Realization of Structural Colors and Dual Image Storage Based on Optical Micro-resonators

Abstract

Recently, structural color has attracted tremendous interest owing to its potential as an alternative to colorant pigment. In contrast to conventional coloration where color is primarily determined by the intrinsic optical absorption of chemical pigments, structural colors arise from the resonant interaction of light with optical nanostructures. Accordingly, structural coloration has several unique features, being differentiated from chemical pigmentation. First of all, any materials can be used for coloration giving more flexibility in material choice. Secondly, overall optical efficiency is relatively high owing to small optical absorption. Thirdly, the size of color pixel can be miniaturized into the scale of diffraction limit. Fourthly, color can change in response to external input by employing active materials. Owing to these attributes, structural colors are anticipated as new platform for the development of highly efficient nanoscale color devices as well as novel application areas that are inaccessible with conventional technologies. Toward the realization of structural colors, there have been largely three strategies according to device platform

photonic crystals, plasmonic nanostructures, and optical micro-resonators. Among them, optical micro-resonators have been considered the most promising owing to high optical efficiency, simple fabrication process, and polarization-independent optical properties. For practical applications, however, several technical issues should be addressed. Firstly, low-cost and high-throughput color integration techniques should be developed. Secondly, the color properties such as angle-dependency and color gamut should be improved. Thirdly, new functionalities such as dynamic tunability should be imposed for use in more broad range of applications. In this thesis, we investigate new device architecture and fabrication techniques to resolve above mentioned technical issues and by extension, develop novel applications by making use of these approaches. First, we present simple and powerful fabrication method based on a series of transfer printings of dielectric materials for the large-area integration of different color elements. By using proposed technique, we demonstrate the prototype of LCDs incorporating color filters based on optical micro-resonators. Toward improving optical properties, we investigate the effect of embedding metallic nanostructure as a phase-shifting element inside resonant cavity. Depending on the geometrical structure of metallic nanostructure, the resonant peak shifts to longer or shorter wavelengths. The ultrathin thickness of a phase shifting element allows for the integration of different color elements as well as the extension of color palette within a uniform thickness of resonant cavity. Moreover, we demonstrate a flexible color element showing no color change during being bent by adopting metallic nanoislands that significantly reduce the angle dependency. Lastly, we also demonstrate novel polarization-tunable color elements having a liquid crystal polymer layer for resonant cavity. Color elements that have different directions of optical anisotropy can be easily integrated on a single substrate by using a series of photoalignment processes. This feature allows for recording two different images within the same device area and resolving them by applying different polarizations. The new device architectures and fabrication techniques presented here would pave the way toward the practical implementation of structural colors for a wide range of visual applications ranging from color image storage to advanced display systems.