Invisible photonic printing, an emerging printing technique, is particularly useful for steganography and watermarking for anti-counterfeiting purposes. However, many challenges exist in order to realize this technique. Herein, we describe a novel photonic printing strategy targeting to overcome these challenges and realize fast and convenient fabrication of invisible photonic prints with good tenability and reproducibility. With this novel photonic printing technique, a variety of graphics with brilliant colors can be perfectly hidden in a soft and waterproof photonic-paper. The showing and hiding of the latent photonic prints are instantaneous with magnet as the only required instrument. In addition, this strategy has excellent practicality and allows end-user control of the structural design utilizing simple software on a PC. R ecently, photonic printing technique based on fabrication and manipulation of photonic structures has received increasing attention across science and engineering disciplines because of its applications in photonic circuits 1 , on-chip photonic devices 2, 3 , reflective display units [4] [5] [6] [7] , outdoor signage and photonic sensors [8] [9] [10] [11] [12] [13] [14] . Various photonic printing strategies have been extensively investigated in order to realize the fast and convenient preparation of photonic prints with good tenability and reproducibility. Successful strategies include the self-assembly of colloidal crystals locally on a substrate [15] [16] [17] [18] , and the change of photonic structure by tuning the refractive index (n), lattice spacing (d), and crystal orientation (h) in a specific region of the pre-made colloidal crystals or related composite materials [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] . Invisible photonic printing, an emerging photonic printing technique with tremendous advantages, is especially useful for steganography and watermarking for anti-counterfeiting purposes. However, it is still not easy to realize because of the many great challenges 29 . First, the mostly reported visible photonic printing creates photonic prints with a permanent contrast of reflection wavelength (Dl?0), while the invisible photonic printing requires the photonic prints with an initial zero contrast (Dl 5 0) and later a non-zero contrast (Dl?0) when the latent information is revealed. However, a large amount of responsive content on the print is required in order to obtain a large contrast under external stimuli. Consequently, this will lead to a large deviation from invisibility due to the gradually increased mismatch of refractive index or lattice constant 30 . Secondly, the obtained latent prints must have long-lasting stability, excellent reversibility and the ability of being quickly released with a convenient and non-toxic means such as heating or magnetic field. Recently, we have been engaged in ongoing efforts aimed at the development of novel photonic materials with special functions 28, 31, 32 . For example, we reported the fabrication of a novel photonic wordpad from fast magnetically induced self-assembly of superparamagnetic colloidals, which is fixed inside a polyacrylamide glycol gel matrix through an instant radical polymerization 28 . Herein, we demonstrate a novel photonic printing strategy targeting to overcome the above challenges and realize fast and convenient fabrication of invisible photonic prints with good tenability and reproducibility. A variety of graphics with brilliant colors can be perfectly hidden in a soft and waterproof photonic-paper (P-paper) utilizing this novel technique. When the functionalized P-paper is put into a vertical magnetic field, only the prints with magnetic-responsive photonic activities are expected to be changed. The pre-hidden patterns will be revealed due to vastly different reflection wavelength contrast between the background and the print. From the perspective of practical printing technique, this strategy is of great interest due to its superior features, including the ability to (1) generate invisible patterns with magnetic showing effect, (2) obtain end-user control of structural design utilizing simple software on a PC, and (3) allow fast, convenient and scalable production at low-costs. Moreover, the adaptation of the P-paper for invisible photonic printing
Another approach is printing with invisible inks that can be revealed under ultraviolet light REF .
Invisible photonic printing: computer designing graphics, UV printing and shown by a magnetic field
Scientific Reports
Scientific Reports
Computer Science Medicine