Tunable Shear-Ordered Face-Centered Cubic Photonic Crystals

Large-area photonic crystals are produced by the shear-alignment of stabilized 720 nm diameter PMMA spheres dispersed in a range of liquids suspended between two parallel glass slides. An oscillatory linear shear aligns the initially small crystallites into a single domain many square centimeters in area. Reducing the shear to one lattice constant per layer produces a face-centered cubic crystal structure. This relaxes to a faulted twinned face-centered cubic structure on removal of the shear. Pure face-centered cubic crystals can be fabricated by dispersing the PMMA spheres directly in an epoxy resin. A 2-dimensional shearing scheme creates a stable face-centered cubic structure which is made permanent by exposing to UV light, thus solidifying the epoxy resin. The diffractive properties of the crystal can be monitored as the shear is applied and used to determine the crystal structure. Further, if the PMMA spheres are dispersed directly into a liquid crystal, the refractive index contrast and hence the diffraction properties can be controlled by temperature and by applying an electric field across the crystal.

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