Reflectance properties of gold nano-cavity spherical and cuboid molded arrays

Gold nano-cavity arrays supported on polydimethylsiloxane (PDMS) have been created using colloidal lithography. PDMS is cured on top of hexagonally close packed arrays of polystyrene spheres of diameter 820 nm resulting in a close packed sphere imprinted polymer block. The depth of the imprints is 200 nm, indicating the whole sphere is not entrapped in the polymer during curing. The spherical nature of the imprint can be deformed by stretching of the flexible polymer, thus creating cuboid shaped arrays. Finally, the arrays are coated with a 100 nm gold layer, which conforms to the polymer surface to create either spherical or cuboid shaped gold nano-cavities. Experiments show that the reflectance properties of the arrays are critically dependent on the shape of the cavity. Spherical shaped cavity arrays display diffuse reflectance peaks at wavelengths slightly shorter than the diameter of the templating sphere, which are absent in the cuboid arrays. Both spherical and cuboid arrays show reflectance which is strongly dependent on the angle of incidence, with the cuboid arrays showing differing spectra depending on the direction of the impinging light with relation to the axis of stretching. The changes in optical behavior between the spherical and cuboid cavity arrays is discussed with relation to the change of shape of the patterning feature at the interface.