In this paper, published in Optics Express, we design and experimentally demonstrate the two-tier manipulation of optical information using holographic metasurfaces. Our devices encodes different information that can be retrieved if two keys are simultaneously used. Here, one key is the wavelength used to probe the holographic metasurfaces. The other key is the medium in which the metasurface is held. Specifically, one image is obtained for visible light at 705nm and air and a different, uncorrelated image is obtained for a wavelength of 750nm in water.
The key element that enables our technology is the use of resonant meta-atoms (the unit cell of the metasurface) which have high reflectivity at the desired conditions. Importantly, the device shows very little crosstalk between the two states, which means that only one image is visible at any one time, for the designed conditions. In the image below, Ms Jianling Xiao shows the operation of the device out of the intended operation conditions, to reveal both the encoded images.