Photoinduced Tuning of Optical Stop Bands in Azopolymer Based Inverse Opal Photonic Crystals

Photonic crystals (PCs) which are arrays of dielectric materials with a one, two, or three dimensional periodicity have attracted considerable attention during the past decade. PCs have been considered to be one of the most promising materials for manipulating light in three dimensional spaces for all optical signal processing and optical integration technology. Through numerous theoretical studies and experimental work, a complete photonic bandgap (PBG) has been introduced for a PBG material to cover all three dimensions. One possible approach is a three dimensional architecture such as artificial opal structures which are constructed from self-assembled colloidal spheres. Such colloidal PCs have been widely investigated for other optical applications due to their low-cost and ease of fabrication. To open the full PBG in 3 dimensions, various attempts have been tried through the infiltration of several high refractive index materials into the nanoscale voids of the colloidal PC. Because the opaline structures based on a colloidal assembly can not have complete PBGs; they exhibit only a pseudo gap or optical stop band in a certain crystallographic axis. Tunable PCs, the bandgap of which can be controlled through external parameters, such as physical extension, chemical treatment, an electric field, and light irradiation, have recently been investigated for use in tunable optical devices. PCs based on colloidal crystals have been good candidates for tunable PCs because of their macroporous structures to easily infiltrate other functional materials. Based on this infiltration technique, the concept of tunable PCs has been proposed, in which the PBG can be tuned as desired by controlling certain parameters such as lattice spacing, refractive index, and spacefilling factor of colloidal PCs. Tunable PCs based on the concept of modifying the lattice constant have been reported. For example, an external mechanical force, the volume phase transition, change of temperature or ionic strength, light irradiation, the order-disorder phase transition, and anisotropic colloidal shape. Other tunable PCs have been reported based on changing the refractive index contrast. Phototunable PCs by refractive index change due to photochromic reaction of a spiro dye, and liquid crystal (LC) infiltrated opal and inverse opal PCs, have been demonstrated for tuning a band through external factors such as electrical field, thermal energy, and photochemical reaction. Kubo et al. recently demonstrated a tunable PC structure infiltrated with photoisomerizable azobenzene dyes blended with LC molecules. More recently, Ozin group demonstrated the tuning of defect modes using a photochemically and ther-