Responsive Photonic Crystals And Its Application In Anti-fraud Printing

Responsive photonic crystals (RPC) are dielectrically periodical structures, which can alter their photonic band structure and change their reflected signal and structural color upon exposure to physical or chemical stimuli. Many kinds of photonic prints have been developed based on the combination of responsive photonic crystals and printing method. Compared with the traditional photonic prints with an unchangeable pattern, invisible photonic prints should be unseen under normal circumstances, but can be recognized by specific method later, and it will be unseen again when the stimulation is removed. So it is an ideal anti-fraud material as the invisible photonic prints have recordable information and self-maintained patterns of structure colors, and the pattern can reversibly transform between showing and hiding.In this paper, three kinds of responsive photonic crystals prints are fabricated through supersaturation induced precipitation and self-assembly of silica particles, which can change their structural colors to reveal the invisible pattern under deformation, soaking and electric field respectively. At the same time, the pattern will be invisible when stimulation is removed. The key point of fabricating invisible photonic prints is the creation of "pattern"’and "background" with different response to the external stimuli. Meanwhile, their photonic structure should be very close at normal state. That is, "pattern" and "background" have a zero contrast (△λ0= 0) at first and a large contrast (△λt≠0) under a particular stimulus to reveal the hidden information. Here, invisible photonic prints which can be revealed by simple method, such as squeezing, soaking and applying electric field have been developed. We investigated the speed and sensitivity of response, the reversibility of showing and hiding, and the resolution of the pattern to optimize the performance of the prints. The invisible prints shown by deformation can be revealed immediately when squeezed or stretched. The pattern of soaking and electric field based invisible prints can also be shown in 10 seconds when corresponding stimulation applied. The resolution of these prints can reach 100 μm. With these merits, it is very suitable to apply these invisible photonic prints to fabricating antifraud labels.