Fig. 1. Illustration of a UEDP-based camouflage metasurface. (a) Schematic of wavelength-dependent light modulation using a metasurface. (b) Two types of common pixel schemes for realizing wavelength multiplexing. A superpixel can be formed either by combining multisized meta-atoms or by combining identical meta-atoms. The near-field interaction among adjacent meta-atoms in one superpixel may cause spectral cross talk, while the color stitching scheme with unchanged density may cause intensity imbalance among different multiplexed color channels. (c) Four basic designs of UEDPs and the schematic of the UEDP-based metasurface consisting of nanofins with varied sizes and filling density. Holographic images can only be observed under specific conditions.

Fig. 2. Simulation on cross-polarization transmission spectra and electric fields of arrayed silicon nanofins with different sizes. (a) Schematic of the basic structural unit using a SiO2 substrate, a single-crystal silicon rectangular column. (b) Dependence of transmission on width and length at an incident wavelength of 633 nm. (c) Dependence of transmission on wavelength and width for a fixed length of 80 and 160 nm. (d) Dependence of transmission spectra on the length of meta-atoms, where W=40nm. (e) Electric field distributions at two different periods, where λ=633nm and L=160nm, corresponding to the red channel.

Fig. 3. Design of a UEDP-based metasurface for integration of four holograms with one color printed image. (a) Simulated polarization conversion efficiency curve for the four types of meta-atoms with the same period, 300 nm. The incidence is circularly polarized light. (b), (c) The SEM images and transmission of three fundamental types of UEDPs with a size of 1.2μm×1.2μm. The period is 300 nm for the UEDPs supporting mixed color and yellow color and 200 nm for the UEDP supporting blue color. (d) Design flow of the UEDP-based camouflage metasurface. The printed image, which consists of gray, yellow, and blue color regions, is at the center. Four printing color channels of A1 to A4 are extracted from the center printed image. The color areas also support their corresponding holographic images of H1 to H4. (e) Flow chart of the modified GS algorithm.

Fig. 4. Experimental demonstration of a UEDP-based camouflage metasurface, i.e., four monochromatic holograms integrated with a color printed metasurface. (a) Optical setup for observation of the color printed and holographic images. (b) The SEM images of the metasurface. (c) The printed image observed under the illumination of a white light. The size of the printed image is 360μm×360μm. (d) The four monochromatic holographic images under the illumination of circularly polarized light at wavelengths of 633 nm (LCP), 594 nm (LCP), 532 nm (RCP), and 473 nm (RCP), respectively. The scale bars in four holographic images are the same: 100μm. f is the distance between the metasurface and the viewing screen.