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.
We use a plane wave expansion method to define parameters for the fabrication of 3-dimensional (3D) core-shell photonic crystals (PhCs) with lattice geometries that are capable of all-angle negative refraction (AANR) in the midinfrared centered around 8.0 μm. We discuss the dependence of the AANR frequency range on the volume fraction of solid within the lattice and on the ratio of the low index core material to the high index shell material. Following the constraints set by simulations, we fabricate two types of nanolattice PhCs: (1) polymer core-germanium shell and (2) amorphous carbon core-germanium shell to enable experimental observation of 3D negative refraction and related dispersion phenomena at infrared and eventually optical frequencies.