Controlling porosity of an interference lithography process by fine tuning exposure time

A method for controlling a density distribution of a three-dimensional photonic crystal template, comprising:

• depositing a photoresist composition comprising a photoinitiator homogeneously distributed throughout a volume of the photoresist composition on a silicon substrate;
• depositing a plurality of light reflective and/or light absorbing particles on the photoresist composition resulting in at least two areas above the silicon substrate having different amounts of the plurality of light reflective and/or light absorbing particles before irradiating,
• irradiating the photoresist composition with at least four laser beams using a multibeam interference lithograph system to generate a periodic interference pattern comprising a periodic pattern of a gradient constructive volume and a destructive volume in an exposed photoresist composition, where an exposure duration is less than 0.3 seconds, and where the photoinitiator is at a concentration greater than a maximum amount that photolyzes in two seconds;
• baking the exposed photoresist composition at a fixed temperature and time independent of the exposure duration to form a gradient-like periodic structure comprising a polymerized and gelled photoresist derived from the gradient constructive volume and an un-gelled photoresist in the destructive volume;
• dissolving the un-gelled photoresist with a solvent to form an un-gelled photoresist solution;
• and
• washing the un-gelled photoresist solution with a washing solvent from the three-dimensional photonic crystal template such that the three-dimensional photonic crystal template has a density gradient that is proportional to the exposure duration and a density of the three-dimensional photonic crystal template under the at least two areas with the plurality of light reflective and/or light absorbing particles is less than at least one area absent the plurality of light reflective and/or light absorbing particles.