Photonic Crystals for Dichotomous Sensitivity to Strain for Sensor and Indicator Applications

Photonic crystals (PC) with structural colors have been used in various sensors or indicators given that they change the colors upon mechanical deformation. However, the color also changes according to the incident or viewing angle, presenting a challenge when the structural color serves as the practical sensor or indicator. Therefore, it is necessary to compensate for this color variation caused by the incident or viewing angle. This study presents devices in which strain-sensitive photonic crystals (SSPCs) and strain-insensitive crystals are fabricated together. The photonic crystals were fabricated by the self-assembly of nanoparticles and the deposition of a thin metallic film, after which they were transferred onto flexible and adhesive tape so that the devices, i.e., the sensors and indicators, could be fabricated. When the nanoparticles were in a close-packed arrangement, the thin film formed into a continuous layer upon a monolayer of the nanoparticles. Accordingly, the lattice constant of the PC did not change while the device were deformed, resulting in strain-insensitive photonic crystals (SIPCs). Meanwhile, when the nanoparticles were not in a close-packed arrangement, as realized by deep reactive ion etching, the thin film formed into separate layers on the individual nanoparticles. The lattice constant of the PC varied in this case when the device were deformed, resulting in strain-sensitive photonic crystals. The devices consist of both types of photonic crystals such that the colors from each PC could be compared to each other, enabling angle-compensating characteristics. In this way, the colors can be used as effective signals of mechanical strain or as chemical liquids at arbitrary angles. They can also be used as an anticounterfeiting stamp. This study thus introduces a fabrication method that therefore enables a choice between strain-sensitive and strain-insensitive photonic crystals.