ lossy metal elements and most PhC structures found to exhibit negative refraction are made of positive index dielectric materials and are two-dimensional. Subwavelength imaging of a 3D object requires a 3D PhC capable of negative refraction.
Inspired by the numerical simulations of Luo, et. al. (Appl. Phys. Lett. 81, 2352 (2002)), we demonstrate the fabrication and characterization of a 500nm-diameter polymer core, 250nm-thick Germanium shell 3D photonic crystal lattice that exhibits negative refraction in the mid-infrared, centered around 8µm. This 3D photonic crystal resembles a BCC lattice of air cubes in dielectric media and was fabricated using two-photon lithography direct laser writing of an acrylic polymer resin scaffold followed by RF sputtering of Ge. The band structure of the lattice was mapped using FTIR spectroscopy reflectance measurements, and negative refraction was observed using far-field IR transmission imaging.