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Among many types of smart materials, responsive photonic bandgap materials, or more commonly known as responsive photonic crystals, which can change their color in response to external stimuli, have attracted much attention due to their important uses in areas such as color displays, biological and chemical sensors, inks and paints, and many active components in optical devices. The unique colors originating from the interaction of light with periodically arranged structures of dielectric materials are often called structural colors, which are iridescent and metallic, cannot be mimicked by chemical dyes or pigments, and they are free from photobleaching unlike traditional pigments or dyes. Many interesting applications have been proposed for responsive photonic crystal structures. For example, they may be used as optical switches for full automation of optical circuits when significant improvements towards the quality of colloidal crystals and their response time are realized. Military vehicles covered with such materials may be able to dynamically change their colors and patterns to match their surroundings. Such materials might also be embedded in banknotes or other security documents for anti-counterfeiting purposes. The hidden information cannot be revealed until an external stimulus such as a pressure or temperature change is applied. The photonic effect can also be used as a mechanism to develop chemical and biological sensors for detecting target analytes by outputting optical signals. These types of crystals may also find great use as active color units in the fabrication of flexible display media, including both active video displays and rewritable paper that can be reused many times.

Compared to photonic crystals prepared by microfabrication methods, self-assembled photonic crystals, in particular colloidal crystals, can be produced at much lower costs and with higher efficiencies owing to the parallel nature of the self-assembly processes. It is also more convenient to modify the building blocks before or after the formation of crystal structures to enable responsiveness to a given stimulus. As a result, the majority of research on responsive photonic nanostructures has been focused on constructing the photonic crystal structures and incorporating stimulus-responsive materials into the self-assembled photonic crystal structures. In principle, the stimulus can be any means that can effectively induce changes in the refractive indices of the building blocks or the surrounding matrix, and changes in the lattice constants and/or spatial symmetry of the crystalline arrays. While various responsive mechanisms have been developed, such as mechanical stretching, solvent swelling, and temperature-dependent phase change, the research activities in the field have been focused on broadening the tunability of the photonic properties, enhancing the response rate to the external stimuli, improving the reversibility, and integrating into existing photonic devices.

This book highlights several recent areas of progress in the self-assembled responsive photonic nanostructures based on a number of different tuning mechanisms. Among all photonic crystal structures, one-dimensional Bragg reflectors that consist of alternative multilayers of two materials with different dielectric constants are regarded as the simplest type of photonic nanostructures. Calvo and Míguez first discuss recent progresses in the development of such materials for potential applications in sensing owning to their ability to respond to changes in the surrounding environment with a modification of their optical properties, generally caused by a variation of either their refractive index, the thickness of the constituent layers, or both. Self-assembled opals of close-packed colloidal crystals from monodisperse colloidal particles have predominantly served as the starting frameworks for constructing responsive photonic nanostructures. Stimulus-responsive materials can be incorporated into the periodic structures either as the initial building blocks or as the surrounding matrix so that the photonic properties can be tuned. Such colloidal crystals may also be used as the templates to fabricate inverse opals. Various versions of tunable opals and inverse opals have been developed that can respond to a wide range of external stimuli such as mechanical stretching, humidity, light, and temperature change, as reviewed separately by Fudouzi, Gu and Stein and coworkers. Since the opal structures themselves are relatively weak due to the fragile contact points between spheres within the structure, many structurally deformable photonic structures have been made from close-packed or nonclose-packed colloidal crystal arrays encapsulated within a hydrogel or polymer matrix that fills the void space surrounding the colloidal crystal, as discussed by Kanai and Takeoka. Through the infiltration of a defect layer of liquid crystals into photonic structures, the optical properties can be reversely manipulated by the external electric fields to realize the electrochromatic effect. The relevant research has been summarized by Ozaki and coworkers. Yin and coworkers also highlight recently developed magnetically responsive photonic nanostructures with widely, rapidly and reversely tunable structural colors across the entire visible and near-IR range, which utilize the magnetic field as the convenient stimulus to tune the optical properties by affecting the lattice constant, the orientation, and the structures of the colloidal assemblies. We hope this book will serve as a useful reference to researchers interested in smart nanoscale optical materials, in particular, responsive photonic nanostructures.

Yadong Yin

University of California, Riverside

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