Percolation lithography: Tuning and freezing disorder in 3D photonic crystals using partial wetting and drying

Although complex, hierarchical nanoscale geometries with tailored degrees of disorder are commonly found in biological systems, few simple self-assembly routes to fabricating synthetic analogues have been identified. We present two techniques that exploit basic capillary phenomena to finely control disorder in porous 3D photonic crystals, leading to complex and hierarchical geometries. In the first, we exposed the structures to mixtures of ethanol and water that partially wet their pores, where small adjustments to the ethanol content controlled the degree of partial wetting. In the second, we infiltrated the structures with thin films of volatile alkanes and observed a sequence of partial infiltration patterns as the liquid evaporated. In both cases, macroscopic symmetry breaking was driven by subtle sub-wavelength variations in the pore geometry that directed site-selective infiltration of liquids. The resulting patterns, well described by percolation theory, had significant effects on the photonic structures' optical properties, including the wavelength-dependence and angular dependence of scattering. Incorporating cross-linkable resins into our liquids, we were able create permanent photonic structures with these properties by freezing in place the filling patterns at arbitrary degrees of partial wetting and intermediate stages of drying. These techniques illustrate the versatility of interfacial phenomena in directing and tuning self-assembly of aperiodic structures.