Design of embedded tri-color shift device

To improve the performance of existing guided-mode resonance (GMR) anti-counterfeiting grating, a tri-color shift device based on a one-dimensional (1D) singly periodic rectangular structure and ZnS film is reported. By turning the azimuths, the proposed device exhibits tri-color shifts of blue, green, and red for both TE and TM polarizations simultaneously. As the natural light can be considered as a superposition of TE and TM polarizations, in order to achieve the azimuth-tuned tri-color shifts of blue, green, and red, the wavebands and magnitudes of the reflection peaks for TE and TM polarizations should be designed at three azimuths, that is, at the first azimuth, high reflectivity in blue band and low reflectivity in green and red band should be reached; at the second azimuth, high reflectivity in green band and low reflectivity in blue and red band should be reached; at the third azimuth, high reflectivity in red band and low reflectivity in blue and green band should be reached. Considering these design goals, the evaluation function is established. By making the rigorous coupled wave analysis, the 0th reflectivity of the device can be numerically solved, which is relative to the incident light parameters (, , , ), the structure parameters (f, T, dg, dc), as well as the refractive indices of all the regions (ni, nc, ns). There is no analytical relationship between these parameters and the 0th reflectivity. So genetic algorithm is used to optimize the evaluation function, and then the optimal parameters of the tri-color shift device are obtained. When T=431.5 nm, dg=124.2 nm, dc=13.1 nm, f=0.5, and =45, at azimuth angle 0, natural light has reflection peaks at 468 nm and 442 nm; at azimuth angle 58, natural light has reflection peaks at 557 nm and 521 nm; at azimuth angle 90, natural light has reflection peaks at 690 nm, 673 nm, 650 nm and 644 nm. As a result, the device exhibits blue, green and red color responses at 0, 58 and 90 azimuth, respectively. The research results are explained in physics. Furthermore, the influences of key parameters on the reflection peaks are investigated. It is found that the reflection peaks of blue, green and red light are red-shifted with the increase of device period, groove depth, coating thickness and the decrease of incident angle. When the period, depth, thickness, and the incident angle are changed by 4.6% ( 20 nm), 27.4% ( 34 nm), 100% ( 13.1 nm), and 11.1% ( 5) with respect to the original designs, respectively, the device can well keep the color-shift effects of blue, green and red. The results above are meaningful in the designing, manufacturing and testing of the device. Compared with the existing GMR anti-counterfeiting grating, the tri-color shift device has high anti-counterfeit and appreciative value because of the harder designing and richer visual effect. Moreover, the 1D simple periodical structure is good for the manufacture of the high-precision master masks, and the device can be massively produced at low cost by the traditional embossing and evaporating technique in the laser holography industry. This tri-color shift device breaks through the limit of bi-color shifting technology, and may have great applications in the field of the optically variable image security.