Large area 3D helical photonic crystals

Abstract

Three-dimensional helical photonic crystals are attractive for chiral metamaterial devices that mix electrical and magnetic responses. In chiral metamaterials, the refractive index for propagation of light along the axis of the helix is different as the handedness of the circular polarization is parallel or antiparallel to the chiral axis. The refractive index is increased for one circular polarization and reduced for the other, if the chirality is strong enough, negative refraction may occur. A novel interferometric lithography (IL) technique utilizing six separate two-beam exposures for fabricating three-dimensional helical photonic crystals is presented. In contrast to most previous demonstrations, which used a two-photon direct-write process, IL is a large-area process readily adaptable to realistic manufacturing constraints. This novel interferometric lithography uses only TE polarized light for maximum contrast and allows for independent dimensional control of the helix period along each lattice axis. Both mathematical models and experimentally realized three-dimensional helical photonic crystals (over a mm2 in area and up to 5 μm tall, with a helix spacing of 1.1 μm and a helix pitch of 1.9 μm on a hexagonal grid) are presented. The helical photonic crystals are formed as thick photoresist structures that can be subsequently used as mandrels for a sol-gel or metal electroforming processes, enabling a high index contrast chiral metamaterial. Optical transmission measurements of these helical photonic crystals are reported at the 1550 nm telecom wavelength.