Fabrication of Photonic Crystals with Tunable Bandgap by Prism Holographic Lithography

Abstract

3-D photonic crystals (PhCs) have been inspiredly studied to control photonic band gaps (PBGs), i.e. reflected light, which are originated from periodically dielectric structures. For soft PBG structures, hydrogel among the polymeric materials has been received much attentions recently because of its tunable optical property, which is induced by gradual or abrupt volume changes under the external stimuli. Also, it is simple to form networks of crossliked hydrogel by photo-polymerization. Meanwhile, holographic interference lithography is a robust tool to create 3-D nanostructures with free-defects by multi-coherent lights. Moreover just by a single light emission, desired pattern can build up over the large area without masks or templates unlike conventional lithography. In this study, the experimental conditions including preparation of hydrogel solution (i.e. ratio of composition of random copolymer, photoacid generator, crosslinker, and solvent), laser exposure dose, and post-exposure baking were optimized for well-defined photonic structures. The fabrication of 3-D porous hydrogel pattern with thermo-sensitivity via a single refracting prism was demonstrated to avoid elaborate beam alignment and total reflection. Based on the unique swelling property of hydrogel as a visco-elatic material, hydrogel PhCs with reversible optical properties have potential applications such as functional microfluidic sensors and adaptive photonic crystal devices. Not only volume changes of structures, but also it showed that modification of surface property by physical grafting could be used for various tunable functions.