Fig. 1. Schematic of the proposed SU-8 grating structures for multilevel optical security print based on different light illumination conditions, namely bright-field reflection, dark-field reflection, and cross-polarized reflection.

Fig. 2. Bright-field and dark-field colors. (a) Bright-field and (b) dark-field reflection micrographs for gratings with gap varying from 500 to 800 nm and width varying from 150 to 750 nm. (c) Bright-field and (d) dark-field reflection micrographs for gratings with period varying from 800 to 2000 nm and grating width fixed at 300 nm. The dose factor is varying from 1 to 3.5 with base dose of 1  μC/cm2. (e) Measured dark-field scattering spectra. (f) Corresponding color coordinates in the CIE1931 color space. (g) Illustration of maximum diffracted angle for wave diffracted to the opposite condenser. (h) Illustration of minimum diffracted angle for wave diffracted to the same condenser.

Fig. 3. Cross-polarized reflection. SEM images of the grating orientations of (a) 0° and (b) 45°. (c), (d) and (e), (f) Corresponding optical micrographs under dark-field illumination and cross-polarized reflection. (g) and (h) Schematics of polarization change upon reflection from different grating orientations.

Fig. 4. Multilevel security print. (a) Bright-field micrograph of grayscale image caused by different density of structures. Dark-field micrographs (top) of the image with bright and vivid colors and part of the corresponding SEM images (bottom) with (b) non-randomization and (c) randomization of grating orientations. (d) Bright-field micrograph of cross-polarized reflection of the image with the hidden information encrypted by the orientations of the gratings. (e) Bright-field micrograph of grayscale image caused by different density of structures. The “SUTD” word was exposed with dose of 2  μC/cm2, causing it to be taller, while the rest were exposed with dose of 1  μC/cm2. (f) Dark-field micrograph of the image with bright and vivid colors caused by diffraction effect. The “SUTD” word appears to be less obvious in the dark-field image. (g) Hidden information in bright-field cross-polarized reflection.