Thermally stable materials for photonic band gap crystals
Colloidal microspheres can be used to "self-assemble" novel structures known as photonic band gap crystals. The optical properties of photonic band gap crystals can be manipulated by tailoring defects in these structures. Here we demonstrate a promising strategy to introduce defects without disturbing the lattice around the defects. We use multiphoton photopolymerization to draw patterns inside the photonic crystal. Because of the high temperature processing steps conventional organic photoresists are not suitable. Instead, we utilized methylsilsesquioxanes. Confocal and electron microscopies were then used to observe the patterned feature inside the 3-D structure.
We expand the scope of the use of these materials to multiphoton lithography that is a fast and easily controllable method to fabricate 3-D photonic crystals. Usage of thermally stable inorganic photoresists makes it possible to obtain photonic band gap crystals using this method. We present photonic band gap crystals using methylsilsesquioxane photoresists and multiphoton lithography. The structures maintain their shape without cracking even at the elevated temperatures necessary for further processing of the photonic crystal.
In addition, we address two problems in self-assembled photonic crystals. First, the growth of opals using large spheres (∼ 1 micron) is challenging because of their fast sedimentation. The mixture of water/n-propanol gives better control over the quality of opals. Second, cracks are generated during high temperature steps due to the shrinkage of silica spheres. We show how to avoid the formation of cracks using silica spheres preheated at high temperatures.