Lithographically-featured photonic microparticles of colloidal assemblies

Abstract

We have described a new and promising strategy for the fabrication of composite and porous photonic crystal microparticles that combines the self-assembly of colloidal particles with photolithography techniques. We fabricated silica/SU-8 composite microparticles with photonic bandgaps via four steps: (1) deposition of the silica colloidal crystals on the photoresist, (2) embedding of the colloidal crystals in the photoresist, (3) UV exposure through a photomask and subsequent development, and (4) release of the microparticles from the substrate. Embedding was performed above the glass transition temperature (T_g) of uncrosslinked SU-8. At such temperatures, capillary forces on the silica particles facilitate the migration of colloidal crystals in the SU-8 matrix. Particle migration ceased when the top colloidal crystal layer was trapped at the interface between air and SU-8. In addition, we also prepared porous microparticles with an inverse opaline structure by dissolving the embedded silica particles from the composite structures. The porous microparticles showed enhanced reflectivity at the bandgap position due to the large refractive index contrast. The bandgap position of the microparticles was controlled by the size of the silica particles, which determined the lattice constant. Bilayered composite and porous microparticles with two distinct photonic bandgaps were also prepared by sequential deposition of colloidal crystals composed of two differently sized silica particles.