Figure 1.

Schematic for hiding a high-resolution grayscale image. Under the illumination of linearly polarized light, two reflected beams with a spatially variant linear polarization profile are generated, which can be used to hide a high-resolution grayscale image (1300 × 1300 pixels and 256 grayscale levels). Only one reflected beam is shown here for demonstration. The two beams are exactly identical except for the propagation direction. The hidden image is revealed by an analyzer (linear polarizer) (a), without which no image is obtained (b). The metasurface consists of gold nanorods with spatially varying orientations on the top, a SiO₂ spacer (85 nm) and a gold background layer (150 nm) on a silicon substrate. The grayscale image is encoded into the polarization profile of the resultant beam at subwavelength scale via the metasurface. The size of each pixel is 300 × 300 nm, and the overall size of the image is 390  × 390 μm.