Micromolding-based design of single-and multi-colored microcylinders for photonic barcoding

Abstract

Photonic crystals are periodic nanostructures of which spatial modulation of refractive index gives rise to photonic bandgap property. When the periodicity is comparable with half the wavelength of visible light, photonic crystals display pronounced iridescent structural colors, which are appealing for esthetic coloration. To make granular format of photonic crystals as color pigments, monodisperse colloidal particles have been crystallized in emulsion drops, which has provided spherical color pigment. However, the spherical symmetry exhibits low reflectivity. Furthermore, it is difficult to integrate multiple domains of distinct color into single granules, restricting the use. In this thesis, I report a pragmatic micromolding technique to fabricate cylinder-shaped photonic mi-croparticles. As a mold, I fabricate elastomer films with cylindrical hole array by soft-lithography. To make colloidal crystals in each hole, I prepare a photocurable suspension containing monodisperse silica particles. The silica particles in the resin spontaneously form a face-centered cubic (fcc) lattice when they are highly concentrated. I fill the holes with the suspension and irradiate them with UV. The silica particles form onion-ring-like configuration within each cylindrical hole, while maintaining overall regularity of colloidal crystals. After removal of the mold, microcylinders are released, which are then treated with hydrofluoric acid to re-move the silica particles selectively. The resultant cylincrical microparticles possess regular air cavity array, which develops pronounced structural colors. Stepwise application of the molding technique enables us to create compartmented microparticles with controlled size and color, providing an application for photonic barcoding