Studies on 3D nanopatterning using photo-responsive organic molecules and its optical applications

Abstract

Structural coloration is a property of the periodic structure produced by various types of materials and easily found in everyday life and nature. As this coloration is originated from the structural property, there are many advantages compared to chemical coloration, which is commonly used in ink. Among the materials for structural colorations, photonic crystals have been considered important research topics to study the fundamental characteristics of light and make useful applications. Since the color from the photonic crystal is closely related to the periodic length of the nanostructure, it is essential to fabricate the periodic three-dimensional (3D) structure in a visible wavelength range. As well-known, conventionally, 3D nanofabrication technology is based on top-down and bottom-up lithographic methods. However, the conventional methods have limitations in terms of cost, difficulty, stability, and reliability in practical application. In this research, we developed a new nanofabrication process for the photonic crystal that can overcome the existing problems. In particular, the helical nanofilament (HNF) liquid crystal (LC) phase made of bent-shaped LC (BLC) molecules is used as a building block to fabricate the desired nanostructure. HNF can form periodic chiral nanostructure after the sequential two-step hierarchical phase transitions driven by saddle-splay elastic deformation. However, until recently, the lack of orientation tools for HNFs in a large area hinders further study and optical applications of this LC phase. To get uniformly oriented HNFs, we used a photoalignment process. New BLC molecules with photo-responsive azobenzene moieties are designed, and the HNFs are uniaxially controlled by the light illumination method. Uniformly oriented HNFs show unique chiro-optical properties with structural coloration. Based on these achievements, practical optical applications have been suggested, such as a chiral sensor, security code, and color filter.