Pixellated Photonic Crystal Films by Selective Photopolymerization

Hierarchical microstructures of colloidal crystals have attracted enormous research efforts due to their potential uses in display devices, optical communications, catalytic supports, biosensors, and acoustic materials. Recent progress in selfassembly approaches for such microstructures has shown that regular arrays of microspheres can be easily grown by several clever ways based on well-established techniques including coating and epitaxial growth. One of the remaining issues in self-assembled colloidal crystals is to create defective or patterned structures for photonic crystals based microdisplay devices, integrated photonic chips, as well as microcavities for extracting or adding light of specific frequencies. Over the years, only a few research groups have suggested suitable fabrication methods for the defect engineering of colloidal crystals, including colloidal crystallization inside microchannels, two-photon polymerization, and surface-charge-induced colloidal crystallization. However, a practical approach for hierarchical patterns with high resolution at the micrometer scale inside photonic-crystal films is still a challenge. Here, we describe a simple and facile method for 2D arrays of pixellated multicolor mosaic cells by selective photon-induced polymerization in colloidal crystal films. To tune the reflective colors of colloidal photonic crystals, it is necessary to find a way to control the refractive-index mismatch, lattice constant (or center-to-center distance between particles, or particle sizes), symmetry (or orientation), magnetic permeability, and precisely positioned defects. Among these factors, one of the most simple and feasible to control is the refractive-index contrast between the matrix and the colloidal particles. Previously, Yoshino et al. reported experimental results on heat-induced color tuning of colloidal crystals embedded in a conducting polymer and several other research groups, including Kang et al., have demonstrated liquid crystal (LC)-based approaches. However, in addition to the manipulation of the refractive index, it is still necessary to be able to pattern colloidal photonic crystals at the micrometer scale for practical applications in photonics devices and microdisplays. To do this, we used the selective reaction of a photocurable prepolymer inside opaline colloidal crystal structures, whereby the refractive indices of the prepolymer and the opaline colloids were perfectly matched in order to avoid scattering-induced blurring of the photoinduced pattern. As shown in Figure 1, we have explored photolithography for the manipulation of the reflection colors of inverse opals by patterning them. Before any patterning experiments could be carried out on the photonic crystals, high-quality silica opal films needed to be prepared using uniformly sized silica spheres. Here, silica opal films were deposited on glass substrates by a vertical dip-coating method developed by Colvin and co-workers, which is an easy way to fabricate colloidal crystal films with a relatively small number of defects over large areas. To obtain a polymeric inverse opal film as a scaffold structure, a prepolymer of a negative photoresist (SU-8) was infiltrated into the silica opal film and hard-baked under UV exposure (step i). The silica colloids were dissolved with an HF solution leaving behind well-ordered macropores in a face-centered cubic (fcc) lattice (step ii). Then, the silica opal film was peeled off the glass substrate and placed upside down onto a wafer so that the smooth and planar surface was facing outwards. This was an important procedure for the subsequent patterning process. The top of the inverse opal film was covered with a hydrophobic film of polydimethylsiloxane (PDMS) and the same negative photoresist was used to fill the macropores (step iii). Capillary forces drove the photoresist into the macropores, and as the infiltration was completed the film became transparent. At the soft-baking step, the film remained transparent, which implied that the refractive indices of the inverse opal scaffold and injected photoresist (SU-8) were almost the same. In our previous work, we reported that the refractive indices of hard-baked and softbaked negative photoresists of SU-8 were 1.5929 and 1.5925, respectively. This small difference in refractive index did not cause any scattering of the UV beam, which would be seen in a corona around the patterns. Finally, UV light was C O M M U N IC A IO N S